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Geodesy in the Year 2000 (1990)

Chapter:GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000

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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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Suggested Citation:"GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000." National Research Council. 1990. Geodesy in the Year 2000. Washington, DC: The National Academies Press. doi: 10.17226/1495.
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GPS-Based Geodesy Tracking: Technology Prospects for the Year 2000 W. G. Melbourne Jet Propulsion Laboratory California Institute of Technology Pasadena, California 91109 INTRODUCTION The twenty-first century is a scant dozen years away, too near for many landmark advances in technology, yet long enough for a few surprises to emerge. To put it into focus one need only look backwards 12 years to the way things were in 1976, the year of our bicentennial. The Navstar/Global Positioning System (GPS) program had already been underway for two years. Meanwhile, we are still awaiting the launch of the first Block II satellite, which will mark the onset of the GPS operational phase. Of course, history is laden with inaccurate predictions of future technology trends based on current knowledge. My message is really ambivalent. Don't expect too much progress in the decade of the 1990s; on the other hand, do expect some surprises because technology often advances in quantum jumps. In the case of GPS, the evidence today unmistakably suggests future trends for high accuracy geodesy and Earth satellite tracking. Given the current performance of GPS, particularly on regional baselines, one can say that GPS has arrived. Its attributes of high accuracy and relatively low cost make it the geodetic technique of choice in the l990s for most of the regional deformation studies that require high temporal and spatial resolutions. It has even greater potential for the future. I would like to discuss this potential in terms of: a. future accuracies of GPS geodetic systems, b. future costs for data acquisition equipment, operations and data analysis, and c. future applications in ground and satellite geodesy. PROSPECTS FOR MILLIMETER ACCURACIES IN GPS-BASED GEODESY What are the limiting error sources today for GPS systems and what are the prospects for their improvement? I think that we should take our cues on these questions from experience with VLBI. The horizontal accuracy from Mark VLBI III systems ranges from around 1/2 - 1 cm on regional baselines to 1 - 1-1/2 cm on transcontinental baselines (Clark et al., 1987; see chapter by Rogers) the vertical accuracy is about a factor of 2-3 worse because of tropospheric water vapor errors and limitations of observational geometry, i.e., one cannot observe below 124

125 one's horizon. Roughly the same ratio between horizontal and vertical appears to hold in GPS accuracies. However, there is some suggestion that the more robust observation scenario with GPS systems, in which GPS satellite and ground receiver clock instabilities are isolated to prevent possible corruption of baseline estimates, yields better accuracies in the vertical component than are obtained with VLBI. Today's evidence (Blewitt et al., 1988) supports the position that GPS horizontal accuracies are comparable to VLBI accuracies on regional baselines. For example, Figure 1 shows the agreement between GPS and VLBI-determined baselines over a range of lengths up to 2000 km for several measurement campaigns spanning 1985-1988. Centimeter-level agreement is the norm for regional baselines. On transcontinental baselines, GPS systems are not quite as accurate as VLBI because of current limitations in the control of GPS reference system errors, which should be remedied in the near future with stronger GPS fiducial and/or global tracking networks. The principal errors limiting GPS performance today arise from: a. mix-modeling and/or mix-calibrating the propagation media, b. antenna multipath, c. antenna phase center variability, and d. reference system errors including the terrestrial reference frame, limitations in alignment with VLBI and SLR frames, and the GPS satellite ephemerides. I will now summarize the present status of these error sources and try to estimate their future course in the 1990s. To reach the millimeter level will also require dealing with error sources that are presently masked by these ''tall poles'' cited above. For example, seasonal environmental effects such as ground water variability can certainly affect the height of monuments at the millimeter level in certain soil conditions. Ocean loading is another. GPS satellite multipath will have to be dealt with on longer baselines. I will mainly focus on the current limitations and the prospects for improving them. PROPAGATION MEDIA Tropospheric water vapor is the major error source for VLBI and GPS systems. Small errors can arise from mix-modeling or mix-calibrating the troposphere and the ionosphere. Dry Troposphere. The total zenith delay of the dry troposphere is about 2 m and is readily determined barometrically to a precision of about 1 mm using standard atmosphere models. Departures from hydrostatic equilibrium of the dry component of the troposphere leads to zenith delay mix-modeling that can amount to a few millimeters in certain dynamical conditions. For baselines longer than the correlation length of these departures from hydrostatic equilibrium, differential delay errors will translate into comparable errors in the horizontal

126 components and into larger errors in the vertical component. Bender (1987) has discussed the potential error arising from the dry component of the troposphere. To reach 1 mm accuracy it may be necessary to utilize synoptic meteorological data under some conditions; improved mapping functions may be required to transfer from the zenith to line- of-sight delays. Ionosphere. There are small departures from the standard dual band plasma frequency correction for group delay and phase advance through the ionosphere. Also, for equatorial and high latitude observations, particularly those made during very high solar activity, ionospheric scintillations can be troublesome. The rapid phase variations and concomitant signal amplitude in the L-band carriers induces tracking errors in-the tracking loops of some receivers and even loss of lock; on the other hand, some receivers of more recent design are capable of tracking signal frequency accelerations of up to several g's, even in adverse signal conditions, without significant error at the millimeter level. Code-tracking receivers definitely have a big advantage over codeless receivers in weakened and highly variable signal conditions. Consequently, this aspect of the ionospheric problem is likely to improve greatly, even with the high solar activities expected in the early l990s, with the advanced receiver designs expected by then. The other aspect is the departure from the dual frequency correction resulting from ionospheric density gradients and from the small corrections in ionospheric refractivity due to the presence of the geomagnetic field. These effects, which depend on the third and higher degrees in carrier frequency, can amount to several millimeters at low elevations during high ionospheric activity. Another error contribution arises from the residual LCP component of the GPS signal which is about a factor of four below the RCP component. The propagation velocities of the RCP and LCP waves differ slightly in a plasma containing a magnetic field. The rejection of the LCP component by GPS antennas is not complete, typically 10-20 db lower (in power). As a result errors relative to the dual band correction up to a few millimeters can result. Additional modeling based on the dual frequency correction may be required to reach the millimeter level (Clynch and Renfro, 1982~. Water Vapor. Water vapor zenith delays range from 5 cm in dry and cold conditions, upwards to 50 cm in tropical conditions. Water vapor is not in hydrostatic equilibrium. As a result, in situ meteorological measurements to infer total water vapor delay can be in error by as much as 100% of the actual delay; they have fallen into disrepute. Currently, stochastic modeling and water vapor radiometry-based calibration are the only viable techniques for dealing with water vapor. Using the best technology currently available, mix-modeling and/or mis- calibrating the differential water vapor delay at the geodetic sites results in expected horizontal baseline errors that range from a few millimeters to perhaps a centimeter depending on the strategies used, on the local meteorological conditions, and on the length of the baseline (Elgered et al., 19881. At these error levels the correlation length

127 for water vapor is well below 100 km for many regions (Treuhaft and Lanyi, 1987~. For the vertical component these error levels should be increased by a factor of three. State-of-the-art water vapor radiometers have a stability over several hours of about lK in measuring sky temperature (Jannsen, 1985~. This translates into an accuracy in zenith delay of about 0.5 cm. Short period stability can be somewhat better but this is less important for baseline accuracy than long period stability. Assuming the WVR error can be characterized as a random walk process with an expected error accumulation of 0.5 cm after four hours of tracking, one obtains about 3 mm as the expected error in the horizontal baseline components. A meteorologically benign region, such as the relatively dry western U.S. or even a tropical area with a uniform marine layer, would be characterized by low temporal and spatial variability in water vapor delay. For these regions, an equally successful approach is to stochastically model the tropospheric zenith delay as a piecewise constant random walk or as a first order Gauss-Markov process. The values of the constants in this time series are adjusted in a batch- sequential least squares process in which both the transition from one value to the next and the length of time over which the vertical delay is held fixed are a priori constrained by the stochastic model. By judicious choice of the values of the parameters in the stochastic model, one can obtain baseline repeatabilities that are competitive with those obtained with WAR calibrations (Herring et al., 1989; Tralli et al., 1988~. Whether this approach will prevail in less benign tropospheric conditions with high spatial and temporal gradients such as those sometimes prevailing at Kokee Park, Kauai is undetermined. A general consensus remains elusive on strategies for dealing with the water vapor problem toward achieving 1 mm horizontal baseline accuracy. A major reason for this is that the variability of water vapor delay, both spatially and temporally, is not well understood as a function of locale, season and current meteorological conditions. Of particular concern is the spectral power of the variability at periods of roughly a half hour and longer; these are the components that can partially mimic the signatures of the geodetic parameters in the tracking data and thus corrupt their estimates. Continuously operating arrays of GPS receivers, to be discussed later, offer some promise in developing a data base that should enable one to quantify the spectral properties of water vapor variability and to develop strategies for exploiting this knowledge. Pushing the WVR-based methodology to yield l mm horizontal baseline accuracies will~be a major technological challenge. To achieve this, the current dual channel WAR will have to be expanded to several channels and the sky temperature measurement accuracies improved to O.1K, a tough requirement. Current WAR system temperatures are several hundred Kelvins. Future W7R's using HEMT technology will probably achieve around NOOK, thus, the precision requirement will be around 0.1%. However, the accuracy requirement of O.1K is more stringent

128 requiring corresponding instrumental and calibration stability over several hours. Further, the retrieval algorithms for inversion of the measured sky temperatures to the water vapor-induced path delay, which involve similar but different integral equations along the line-of- sight, will have to be greatly improved over current algorithms; they are likely to become highly dependent on the region and the local meteorology. Also, mapping functions to map WVR line-of-sight delays to the local vertical or to the GPS directions will need further development. To achieve millimeter-level performance by WVR calibration is going to be very expensive in the short term. Current W7R's cost around $125K and that price is likely to go up unless major strides are made in solid state digital RF systems; that is an ambitious undertaking today at these relatively high frequencies (20-60 GHz). However, if present development trends persist in microwave monolithic integrated circuits (MMIC technology), the mid-l99Os may see virtually all-digital W7R's including digital sampling at these RF frequencies enclosed in a highly stable oven that occupies a volume of less than 106 cm; in addition to costing an order of magnitude less than current designs, it would also promise an order of magnitude improvement in instrumental stability. Using a phased array antenna for sky coverage would also improve performance and reduce cost. Nevertheless, the cost trend for the WAR is likely to move opposite to that of the GPS receiver technology in the early 1990s; the current factor of two in their relative unit cost is likely to grow to ten. Unless their costs can be reduced by an order of magnitude, WAR systems will not be affordable in large numbers and would be concentrated mainly at VLBI fiducial and other sites where the highest accuracy is sought. While it may be theoretically possible for GPS-based geodetic systems to achieve millimeter horizontal baseline accuracies with major capital outlays, as a practical matter we may be mostly limited to accuracies attainable with the stochastic modeling approach. Stochastic modeling will reach an impasse at some accuracy level. The short period (t<l/2 hour) spectral components will effectively increase the carrier phase measurement noise; the longer period components can correlate with baseline offsets. These limiting accuracies are really unknown at this time because other error sources have masked the inherent accuracy of the stochastic approach and there are additional refinements to it that are yet to be made. I am optimistic, however, that water vapor-induced baseline errors can be driven well below 5 mm for many applications and that 1 mm can be approached, given sufficient averaging time. The question still unanswered is: what is the minimum averaging time to reach 1 mm, or alternatively, what is the expected baseline accuracy versus averaging time due to mix-modeled water vapor errors? This may be the limit on resolving short period geophysical signatures in the tracking data.

129 ANTENNA MULTIPATH Antenna multipath refers to the perturbation of the received signal due to the presence of reflective objects in the vicinity of the antenna. These spurious signals are characterized by additional delay and usually smaller amplitude relative to the direct signal, and they contain phase shifts. They combine interferometrically with the main signal to cause phase and amplitude variations; these are processed by the receiver and manifest themselves in the measurements as carrier phase and group delay errors. Thus, both the carrier phase and the pseudorange measurements are corrupted by multipath, although in fundamentally different ways. Their manifestation also depends on the signal processing architecture of the receiver. Carrier Phase Multipath. Carrier phase multipath depends on the signal processing scheme used to recover the carrier. For receivers using a half chip early/late gate correlation approach multipath can arise from objects up to distances of 450 m for C/A-based recovery and 45 m for P- code-based recovery. Figure 2 gives an example of multipath in carrier phase measurements. Here, the plots are the post-fit smoothed residuals of double-differenced carrier phase measurements of two GPS satellites from two ground receivers. Ionospheric effects have been eliminated with dual band measurements. The residuals are shown for two separate diurnal passes (an arbitrary vertical offset and the siderial period adjustment in the abscissa are included for illustrative convenience). The ground receivers were TI-4100s with the standard TI-4100 antenna. The RMS variation about zero mean is around 1-1/2 cm. Notice the high degree of diurnal repeatability; the cross correlation function peaks strongly for a zero time lag. Multipath in the carrier phase measurements will certainly have to be dealt with if 1 mm baseline accuracies are sought. It is the long period spectral components of multipath variability, hence, reflections from nearby objects, i.e., roughly 1/2 hour and longer, for which one needs to be concerned. The shorter period components will tend to average out over several hours of observations and will have little corrupting influence on the baseline estimates. The high degree of diurnal repeatability in multipath suggests that a major fraction of it could be calibrated out by building up an empirical template from a number of successive passes.: But one would be vulnerable to 1. The Navstar/GPS Joint Program Office has recently announced that it is considering raising the GPS satellite altitudes about 50 km, which would increase their period by two (2) minutes. This would cause the ground tracks to drift westward 1 degree per day; thus, exact daily repeatability would no longer hold and would be replaced with an annual repetition. Near exact repeatability might occur at intermediate intervals but it would involve different satellites. Although this adjustment breaks an undesirable resonance and results in improved station keeping; however, it would be unfortunate for those wishing to calibrate diurnal multipath effects.

130 environmental changes around the antenna site during the intervening periods between revisitations with a GPS receiver; these could lead to different multipath signatures and hence, to multipath-induced baseline errors that are different on the two visitations. Thus, short-term baseline repeatability over a few days might be an optimistic measure of long-term repeatability spanning several years of site revisitations. These considerations also lead one to require standardization and configuration control of antenna/backplane assemblies over a span of years because of internal multipath in these systems. Counting on common mode error cancellations is a dangerous game when the outcome depends on the stability of two big numbers that are differenced! Pseudorange Multipath Errors. Multipath error in the pseudorange depends on the dispersive character of the multipath effects across the power spectrum of the pseudorange PRN code. Pseudorange measurements are made by matching local models of the ranging code generated at the receiver with codes being received from the GPS satellite. In general, the received signal is a combination of direct and reflected signals with different delays, phases and amplitudes. The effect of this combined input is to distort the shape of the correlation function between the model and received signals. This shape distortion maps into error in determining the point of maximum correlation and this mapping function will depend on the particular correlation scheme used by the receiver. As an example, for a half chip early/late gate scheme, a peak error of ~5 ns occurs when a multipath signal is present with an amplitude of 0.1 and an additional delay of ~0.5 chip (1 P-code chip = 97 us) relative to the direct signal. Because the effective "wavelength" of P-code pseudorange is two orders of magnitude greater than the carrier wavelengths, the pseudorange mulipath error is typically a factor of 100 greater than carrier phase multipath error. An important exception is the case where the reflecting objects are within 1 carrier wavelength of the receiving antenna. In this case, the carrier phase multipath is reduced by less than order of magnitude relative to pseudorange multipath. Figure 3 illustrates P-code pseudorange multipath effects for a typical environment using a TI-4100 antenna. The integration time for the points in this figure is two minutes; thus, thermal noise is an insignificant contributor to these residuals. Successive daily plots will attest to these residuals being primarily multipath-induced. RMS levels for P-code pseudorange typically are in the 1-2 m range for TI- 4100 measurements; in some cases they are worse, even causing loss of lock; in some cases they are better but rarely, if ever, are they below 50 cm. Multipath errors need not be anywhere near as high as those in Figure 3. With proper care in preparing a site with low multipath environment and with use of advanced antennas and, most importantly, backplane designs, multipath can be reduced by over an order of magnitude. Figure 4 shows the multipath level in a recent experiment

131 using a Rogue receiver which has a Dorne & Margolin antenna coupled with a choke ring backplane designed at JPL (see insert on Figure 4~. These results are from a single receiver tracking a single GPS satellite whose elevation with time is also shown. Here, the residuals are plotted in the form of two-minute points, the nominal output rate of the Rogue receiver, and also as half-hour running average points. The ordinate on this plot is the linear combination of P:, L: and L2 that eliminates both the ionospheric and range delays, where P: is the L1 pseudorange and L: and L2 are the carrier phase measurements, all expressed in range units. This linear combination should be constant with time; variations are measures of multipath, which for this combination is strongly dominated by that on P:. Also shown are two bands pertaining to performance specifications that we have placed on our antenna/backplane development effort. These specifications can in part be described by the requirement: multipath spectral power shall not cause P-code pseudorange residuals to exceed 30 cm for all periods of five minutes or less, nor exceed 5 cm for all periods of 30 minutes or longer. The latter condition is the most stringent one and the most relevant to baseline recovery accuracy. Except for very low elevations this test amply satisfied our long period multipath requirement. Future innovations on this design are likely to further improve these results. The particular test shown in Figure 4 demonstrates two points: (1) dramatic improvements in multipath can be achieved, so that smoothed P- code pseudorange accuracies below 5 cm appear feasible, and (2) high frequency multipath, which tends to originate from more distant objects, can be averaged down to relatively low levels. The very high repeatability of these residuals on a daily basis (with peak cross- correlation values well over 90% for this particular experiment) also suggests an approach for calibrating multipath effects in a stable environment. The Utility of Pseudorange. A brief digression on the value of high accuracy pseudorange would seem to be in order. Most GPS practitioners make little use of the P-code pseudorange for high accuracy applications. The reasons for this have been, (a) its relatively coarse (meter-level) accuracy, primarily because of multipath and partly because the architecture of the TI-4100 receiver, the standard of the 1980s, through its multiplexing and baseband processing design, resulted in effectively SNR-limited P-code performance, and (b) the uncertain future availability of the P-code on the Block II satellites through possible actuation of the anti-spoofing (AS) function. Receiver manufacturers have exploited that uncertainty by developing systems for the civilian market that avoid use of the P-code, using instead the C/A code to recover L: and various nonlinear detection schemes (e.g., Costas loops, signal squaring, delay and multiply, code enhanced squaring) to recover the second harmonic of L2 and in some cases P1, Pa, L1-L2 and P1- P2. Codeless recovery pays a price in the form of higher SNR-driven measurement errors, which is particularly significant for the P-code, and higher thresholds for signal detection and tracking the L2 carrier. However, receivers of the late 1980s and the l990s are and will achieve nearly an order of magnitude improvement in SNR performance through elimination of multiplexing, use of the P-code, use of double sideband

132 in-phase and quadrature processing, lower system temperatures, and elimination of interchannel biases through all-digital processing. Moreover, the average received signal strength (C/No) of the P-code modulation from the Block II satellites may be about 3 db higher. The DoD has progressively relaxed and/or clarified its position on the operation of AS over the last several years; its current announced position is that, except in times of national emergency, AS will be off except for periods of test and training which would be scheduled relatively infrequently and announced in advance. The DoD policy on Selective Availability (SA) is another matter to be discussed later; but SA does not normally render inoperative the P-code tracking functions of receivers without decryption capability. However, receivers without decryption capability (i.e., without the so-called W-chip and the crypto-keyed information), regardless of whether they are P-code trackers or not, would not be able to recover the corrections to SA in real-time. The discussion earlier suggests that 5 cm differential P-code pseudorange accuracies or better on 30-minute averages will be attainable for receiver and antenna/backplane systems that are well- designed and well-sited, and that it will be largely available to civilian users. Of what use is this capability given the already very high accuracy of carrier phase data which would easily be accurate to sub-millimeter levels if it were not limited by multipath and the troposphere? A number of applications come to mind. 1. Tracking GPS satellites with dual band carrier phase and 5 cm pseudorange would yield highly accurate global ephemerides obtained with far less tracking per pass than is required today. As a rough estimate, 5 cm is about 1 part in 108 on a baseline between two tracking stations 5000 km apart. If the effect of the satellite multipath on baselines of this length can be controlled or calibrated to 5 cm or better, one would expect to obtain ephemerides from a global network of GPS tracking stations that surpass 20 cm in accuracy, a factor of 5-10 better than today. 2. Carrier phase data that are phase connected, i.e., that have no cycle dropouts, provide range change information to nearly perfect accuracy but cannot measure absolute range unless the cycle ambiguities are resolved. Pseudorange measures absolute range but cannot compete with carrier phase in measuring range change. This suggests a synergistic approach (Hatch, 1982~. By combining these data types one can map a whole time series of pseudorange observations to a common epoch with nearly perfect accuracy using the continuous and phase connected carrier phase observations. Thus, all of the pseudorange observations made over a span of hours can be brought to bear on a single time point and their errors thereby averaged down. For that epoch, geometric positioning is accomplished with an effective ranging accuracy that should be far better than the accuracy of the individual

133 pseudorange points. This quasi-geometric point positioning approach will have major applicability to tracking from dynamical platforms such as ocean platforms and buoys, aircraft and spacecraft, and in kinematic ground tracking. It will also be used for high accuracy orbit determination of future Earth satellite missions such as TOPEX/Poseidon which will carry a GPS flight-rated receiver (Yunck et al., 1985~. This approach obviates the need for extensive dynamical models to generate the motion of the user platform. 3. It is well known that accurate pseudorange improves the accuracy of baseline determinations, particularly the east-west component (for most regions except very high latitudes) (Melbourne, 1985~. Coarse pseudorange accuracy, e.g., 100 m, provides sufficient accuracy in clock synchronization so that negligible error is introduced in the ephemeris look-up epoch. From that level down to 1 m little improvement is realized. However, for pseudorange accuracy below a carrier wavelength, baseline accuracy typically improves by factors of 1 to 5, depending on the component and the situation. This is because with carrier phase data alone, the baseline estimation process must include the carrier cycle ambiguities or biases which can typically be estimated to an accuracy of about 1/2 carrier wavelength or better. In those situations where it is not possible to exploit the integer character of the cycle ambiguities and hence to fix their values, introducing pseudorange with an accuracy that is comparable to or better than the a posterior) cycle ambiguity accuracy adds significant information bearing on their values; it aids in breaking the high correlations that tend to prevail between the biases and the eastern component of the baseline. Figure 5 gives an example of the effect of introducing pseudorange into the estimation process with increasing accuracy ranging from no pseudorange to carrier range, the latter being the case where the cycle ambiguities are exactly known. 4. Pseudorange can also greatly assist in resolving the carrier cycle ambiguities. Cycle ambiguity resolution strategies without highly accurate pseudorange have had moderate success for well designed networks that contain a range of different baseline lengths. These techniques have been applied to networks up to about 1000 km in extent (Blewitt, 1989; Dong and Bock, 1989~. But these schemes usually have not been able to correctly fix 100% of the cycle ambiguity integers in any given measurement session mainly because of ionospheric delay and ephemeris errors. Aiding the ambiguity resolution process with accurate pseudorange would virtually assure a 100% success rate at all baseline lengths. It can be shown (Melbourne, 1982) that the cycle ambiguity integers for L1 and L2 are given by the expressions: (1) nigh = 4.091*P: - 3.091*P2 - L: (2) next = 5.091*P~ - 4.°91*P2 - L2 where L and P are the carrier phase (module 2~) and P-code pseudorange measurements, respectively, all expressed in range units; and ~ is the

134 carrier wavelength. Here, n, P and L may be considered as residuals, and/or as singly or doubly differenced. In the absence of measurement errors the left hand side of these equations are constants with time for a series of phase-connected carrier phase observations and P-code measurements. This suggests reducing the effect of measurement errors by averaging this system over an entire pass to obtain an ensemble mean for (n:,n21; short period measurement errors (including multipath) will average out, but long period effects will likely remain. If the long period effects are sufficiently low, the ensemble mean should lie close to an integer doublet and hence can be fixed. However, it can be shown that for a greater than 95% confidence level of picking the correct integer values for (n',n2), the standard deviation of the ensemble mean must be less than 1/4 in each component; inspection of the above system of equations reveals that this is rather ambitious, requiring that the residual systematic pseudorange errors and averaged down multipath errors must in aggregate be no larger than about 1 cm. Nevertheless, Figure 4 suggests that the Rogue receiver/antenna system came close to achieving this goal in that particular experiment. The ordinate of Figure 4 is merely the linear combination of Equations (1) and (2) that eliminates P2. There exists a similar plot for the combination eliminating P: and it shows comparable performance. One can infer from these two plots that the uncertainties in the ensemble means for the double differenced versions of n: and n2 (for white Gaussian distributed errors) would be around 1/3 for this experiment, close but not quite close enough. Short of an all-out direct assault on the cycle ambiguity problem as described above, to what other use could accurate pseudorange be made? Even if the pseudorange measurement accuracy were not quite sufficient to reliably resolve ambiguities, it certainly would provide a narrow bound on their possible integer values. This information should improve the efficiency of current carrier cycle ambiguity resolution techniques. Also, most techniques first try to fix the value of the difference, n~- n2, the so-called wide lane cycle integer. This corresponds to a wavelength of 86 cm and is therefore relatively easier to resolve. Without P. and P2 the uncertainty in the differential ionospheric delay limits the reliability of this fixing to baselines or a maximum length that depends on solar activity, time of day, elevation angles of the GPS satellites, and geographical location. About 200-300 km appears to be a typical upper limit for campaigns that have been conducted in the continental U.S., Mexico and Central America. For receivers that do not use the P-code to recover L2, but rather, alternative nonlinear techniques that recover the second harmonic of L2, a wide lane ambiguity length of only 43 cm results, which is twice as vulnerable to ionospheric mix-modeling. With the P-code pseudorange the wide lane ambiguity can be fixed with very high reliability quite independent of baseline length (Blewitt, 19891. The above cycle ambiguity equations can be combined to yield (3) n:-n2 = 0.00652*P: - 0.00509*P2 - Lo + L2

135 where the units of P and L are now in centimeters. These are very small coefficients on P: and P2; the 95% confidence level requirement for correctly fixing n:-n2 is easily met by the measurements in the experiment shown in Figure 4. Even a pass of pseudorange from the TI- 4100 usually can be averaged to satisfy the requirement. Having fixed n:-n2, one can use Equation (3) as an a priori constraint in estimating the individual integer ambiguities from the carrier phase observations (Blewitt, 1989~. Finally, it should be noted that the variations in Figure 4 are almost entirely due to multipath. Hence, if the antenna/backplane physical and electrical configuration is held invariant and the environment remains unchanged, one should obtain nearly identical residuals the next day but four minutes earlier because of the sidereal period of the daily exact repeating ground tracks of the GPS satellites. This means that whatever the values of the cycle integers that were chosen today, they are going to be the same tomorrow. Thus, the near repeatability of the multipath signatures allows one to connect all of the cycle ambiguities together over successive daily passes, thereby greatly reducing the number of degrees of freedom in a multi-day estimation solution. This technique would allow one to phase connect two carrier tracking segments interrupted by an extended outage using the information obtained from neighboring days. If cycle ambiguities can be resolved on continental-sized baselines through a combination pseudorange and carrier phase-only techniques, it would result in a marked improvement in tracking accuracy for GPS ephemeris production. One would have carrier ranging effectively accurate to the limits of tropospheric mix-modeling, that is, sub- centimeter accuracy. The limiting error source in this case probably will be the mix-modeling of the phase center variation of the GPS satellite antenna array. This varies with Foresight angle at the satellite and the latter ranges from zero up to a maximum of 14.3 degrees. The differential effect is baseline length dependent and negligible for regional baselines. By tracking the GPS satellite from rise to set using a high-gain steerable antenna (thereby minimizing ground multipath), one can obtain from the pseudorange and carrier phase measurements an estimate of multipath effects originating at the satellite. Recent tests have placed an upper limit of 5 cm variation over a several-hour pass (Young et al., 19851. 5. Quite apart from accuracy considerations there are a number of operational and data processing benefits to be accrued from receivers with pseudorange that should be evident from our previous discussion. The temporal invariance of the left hand sides of Equations (1) and (2), including invariance to clock instabilities and also to SA-induced dithering, suggests a very efficient technique for data editing, e.g., detecting and removing cycle dropouts (Lindqwister et al., 1988), and for real-time recovery from loss of lock by the receiver without loss of carrier phase connection. The latter will be quite useful in kinematic

136 surveying applications for dealing with signal loss resulting from obstructions, high accelerations and jerk. Real-time data compression by the receiver is another application afforded by accurate pseudorange. This has been practiced for some time by the USAF GPS Monitor Stations under the name, ''Carrier-aided Pseudorange''. Greatly shortened tracking sessions will achieve the same system accuracy as the current much longer tracking scenarios. ANTENNA PHASE CENTER VARIABILITY Antenna phase center variability arises from internal reflections and diffractions within the antenna/backplane assembly that shift the received phase as a function of signal direction. Errors arising from this effect should really be a non-problem at present system accuracies; unfortunately' it currently appears to be one. Unless rigorous standards and configuration controls are agreed to by the community, it is almost certain to preclude achievement of 1 mm baseline accuracies in the future. The phase shift from an omni antenna typically varies at the 1-2 cm level with wavefront direction (and is therefore indistinguishable from carrier multipath). This variability differs depending on the antenna design, the antenna and backplane physical and electrical configuration, and the local electrical and physical environment (Sims, 1985; Tranquilla, 19869. The mean phase center obtained by averaging over the solid angle above some elevation cutoff, e.g., 10 deg. and chosen to minimize the RMS variations, is one accepted definition of the phase center. The actual mean phase center, which results from averaging over a daily tracking session and which therefore depends somewhat on the tracking scenario and on the configuration of the GPS constellation, may differ from the conventional definition. Mis-modeling phase center variability can lead to errors in baseline estimates. The high accuracy attained to date with GPS geodetic systems is largely due to common mode error cancellation: antennas of the same manufacture and configuration have very similar phase center properties and thus their differences are slight. Using receivers with different antennas is an invitation to trouble in the form of significantly degraded repeatabilities (at the one centimeter level or higher) in baseline solution comparisons made over successive experiments, even though intra-experiment repeatability might be substantially better. Moreover, even with antennas with identical phase center variability with azimuth and elevation, one is likely to incur a baseline length dependent error owing to the differing direction of a GPS satellite as viewed at the ends of the baseline. There are obvious calibration experiments to tie different systems together and to convert from one scenario to another that one could perform in addition to standardization wherever practicable.

137 REFERENCE SYSTEM ERRORS The strategies for defining and maintaining a terrestrial reference frame follow two rather different approaches. Simply put, one approach uses the fiducial concept (Davidson et al., 1985; Bock et al., 1986; Beutler et al, 1986; Lichten and Border, 1987) and the other uses instead the dynamical framework of the GPS constellation. The Fiducial Network. The fiducial concept uses strong a priori information on the location of certain fiducial sites spanning a network of GPS stations. The a priori data are baseline vectors between GPS sites that are collocated with VLBI sites, augmented with geocentric coordinates obtained from collocations of VLBI and SLR. By processing concurrent tracking data from the fiducial and the geodetic sites and incorporating the a priori site coordinates, one effectively aligns the GPS orbits with a reference frame defined by these fiducial points and thereby transfers the fiducial information via the GPS ephemerides to the geodetic sites within the network. There are variations on this approach, mainly concerning how the ephemerides are generated and controlled, and how their information is transferred to the geodetic sites; but strong a priori fiducial information from VLBI and SLR is the hallmark of the fiducial approach. Systematic errors in the fiducial approach arise from: (a) errors in the alignment of the GPS receiver phase center with the intersection of axes of the VLBI telescope, (b) errors in the VLBI/SLR-derived fiducial coordinates, and (c) errors in the observations and modeling used in the GPS ephemeris generation process. Alignment errors arise from the tie of the GPS phase center to a nearby benchmark and from the benchmark to the intersection of axes of the telescope. With a common environmental configuration and with identical equipment design and manufacture at each end of the baseline, the error in tying the antenna phase center to a benchmark largely cancels in baseline results (probably completely at the centimeter level but probably not at the millimeter level); but those arising from surveys between neighboring benchmarks and the intersection of axes, particularly of large antennas, have had a troublesome history over the past four years. Even today, residual errors of 1 cm are likely to be lurking in these ties; reducing this to the millimeter level will be a major undertaking. The repeatability of the lengths of about 150 baselines determined by VLBI over the past four years can be expressed as Repeatability = (5 ~ 3*B*10-3) mm where B is baseline length in kilometers (Clark, 19881. This suggests

138 that the accuracies of relative fiducial coordinates obtained from VLBI observations on regional scales are probably sub-centimeter in the horizontal components and probably 1-2 cm on transcontinental baselines with robust observation sets and with accompanying Earth orientation observations good to the milliarcsecond level. Improving these system accuracies to the millimeter level on regional baselines will require significant advances in dealing with the water vapor problem and in the development and deployment of VLBI data acquisition systems that span a wider bandwidth for group delay measurements than the Mark III system. Doubling the current -400MHz spanned bandwidth should reliably yield carrier phase delays at S and X-band. That would give VLBI about an order of magnitude improvement in precision over the current Mark III system in measuring delay (although not that much improvement in system accuracy). Thus, GPS reference frame definition and maintenance using the fiducial approach on regional applications is likely to be paced by system improvements in VLBI. In addition to VLBI-derived site coordinates, there are errors arising from aligning this fiducial frame with a geocentric frame derived from SLR. At current accuracy levels this is a minor error source for regional applications but troublesome when comparing results from different groups. Standardization will be welcome in the area of reference frames. Other error sources arise from mix-modeling Earth platform processes such as ocean loading, Earth tides and Earth orientation. These currently cause errors in horizontal baseline estimates that are probably at the level of a few millimeters; they will need to be refined to achieve millimeter accuracy. GPS ephemeris errors for well-modeled data analysis systems appear to be somewhat data noise-limited in regional fiducial applications, particularly for GPS satellites with poor viewing geometry. It was just noted that the VLBI-derived horizontal components of the baselines used in fiducial control (typical lengths of 1000-3000 km) are probably accurate to better than 1 part in 108. That should translate into decimeter-level ephemeris accuracies. However, the best ephemerides produced to date, based on multi-day tracking arcs, stochastic modeling of non-gravitational accelerations and the tropospheric variability, and other sophisticated refinements, appear to have accuracies, when averaged over the entire orbit, that range between 0.5 m and 2 m or 3-10 parts in 108. Ephemerides based on regional tracking obviously will be less accurate over parts of the orbit not tracked so this is not really a fair comparison. Accuracies over the region tracked appear to be a factor of three to ten better depending on the strength of the viewing geometry (Wu et al., 19881. This would place the regional ephemeris accuracy in the range of 1-2 parts in 108 (Lichten and Bertiger, 1989), perhaps a factor of two larger than one would expect with perfect tracking accuracies. This is probably largely due to the use of carrier phase data without resolution of cycle ambiguities, which causes a weakening of orbit accuracy analogous to that experienced in the eastern baseline component. In fact, covariance studies with ambiguity resolved

139 carrier phase measurements show that data errors in this case are less significant and, in fact, the terrestrial reference frame then limits ephemeris accuracies. Ephemeris errors resulting from a priori fiducial location errors tend to be magnified by an order of magnitude, particularly over regions outside of the fiducial network. As mentioned earlier, the use of very accurate pseudorange in addition to carrier phase should significantly improve ephemeris accuracies either by improving the a posterior) values of the cycle ambiguities, or by directly facilitating carrier cycle ambiguity resolution, or by tying cycle ambiguities together from day-to-day. In addition, the current success at resolving cycle ambiguities by optimal network design (Blewitt, 1989; Dong and Bock, 1989) also augurs well for success on transcontinental fiducial networks; but networks spanning ocean basins may be a bit more challenging. The Global Framework. With the full deployment of the Block II constellation in the early 1990s one is likely to see an alternative approach to GPS reference frame maintenance, which uses the dynamical framework provided by the GPS constellation. For lack of a better name I have dubbed it the Global Framework. A dynamical approach to reference system maintenance does not, strictly speaking, require fiducial information. Instead, it relies on the dynamical consistence of the GPS constellation for the reference frame as embodied in the GPS ephemerides, in the set of derived station coordinates of a globally distributed GPS tracking system, in the Earth orientation parameters obtained from a global monitoring system, and in the transformations between the conventional inertial system and the conventional terrestrial system. Why would the Global Framework be preferred by some over the fiducial approach? It would have some disadvantages, notably its reference system would not be under the total control of individual users. To make it work an international organization would be required to maintain and operate it. That in itself will be challenging. One reason to prefer it is its potential stability and amenability to conventional standards for its configuration and operation, and for the distribution and timeliness of its data products. Another is its potential for enhanced accuracies, less so over regions spanned by robust fiducial networks; but substantial increase in accuracy would be realized over regions that have sparse fiducial coverage such as third world and oceanic regions (Freymueller and Golombek, 19889. Global tracking would strongly control ephemeris error build-up over continental areas that may be small enough to permit cycle ambiguity resolution on these long baselines. Another compelling reason will be the cost advantages to its users if the reference system can be maintained and the tracking system operated through international agreements and funding. Operationally, this dynamical approach is similar to the way the USAF maintains the operational GPS orbits and obtains the broadcast

140 ephemerides (and the way LAGEOS orbits are maintained by SLR). However, we are talking here about something rather more ambitious. The ephemerides, the tracking station locations, and the relevant Earth orientation information are generated from continuous tracking, both carrier phase and high accuracy pseudorange, by a set of globally distributed tracking stations. These stations are closely enough spaced to have ample satellite mutual visibility for eliminating clock errors, something the current USAF network would have trouble with (see Figure 6~. About a dozen stations would suffice if they are roughly uniformly distributed. With assumptions about tracking and modeling accuracies comparable to those described herein, and with realistic modeling of non-gravitational accelerations and antenna phase center variations on the satellites, covariance studies predict that such a system would yield GPS ephemeris accuracies after two days of tracking that are below 1 decimeter and station location accuracies of 1-2 cm, comparable to that of VLBI and SLR (Melbourne et al., 1988~. Thus, the GPS tracking system would be capable of determining its own station locations and need not rely on the a priori information supplied by VLBI and SLR. The system would also solve for the offset of its reference frame from the geocenter and it would determine its own scale. Figure 7 shows that the geocentric offset is determined to better than 5 cm accuracy after only 1 day of tracking (Wu and Malla, 1988~. In short, this system appears capable of providing on a continuous basis a reference frame whose accuracy may well approach 1 part in 109 by the mid-199Os. A wild card for the Global Framework will be the advent of the low Earth orbiter (LEO) bearing a high accuracy GPS receiver that is concurrently tracking. Two examples are TOPEX/Poseidon in 1992 and the Eos platforms planned for the 1990s. GPS tracking from the LEO not only enables its positioning, it also provides an additional GPS tracking source for the ground network. The LEO mediates the transfer of tracking information between ground stations by simultaneously observing GPS satellites that are not co-visible from the ground stations, thereby expanding the range of tracking intervals over which concurrent observations (and hence clock cancellation) can be made. Also, the LEO to ground station baseline has on average a large ''vertical'' component and is corrupted by tropospheric errors at only one end of the baseline (see Figure 8~. These result in improved accuracy of the vertical components of the baselines within the ground network by a factor of two in certain cases and it reduces the sensitivity of the vertical components of the ground baselines to mix-modeled tropospheric delay. In its extreme form-the only non-GPS observational information that the Global Framework would need is the longitude of a single GPS tracking station and UT1 to tie down the degree of freedom associated with an unobservable rotation of the combined GPS constellation and Earth system. This is exactly how SLR works. Polar motion could be detected through the diurnal rotation of the Earth. Obviously, it would use conventional standards such as the nutation series and the constant of precession. For user convenience, UTC would be supplied and a

141 current tectonic plate motion model could be included as part of the reference system. Also, one should collocate the GPS tracking stations at VLBI sites for operational and logistical reasons including use of their ultra-stable frequency standards and timing. (Using a linear model for clock variability instead of a white noise model improves the data noise component of baseline estimate accuracies by upwards to 40%.) Earth orientation information from an Earth monitoring system such as the International Earth Rotation Service (IERS) should be incorporated to maintain alignment with the Conventional Terrestrial System; and it would be foolish not to use the a priori site location information provided from the VLBI and SLR systems to minimize the effects of systematic errors. But this information need not dominate the solution set for station locations as it does in the fiducial approach. In fact, there is not compelling reason for 100% congruency between the GPS and VLBI/SLR frames. We can and no doubt will live with constant offsets between these frames without undermining plate tectonic and crustal deformation results. Offsets that vary with time will be the troublesome ones that should be minimized. Operating in the SA Environment. As a postscript, I would like to add a few words about the prospects of operating high accuracy geodetic systems in the Block II era when the Selective Availability will be on. The DoD's current plan is to have SA operating on all Block II satellites except perhaps four, which would be used for time transfer functions. The levels at which SA might be operating and the technical details of its implementation will be found in a classified document issued by the USAF NAYSTAR/GPS Joi nt Program Office (Anon., 1989~. SA alters the broadcast ephemeris, the clock correction relative to GPS time, and the P-code clock frequency (10.23 MHz) in such a way to degrade point positioning (and velocity) accuracies of unauthorized users of the P-code to about 100 m 2 arms, about an order of magnitude worse than its nominal accuracy. For authorized users, that is, those having access to the Precise Positioning Service (PPS), their receivers would be equipped with a decryption capability that would enable real- time corrections for SA and the ability to track the P-code when the AS function is on. For high accuracy users involved in non-real time relative positioning, such as the geodetic community, the effects of SA are entirely different. In the first place, these users will be insensitive to errors in the broadcast ephemerides and clocks because they are not used for high accuracy non-real time data processing. SA dithers GPS P- code clock frequency in a pseudo random fashion but the clock epoch varies within certain bounds as prescribed by the 100 m 2 arms point position accuracy requirement. This means that the effects of SA clock frequency dithering manifest themselves as GPS satellite clock epoch errors and, potentially, as a mean error in the length scale when the carrier phase and pseudorange measurements are converted from units of light-see to centimeters. A mean scale error could arise if the mean clock frequency, averaged over the entire observation set in both time

142 and satellites observed, differed significantly from 10.23 MHz. (The L1 and L2 carrier frequencies are coherent with the P-code clock frequency and hence a variation on the latter produces identical proportional variations on the former.) The length scale error, which causes a baseline error that is proportional to length, is likely (but not certainly) to be negligible for most geodetic applications. For very noisy frequency standards aboard the satellite there could be small effects arising from the finite speed of light, which leads to the so- called retarded baseline effect. During the delay in arrival of a common wavefront at the two ends of the baseline (<20 msec), frequency instabilities of 0.001 ppm can lead to millimeter-sized effects; but these levels are very unlikely in nominal conditions. The clock epoch errors can be isolated for all GPS satellites co-visible from two or more ground receivers; they would be treated just like clock errors are treated today and would have no effect on baseline recovery accuracies. Another aspect of SA is its potential impact on the carrier tracking integrity of all receivers not operating with a PPS capability and on subsequent editing and data compression procedures. Qualitatively, SA frequency dithering appears to the user as if the GPS satellite had a very noisy frequency standard driving the P-code clock frequency. This means that frequency dithering could limit the tracking loop bandwidth or the length of time over which carrier tracking can be integrated without potential loss of lock. This is unlikely to be a problem in most applications and in most conditions under which SA would be operating; but users should familiarize themselves with (Anon., 1989) to assure themselves. Obviously, receiver architectures using the P-code to recover the L2 carrier will have better threshold characteristics than codeless designs. For post-processing such as data editing and smoothing, SA would limit the maximum tracking sample interval without risking carrier cycle dropouts if a single-station/single-satellite data processing strategy were followed. One could recover the clock errors, if desired. The fiducial sites are particularly suited for this; they frequently can provide H-maser timing to the GPS receiver; also, the fiducial baseline vectors are known a priori and thus, their solutions in the estimation process should be essentially invariant to any excursions in mean clock frequency. For example, one could treat the SA frequency dithering as a piecawise constant stochastic process and the clock epochs as a pseudorandom walk process. Using standard data processing operations on the tracking data information matrices such as double differencing, Householder or Givens transformations, triangularization, etc., the clock and frequency parameters could thereby be isolated from the geodetic and GPS ephemeris parameters and solved individually for their estimates relative to a master clock and frequency standard at a fiducial site. Thus, one could recover, at least in some lumped form, the clock epoch variations that result from SA or from any other sources of clock instability. From SNR and onboard oscillator stability considerations, one concludes that it is not feasible to recover the

143 clock epoch state frequently enough to determine the actual SA-induced clock frequency transitions, which, in any case, would be treading on dangerous ground security-wise if it were done. Even the lumped values, say over several minutes to hours, might be sensitive if publicly released without an appropriate delay. It should be clear, however, that any agency, U.S. or foreign, could set up a fiducial network coupled with a geostationary communications satellite that would disseminate the SA corrections and improved ephemeris and clock information to its regional users in virtual real-time. Such a regional differential real-time positioning system would readily provide meter- level navigation accuracy. To summarize, what are the prospects for GPS system accuracy in the 1-3 mm range by the end of the l990s? My opinion is that they are very bright. If history provides any precedent, we should take a lesson from it. The system accuracies of VLBI and SLR have improved at a rate of one order of magnitude per decade for the past 20 years since the Williamstown Conference (Kaula, 1969), which first galvanized NASA into creating major development and scientific programs for these techniques. Besides their historical records, there are a number of innovations on the horizon for these systems that should perpetuate this trend in accuracy. For GPS, these innovations include: a. high performance receivers and antenna/backplanes for sub- millimeter instrumental accuracy in carrier phase and centimeter-level accuracy in smoothed pseudorange (including multipath), high performance all-digital WVRs capable of millimeter accuracy in zenith delay. b. improved tropospheric modelling including a better understanding of the spectral content of its variability, better calibration, and better understanding of site and seasonal dependence. c. improved modeling of environmental factors that may cause variability in ground monument location, e.g., seasonal variability of the ground water table, d. improved calibration of GPS satellite multipath, e. more robust and accurate reference systems including a global tracking system for timely dissemination of reference frame data products as well as the tracking data, f. conventional standardization of procedures and of the configuration of data acquisition and information systems, and g. use of low Earth orbiters flying GPS receivers in conjunction with ground arrays to reduce the sensitivity of baseline estimates to tropospheric mix-modeling, to improve the vertical components, and to enhance the temporal resolution of baseline variability.

144 My view is that GPS system accuracy will probably continue to match future VLBI performance, particularly in regional applications, and that it will do so at considerably less cost. Table 1 provides a rather speculative and personal view of where GPS system accuracies might lie at the end of the century. Error budgets are provided in two columns: one for today and one for a decade hence. These figures represent the final error contribution (l-sigma) of each line item to the horizontal baseline components, after all calibrations, modeling and estimation procedures have been applied using state-of-the- art procedures. If present trends continue, I would expect the vertical component to be about three times worse, but the LEO satellites carrying GPS receivers might enable one who uses this tracking information to reduce it to only a factor of two. It should be noted that an error contribution from pseudorange has not been directly included; I have assumed that its role is to facilitate cycle ambiguity resolution. One should discriminate between short-term baseline error arising from observation sessions ranging from minutes to hours and to that attainable with long-term averaging over days; this has been reflected in Table 1. PROSPECTS FOR LOW COST GPS SYSTEMS While GPS capital equipment costs are already an order of magnitude lower than those of VLBI and SLR, we are likely to realize another order of magnitude reduction over the next five years; that is nearly a 40% reduction per year. Thus, we face a scenario in the 1990s where GPS capital equipment will become a minor cost driver for GPS systems. Two other elements are major cost drivers in GPS now and will become more so unless significant improvements are made in their efficiency. These are the workforce costs associated with measurement campaigns and with data analysis. It is illustrative to show how costs for the GPS geodetic systems activities at JPL are currently distributed. For JPL, which NASA views as its principal R&D center for developing GPS technology for geodesy and satellite tracking, the FY88 budget distribution for GPS geodesy was as follows: Campaigns: Planning, Preparation, Field Operations ~ ------ 25% Data Analysis: Systems Analysis, Baseline Recovery ---------- 31% Technology Development: Hardware & Software ----------------- 30% Procurements: Field Equipment, Computers -------------------- 9% Management: -------------------------------------------------- 5% For this fiscal year JPL launched one major campaign, the CASA UNO campaign in Central and South America, which also included a global tracking experiment (Neilan et al., 1989~. This campaign accumulated approximately 600 station-days of observations over a three-week period and involved 43 GPS receivers. This breakdown shows that nearly two- thirds of JPL's budget was directed in FY88 at supporting scientific applications including Campaigns, Data Analysis and Procurements. Our

145 procurement costs were low because our strategy has been to borrow rather than buy GPS receivers on account of the rapidly moving technology. The point is that, if we are going to do more science with the GPS in the future, we need to invest in R&D that not only reduces the costs of data acquisition equipment, but also the costs of field campaigns and data analysis. I would like to discuss the prospects for getting each of these three cost drivers lower in the 1990s. The keys to major reductions in campaign and data analysis costs are reducing the capital cost of the GPS receiver and expanding its functionality. A low-cost receiver means that you can buy and deploy a large number of them quadratically increasing the number of baselines recovered in a given time and correspondingly reducing labor costs per baseline. It also means that you can leave them in the field unattended. Expanded functionality means that you can do more things automatically in the field and less at home, and you can do them with fewer people both in the field and at home, thereby greatly reducing data processing costs. Lowering the Cost of GPS Receivers. Major advances in VLSI and solid state technology made during the 1980s are pivotal in reducing the capital costs of GPS receivers in the 1990s. These advances will result in greatly reduced component and fabrication costs. The TI-4100, the workhorse of the 1980s, is derived from late 1970s technology. It has a lot of analog circuitry. For transistors it uses bi-polar TTL technology extensively, which, by present standards, is very power consumptive, drawing power in both of its logic states. The multiplexing architecture of the TI-4100 baseband processor may have resulted as much from the designers trying to keep the total power requirement of the receiver down as from their desire to minimize costs and possible inter-channel biases inherent in non-multiplexing analog schemes. But a decade makes a big difference in solid state electronics. State-of-the-art CMOS devices consume an order of magnitude less power at the same clock speed as their 1970s counterparts, mainly because of their compactness and because their power consumption is confined to state transitions. Also, they run a lot faster. Gate array chips providing semi-custom VLSI contain as many as 100,000 logic gates today and they are growing; their use in 1980 was virtually nil. Equally important is the availability today of powerful computer aided tools (CAD/CAM) for designing these complex chips and validating their performance before committing the design to fabrication at a foundry, and for their manufacture. The upshot of these and other innovations, notably the commercial availability of much more power, faster, and cheap microprocessors, is that the top-of-the-line geodetic-quality receiver by the early 1990s will be entirely solid state with the exception of its power supply and antenna; it will be virtually all-digital 7 and it will have unprecedented computational power. Its all-digital baseband processors will have more parallel channels than GPS signals in the sky and hence, _ O

146 no multiplexing and no inter-channel biases; it will feature high performance microprocessors with clock speeds in the 30 MHz range and upwards (e.g., Motorola 68030) with integrated floating point units, and it will include 10-100 Megabyte memories, both volatile and nonvolatile. In short, the GPS receiver of the l990s is going to look and operate more like a computer than like a traditional receiver (and its price history will also resemble that of a computer, a curve of very steep descent). It will be compact, light, and cheap. Because I am familiar with the Turbo-Rogue, an advanced all-digital GPS receiver under development at JPL, I would like to use it for illustrative purposes; however, there are a number of other fine receivers with various features under development within Industry that could be cited as well. The Turbo-Rogue, which is a highly compact field receiver designed for unattended operations, is based on the Rogue architecture (Thomas, 1988~. A high level block diagram is shown in Figure 9; some of its performance specifications are included. This receiver will provide comparable accuracy in pseudorange and carrier phase but it will have dramatically expanded on-site computational and functional capabilities. If our design goals are met, the receiver will weigh about 10 kg including the antenna/backplane assembly; it will require about 25 watts of power, and it will cost about $10K. If our schedules hold, it will be available in early 1991. I would like to give two examples from the Turbo-Rogue development to illustrate how these design goals of low weight, power, and cost are going to be achieved. The first deals with the front end of a GPS receiver, the RF section after the preamplifier and before the baseband processor within which the signal is down-converted successively from RF through IF to an appropriately filtered baseband frequency, and then sampled and digitized. Heretofore, this section has been analog and included synthesizers, mixers, filters, a sampler, and an analog to digital converter (see Figure 10~. It has been power consumptive, bulky, expensive, and a potential source of instability and performance degradation through dispersive phase variations from the analog filters. The Turbo-Rogue front end will bypass all of that with a newly developed Gallium Arsenide custom-designed chip that allows direct digital sampling of a very broad band (600 MHz) RF signal which is passed to the baseband processor with intervening filters (Thomas et al., 1988~. Figure 11 diagrams the high level organization of this digital front end chip (DFE) showing its four major functions: a RF sampler and A/D conversion section, a reference clock synthesizer section, a control logic section, and a digital processing section. The input is the RF analog signal; the output is a pair of 4-bit digital samples, one in phase and one in quadrature, each at a controlled rate that can range from 10 to 600 megasamples per second. Through software control the device will operate over a wide range of RF frequencies including the band covering the GPS frequencies at L1, L2, and L3. It uses a unique fractional rate or commensurate sampling scheme on the RF

147 that is controllable by software and that can satisfy the Nyquist sampling criterion for the input bandwidth. Being virtually all- digital, it avoids nearly all of the error sources associated with analog systems. Its power consumption is about 6 W. roughly 25% of the analog front end on the Rogue. Its reproduction cost is about $100 instead of $20,000 associated with high quality analog front ends. Its volume is 200 cm3 compared to approximately 32,000 cm3. With this device the antenna/backplane, the preamp, and the DEE, all relatively low-cost and low-power items, can be physically located together (with, say, a solar-based power supply) at a remote but desirable site (e.g., low multipath, good field of view); its digital samples can be sent by a fiber optic link to the receiver, which may be kilometers away. A single receiver could service an array of antennas via multiple fiber optic links. The second example is a highly compacted gate array that is under development for the Turbo-Rogue baseband functions including doppler modeling and stopping, code generation, cross-correlation, and accumulator processing. The Rogue baseband processor (Figure 9) incorporates a three-chip set of CMOS gate arrays, a shared Motorola 68020 CPU @ 12 MHz and a number of memory units to provide all of the GPS signal processing logic for a single satellite including C/A, Pi, P2,L, and L2. It will also provide a codeless-derived measurement of P~- P2 and L:-L2 for ionospheric calibration in the event that AS is on. These three VLSI chips were fabricated with 2.0 micron technology and comprise about 18,000 gates of digital logic. A Motorola 68020 has about 12,000 gates. An eight-satellite Rogue baseband processor requires 24 of these chips plus five Motorola 68020 CPU chips; four CPUs are for correlation functions and one CPU is for executive functions. Each chip costs about $150. A total of nine boards is needed to support these functions in the Rogue: one CPU board and one correlation board per pair of satellites and one CPU executive board for the entire baseband processor. The power consumption of all this logic is about 80 W. The Turbo-Rogue baseband processor inherits most of the digital design features of the Rogue receiver. However, by taking advantage of recent advances in semi-custom VLSI technology, the processing hardware becomes more compact. The Turbo-chip uses about half of the logic gates on a highly compacted array with 129.000 gates, which is a sub-micron high-speed CMOS device. Each chip will handle all the GPS signal processing for two satellites in addition to taking over a number of tasks previously done by the CPU. Therefore, only four Turbo-chips and two CPUs will be required for eight satellites, at a cost of about $300 each. Although chip costs are not significantly less, fabrication costs decrease significantly due to fewer CPUs and fewer semi-custom chips; there is only one board in the Turbo-Rogue for baseband processing instead of nine. These advances alone reduce the volume, power consumption, and cost of the Turbo-Rogue baseband processor by a factor of ten.

148 Prospects for Greater Functionality: The Smart GPS Receiver. One of the two CPUs in the Turbo-Rogue, which is likely to be a Motorola 68030 @ 25 MHz or possibly an 88000 series, is the host processor for the receiver. It provides the executive control for the baseband processor and for its interfaces with the front end, the timing reference, and its output functions. The host CPU also provides all user interface and outside communications functions. It performs all of the tracking data processing functions required by the receiver: initialization and point positioning/timing, time tagging, data editing, data compression, sampling, cycle dropout recovery, receiver diagnostics, and management of ancillary calibration data (e.g., meteorology). It also provides the receiver interface for data transmission and remote command and control. The host CPU can be aided by special purpose hardware performing repetitive tasks that would otherwise be done with software. The host CPU is central to the concept of the "smart receiver", a GPS receiver that is highly autonomous and adaptive, and one that can be used in a wide variety of high performance applications. Here are some of the operational features that I would expect smart GPS receivers of the 1990s to have: 1. Extensive on-board processing with special purpose hardware, 2. Adaptive data editing, compression and sampling strategies, 3. Extensive data quality checks and receiver diagnostics 4. Pseudorange-based cycle ambiguity estimation, 5. Carrier range-based point positioning and timing, 6. Real-time pseudorange-based carrier cycle dropout recovery, 7. Autonomous operation with remote data links and control, 8. Cold start acquisition in seconds, 9. Kinematic operations with real-time recovery from dropouts, 10. Operations under high dynamic variability without dropout. Reduced Operations Costs. These features and others will lead to markedly lower field operations costs as well as reduced analysis costs. These operations costs are $400-$700/station-day today, depending on the difficulty of the campaign and on the unit labor costs (e.g., graduate students versus professional surveyors). The smart receiver with real- time adaptive capabilities will greatly improve productivity in the field: more sites surveyed in less time. Kinematic surveying alone will revolutionize the field of surveying. The single most important field operations cost-reduction feature of receivers of the 1990s will be their ability for autonomous operation. This feature plus the low unit receiver cost means that networks of permanently installed, continuously operating and remotely monitored receivers can be maintained with virtually no human intervention in the field. Reduced Data Processing Costs. A number of features of smart receivers will greatly relieve post-processing chores and therefore reduce the costs of data analysis. The presentation of very clean phase-connected

149 carrier data, pseudorange, and integrated calibration data as a result of extensive on-site processing will greatly expedite initial post- processing. Automated on-site data compression will also reduce the post-processing load. The availability of ambiguity-resolved carrier phase is another benefit. The availability of reliable and accurate ephemerides would also lead to cost reductions. Probably the biggest factor in reducing data analysis costs is establishing standardization across many system elements: hardware and software configurations; data formats; procedures in site selection, monumentation, data acquisition and data processing; tracking network; reference system; scenarios; archiving; and so on. Nothing takes more time in data analysis than encountering and dealing with the unexpected. Through this kind of discipline and technology advances, the prospects in the l990s for substantial efficiencies in data analysis are very bright. End-to-end data processing from a major field campaign lasting several days now takes several months to complete; it should take only days. Indeed future arrays operating continuously must do so without accumulating a backlog. FUTURE APPLICATIONS OF GPS IN GEODESY AND SATELLITE TRACKING The scientific goals in geophysics and physical oceanography will profoundly influence technology directions in the l990s for GPS-based geodetic and satellite tracking applications (Mueller et al., 1989~. Many of these goals can be transformed into geodetic measurement and/or tracking system requirements, and summarized as follows: 1. Relative velocity vectors of crustal blocks at 1 mm/yr accuracies are required in plate boundary deformation zones with spatial and temporal resolutions ranging over 10-1000 km and from hours up to years. Departures from long-term averages based on tectonic models, including transients, are required. 2. Relative positioning accuracies on land approaching 1 mm are required for intraplate deformation studies and refinement of constraints from tectonic models. 3. Earth rotation vector accuracies of 0.1 mas, measured down to sub-diurnal periods, are required. 4. Seafloor position accuracies of 1 cm are required to support deformation studies in this difficult environment. Potential experiments include monitoring the detailed strain field around spreading centers, subduction zones including the fore and back-arc environment, strike slip zones such as the southern California borderland, and major marine transform faults. 5. A global gravity model with accuracies in the 1 mgal region and 100 km resolution is required.

150 6. Global ocean circulation studies require satellite altimetry missions that yield ocean topography at 1 cm accuracy with spatial resolution down to 10 km and temporal resolutions of one week spanning several years; comparable accuracies are required for the spectral components of the ocean geoid to derive ocean circulation models from the topography. For altimetry missions this requirement also translates into 1 cm orbit determination accuracies in the radial components of the satellite orbit. One of the most challenging of the geophysical goals is No.1 pertaining to regional deformation; this requires the deployment of geodetic arrays with unprecedented spatial and temporal resolutions and with very high accuracies. The extensiveness of the measurement programs implied by this requirement, if carried out on a global basis, would lead to enormous costs if carried out with SLR and VLBI. This leaves only the GPS as the economically viable space technique because of its low cost and high accuracy potential. For GPS these requirements can be grouped into the following applications categories: 1. Arrays for monitoring crustal deformation rates, 2. Networks for maintenance of terrestrial reference systems including GPS ephemerides and fiducial site coordinates plate motions and Earth orientation variability; 3. Tracking and navigation systems for satellite dynamics and for satellite and/or air-borne remote sensing applications; 4. In-situ applications: kinematic surveying, translocation, etc. I would like to briefly discuss the various GPS applications as they fall into these four broad categories. Most of the applications of GPS to date have been in array form; but their density is relatively sparse, they are infrequently operated, and they are very labor-intensive. The continuous array will redress many of these limitations and by year 2000 one would expect to find dozens of these arrays globally dispersed. Continuously Operating Remotely Monitored Arrays. Certainly the continuously operating array is an idea whose time has come (Ladd, 1987; Shimada et al.' 1988~. Here, an array of unattended receivers is connected via a communications link to a remote data processing or network Center. The receivers track continuously and the tracking and ancillary data are collected via the link and processed at a real-time rate by the Center. The Center also controls the elements of the array and monitors their performance. The low cost, high accuracy and smart GPS receiver, discussed in Section III, is required for these arrays. Low cost means that dense arrays are economically feasible; high accuracy means that very weak geophysical signatures might be discernible; and "smart' means that the autonomous and adaptive

151 capabilities of the array receivers make unattended operations feasible and render the enormous real-time data processing tasks economical. Specific examples of on-site receiver data processing and management functions are editing, phase-connecting, calibrating, compressing, and data transmitting. P-code pseudorange is essential for some of these tasks. The receiver could automatically adjust its sample rate upon detection of anomalous doppler rates. Post-processing by the Center would be greatly streamlined over current off-line capabilities, which run at roughly one-tenth the real-time rate. Powerful parallel processing architecture will be commonplace; sophisticated post-filter editors will be used for combining separate network information, e.g., yesterday's solutions and information matrices with today's, or information from one array with that from another or from a reference or global tracking network, the Center will provide highly interactive capabilities for users and efficient and automated data management and archival systems. The potential applications for continuously operating arrays include: In California, arrays containing 100-1000 elements for real-time monitoring of pre-seismic, co-seismic and post- seismic strain with densification in problem areas; 2. Arrays for remotely monitoring volcanic activity; 3. Arrays of tethered and free-floating ocean buoys for ocean circulation, sea level and tidal studies; 4. Sparse arrays, e.g., inter-island geodesy over oceanic basins; 5. Sea floor geodesy: for positioning acoustic-based ocean platforms and tying sea floor monuments to a GPS network (Spiels, 1987~; Skeptics of the utility of continuous tracking contend that it might be compromised by water vapor variability. On the other hand, continuous tracking will certainly yield a great deal of information about the spectral content of tropospheric variability and should lead to better strategies for reaching the shortest temporal resolution of potential geophysical signatures. Kinematic Surveying. Kinematic surveying depends on maintaining cycle ambiguity-resolved carrier phase in the field so that carrier range- based geometric point positioning can be continuously achieved under kinematic conditions (Remondi, 1985; Made r et al., 1989~. Its accuracy potential is at the millimeter level for relative positioning. It is in its infancy today and susceptible to signal outages and loss of phase connection. But with the coming advent of the smart receiver with P- code pseudorange and carrier phase capability, I would expect kinematic surveying to be widely practices in the 1990s. Its potential for extraordinary cost effectiveness will lead to a wide range of applications with accuracy requirements ranging from 1 ppm (i.e., standard surveying) to 0.01 ppm or better. Its theater will be virtually unlimited in extent. (The smart receiver with good

152 ephemerides should enable the user to transfer cycle ambiguity resolution from one satellite to another upon rise or set, and from one day to the next.) Kinematic surveying opens the way for obtaining geodetic measurements at multiple sites around a primary site all within one observation session; there will be no need in the future to dwell at a primary site for multiple days, or even for hours. Future kinematic surveying will also be effected with a single roving GPS receiver operating in conjunction with a remote array, thereby enabling virtual baselines among sites occupied by the rover to be measured with carrier range accuracy using only the one receiver. GPS Global Tracking System. This topic has already been partially addressed previously under Reference Systems. Here, I will briefly describe the management and operation of a global tracking system, and enumerate its major uses and advantages. In a sense, a global tracking system is another array but on a global scale; it will have many of the features of a continuously operating array and incorporate many of its efficiencies. It will coexist with other global tracking systems such as the five-station USAF network for monitor and control of the GPS and for the broadcast ephemerides, timing, health status, and so on. For its surveying and mapping programs, the Defense Mapping Agency maintains an additional five tracking stations that complement the USAF sites. Other civilian users for surveying, transportation and navigation are likely to generate their own systems, but these accuracy requirements probably do not exceed O.1 ppm and in many instances are in the 1-10 ppm range. Here, we are addressing a system that will maintain the terrestrial reference system for GPS at accuracy levels approaching 0.001 ppm. It will also feature GPS tracking from low Earth orbiters to augment the ground network. It will be reliable, its products timely, and its configuration and operation standardized by international agreement through organizations such as the IUGG/IAG. Figure 12 provides a flow diagram showing the major elements of an internationally organized global tracking system and the different classes of users. The System would have four major elements: 1. An International Governing Board: 2. An Operational Center and possibly sub-Centers functioning on a regional basis; 3. A network of tracking stations hosted by the participating country/organization; 4. One or more Computational Centers. The GPS Global Tracking System would deliver tracking data, ephemeris and reference frame data products, information matrices, and calibrations. The users, if they were custom users, might further process the tracking data or the ephemeris solutions and information matrices to refine the accuracy and time resolution of solutions from their own networks. They might return some of their products to the Computational Centers for further integration. For example, the IERS would use these products to further strengthen its Earth orientation

153 monitoring function. Tracking information from continuous arrays and from LEO's carrying GPS receivers might be sent to the computational centers to enhance the accuracy of their solutions. The standard user would use the ephemeris products and station coordinates as they stand. For these users the Global Tracking System would provide significant cost-savings as well as accuracy, stability, and reliability. Is implementing, maintaining, and operating another civilian global tracking network worth the cost? Can nearly equivalent accuracy and stability be achieved by less ambitious measures? If GPS applications were strictly regional arrays with strong fiducial constraints, the utility of a global network would be dubious. A priori ephemeris values and covariances supplied to such a region appear to improve overall system accuracy; but this improvement is marginal and tends to apply only to the longer baselines if multi-pass regional tracking is obtained (Lichten et al., 1989~. On the other hand, preliminary results from the global tracking network used in the CASA UNO experiment suggest that it facilitates cycle ambiguity resolution on the continental-sized baselines of that experiment. If this is true, it would lead to significant accuracy advances on these baselines. In summary, the major advantages of a global tracking system are: Cost savings through reduced operations and computation tasks; 2. Standardization of the Terrestrial Reference System; 3. Standardization of the data management and information systems; 4. Timely tracking and ephemeris products through a world-wide distribution systems; 5. Increased accuracy, particularly for regions with weak fiducial constraints and for oceanic basins; 6. Shortened resolution time for regional arrays; i.e., shortened tracking time for a regional array to achieve a given accuracy level; 7. Complementary Earth orientation information for the IERS; 8. Ground-tracking network for differential GPS applications to Earth satellite missions. 9. Enhanced accuracy and temporal resolution from concurrent tracking by LEOs. Although the GPS and the SLR depend on VLBI for maintaining the inertial reference system and for eliminating possible longitude/right ascension drifts inherent in dynamical systems, the introduction of GPS into Earth orientation monitoring will fundamentally change the mix of these three techniques for this application. GPS tracking, with its effective ranging accuracy at the centimeter level and range change accuracy at the millimeter level, will provide complementary and accurate short period information, and longer period information of comparable accuracy.

154 Satellite Precision Orbit Determination (POD). By placing a high performance flight-rated GPS receiver aboard an Earth satellite and using a globally distributed ground-tracking network, one can achieve sub-decimeter positioning of the satellite in the terrestrial frame defined by the ground network (Yunck et al., 1985~. The first mission to use this approach as a flight experiment will be TOPEX/Poseidon, a NASA/CNES oceanographic satellite to be launched in 1992 (Melbourne and Davis, 1987; Carson et al., 1988~. Here, the requirement for high accuracy in radial position stems from its use of precise satellite altimetry. Its three-year mission is to map the ocean topography to decimeter accuracy or better with mesoscale resolution; this information coupled with an improved ocean geoid, will enable determination of geostrophic currents and will lead to better understanding of both variable and mean ocean circulation from mesoscales up to global scales. Obviously, the requirement for decimeter accuracy altimetry or better implies the need for decimeter or better orbit accuracy. The GPS is likely to be the best way to reach sub-decimeter accuracies. Future remote sensing missions of the late l990s such as Eos will also carry GPS receivers. Ultimately, centimeter accuracy should be achieved using precision orbit determination strategies that synergistically combine carrier phase and pseudorange. A critical component in GPS-based satellite POD is the GPS Global Tracking System; the ground stations of this network must concurrently track the GPS satellites to achieve these orbit accuracies in differential positioning. Platform Positioning. In addition to positioning, GPS will provide attitude information for those remote sensing missions accomplished either by satellite or by aircraft. Three or more GPS antennas multiplexed to a receiver will provide the attitude, typically to 100 prad accuracy. For dynamic platforms such as marine platform for sea floor geodesy or an Earth satellite platform such as NASA's planned Earth Observation System (Eos), P-code pseudorange will be a vital aid to carrier phase. In the case of Eos, the GPS will also be used for recovery of stratospheric temperatures by tracking GPS satellites during their occultation phases. Also, ionospheric tomography studies will be undertaken as well as satellite-based real-time geodesy (Melbourne, et al. 1988~. By combining kinematic tracking, multiplexed antenna arrays for attitude determination, and gravimeters, one should be able to mount airborne and shipborne experiments for regional gravity logging with accuracies and productivity that will be unprecedented. Synthetic Aperture Radar (SAR) Missions. In addition to platform attitude, the GPS will provide positioning to an accuracy that is a small fraction of the radar wavelength, an important measurement for SARs. This positioning accuracy also allows radar data from successive overflights to be coherently combined. For target areas remaining invariant between overflights one obtains synthesized radar images of very high resolution equivalent to an aperture the size of the

155 separation of the overflights. Indeed, the loss of coherence within a target area between overflights can also provide useful information. Gravity Recovery with GPS-based Satellite Geodesy. A satellite-borne GPS receiver operating differentially in conjunction with a ground network can provide significant improvement in our geopotential models. Gravity perturbations, particularly as signatures manifested in the highly precise carrier phase-tracking, will be detected. Their determination will be enhanced by successive overflights of the same geographical area. This will be first performed with TOPEX/Poseidon to improve its overall orbit determination accuracy and to improve the ocean geoid. Because the GPS provides continuous and virtually omni- directional tracking, its gravity information will complement existing gravity models, which are currently less accurate in certain geographical regions as a result of a paucity of laser-tracking data from earlier missions over oceanic regions and most of Asia. However, TOPEX/Poseidon will fly at 1334 km, thus its resolution of the spectral components of gravity will be limited to wavenumbers of about 25 or lower, plus a few resonance terms (Yunck et al., 1985~. Its results will also be vulnerable to mix-modeled non-gravitational accelerations. Gravity Probe-B, a proposed NASA mission to be launched in the mid- 1990s, offers much better prospects. Principally a mission to detect gyroscopic effects predicted by general relativity, it will be drag-free and fly in a polar orbit at 600 km altitude (Proceedings of SPIE, 19869. By also flying a GPS receiver, which is now planned, it should recover valuable gravity information up to a wavenumber of about 60. This maximum wavenumber falls short of the 200-300 that could be achieved with the Gravity Research Mission using a micron accuracy microwave satellite-to-satellite tracking technique, or alternatively, with super- conducting three-axis gravity "radiometer. But missions with this kind of technology are not likely to fly until near the end of this century; so these earlier but less precise results are likely to serve as interim measures. Figure 13 provides an estimate of the gravity recovery potential of GP-B for a six-month mission. Another mission being developed by ESA, Aristoteles, will fly a gravity "radiometer and may also carry GPS receiver. This satellite will be placed in a 200-250 km orbit which may yield additional resolution. Unfortunately, Aristoteles is not currently scheduled to carry a drag-free or drag-compensated system; thus non-gravitational atmospheric drag and radiation pressure effects are likely to corrupt the gravity coefficient estimates, but the magnitude of this corruption versus spectral component is uncertain at the present. SUMMARY Most of this discussion has focused on the state of affairs in high accuracy differential applications of the GPS as they are now and will be a few years ahead; viewed from a distance, CY1995 is much easier to

156 see than CY2000. For example, it is likely that other navigation satellite systems will play a complementary role in geodesy, such as GLONASS of the Soviet Union, and also flight systems such as PRARE and DORIS; but the details are less clear. There are also a myriad of surveying and navigation applications for GPS that are outside of our realm; these will undergo explosive growth in the next several years. But for our class of GPS applications, these important milestones will almost certainly be passed by CY2000 if present trends continue: 1. System accuracy for geodetic applications should improve by a factor of five. For long period resolutions where the tropospheric error contribution will be less, system accuracies should approach 1 mm in the horizontal components of regional baselines. For short period resolutions, it may be possible to surpass 3 mm. 2. Data acquisition hardware costs should drop by over an order of magnitude with top-of-the-line GPS receiver costs of less than $3000 in 1988 dollars; indeed the standard C/A-only receiver for coarse positioning will probably cost less than today's scientific calculator. 3. All-digital WAR instrumentation will be widespread. Its cost will be below $10,000 using MMIC technology and phased array antennas. By employing highly stable but small ovens their calibration system should enable measurements of sky temperature to 0.1 K accuracies . 4. Data processing and field operations costs should be over an order of magnitude lower than today's costs. 5. Continuously operating and remotely monitored arrays with unattended operations and automated data processing should become widespread for a broad class of applications including regional and global arrays, seafloor geodesy, satellite tracking for remote sensing missions, and ocean and ice circulation. 6. Kinematic surveying will have revolutionized surveying methodology and reduced costs by over an order of magnitude. It will enable the practical realization of regional footprints around primary geodetic sites for monitoring local strain distribution. 7. An internationally sponsored and cooperative GPS global tracking network will be operating to maintain a terrestrial reference system approaching an accuracy of 0.001 ppm, to support Earth orientation monitoring, and to support ground programs and Earth satellite missions requiring high accuracy positioning. 8. Lcng-life low Earth orbiters such as TOPEX/Poseidon and the Eos platforms will bee' GPS receivers to augment the ground-tracking network and to further improve system accuracies.

157 Finally, having dwelled at length on the attributes of GPS for scientific applications, it would be remiss not to cite at least one of its deficiencies. The designers of the GPS were concerned with developing this nation's next generation satellite navigation system, not with its scientific applications. The P-code signal structure was designed to produce sub-dekameter navigation accuracies, not sub- centimeter geodetic accuracies. The effective wavelength of the P-code is over two orders of magnitude longer than the carrier wavelengths. For carrier cycle ambiguity resolution with high confidence, the P-code measurement accuracy in its averaged form must be below 1 cm, or about 0.02% of its effective wavelength. This is a burdensome requirement that is difficult to reliably meet today. One improvement is to add another tone or spread spectrum component in the GPS signal structure, coherent with 10.23 MHz P-code clock frequency but having a wider frequency base. An increase of at least a factor of five would lead to a ranging system with greatly reduced susceptibility to multipath, which with due care, would virtually assure 100% success in resolving ambiguities independent of network configuration or extent. Another very promising option is to use the already existing L3 carrier for navigation purposes instead of just for telemetry (Melbourne, 1985~. It is plausible that by CY2000 the DoD will have begun implementing the GPS Block III satellites. Once can hope that they might reflect a few improvements promoted by the high precision civilian community. ACKNOWLEDGEMENTS I wish to express my appreciation to several individuals for their varied contributions to this work in the form of discussions and critiques, figures and technical data. They are: Sassan Bassiri, Geoff Blewitt, Tim Dixon, Michael Janssen, Steve Lichten, Tom Meehan, Ruth Neilan, Benno Rayhrer, John Scheid, and Larry Young, all from JPL. This work was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract from the National Aeronautics and Space Administration.

158 REFERENCES Anon., ICD-GPS-207: NAVS TAR GPS Selective Availability and Anti- Spoofing Receiver Design Requirements, Current -Issue, NAYSTAR/GPS Joint Program Office, USAF Space Command, E1 Segundo, CA,-1989. Bender, P., Atmospheric Refractivity Uncertainties, 1987. Beutler, G., W. Gurtner, M. Rothacher, and I. Schildknecht, Evaluation of the March 1985 High Precision Baseline (HPBL) Test: Fiducial Point Concept Versus Free Network Solutions, EOS Trans., AGU, Vol. 67, p. 911, 1986. Blewitt, G., S. Lichten, P. Kroger, M. Kornreich, U. Lindquister, L. Skrumeda, and W. Bertiger, Accuracy and Long-Term Stability of GPS Baseline Estimation, EOS Tran. AGU, Vol. 69, No. 44, p. 1151, 1988. Blewitt G., Carrier Phase Ambiguity Resolution for The Global Positioning System Applied to Geodetic Baselines up to 2000 KM, J. Geophys. Res., in press, 1989. Rock, Y., R. Abbot, C. Counselman III, and R. King, A Demonstration of 1-2 Parts in 10 Accuracy Using GPS, Bull. Geod., Vol 60, pp. 241- 254, 1986. Carson, L., R. Davis, G. Geier, L. Halley, G. Huth, T. Munson, Design and Predicted Performance of the GPS Demonstration Receiver for the NASA TOPEX Satellite, Proc. IEEE PLANS 88, Position, Location, and Navigation Symposium, Orlando, PL, 1988. Clark, T., D. Gordon, W. Himwich, C. Ma, A. Mallama, and J. Ryan, Determination of Relative Site Motions in the Western United States Using Mark III Very Long Baseline Interferometry J. of Geophys. Res., Vol 92, No. B12, 1987. Clark, T., Private communication, 1988. Clynch, J., and B. Renfro, Evaluation of Ionospheric Residual Range Error Model, Proc. Third International Geodetic Symposium on Satellite Doppler Positioninc, Vol. 1. p. 517, Las Cruces, NM, 1982. Davidson, J., C. Thornton, C. Vegos, L. Young, and T. Yunck, The March 1985 Demonstration of the Fiducial Network Concept for GPS Geodesy: Preliminary Report, Proc. First International Symposium on Precise Positioning with the Global Positioning System, Positioning with GPS-1985, Vol. 1, p. 603, Rockville, MD, 1985. Dong, D., and B. Bock, GPS Network AnalYsis with Phase Ambiguity Resolution Applied to Crustal Deformation Studies in California, J. Geophys. Res., in press, 1989.

159 Elgered, G., J. Davis, T. Herring, and I. Shapiro, Methods of Correction for the "Wet" Atmosphere in Estimating_Baseline Lengths from VLBI, Proc. IAU Symp. No. 129, May 10-15, 1987, p. 543, Cambridge, MA, 1988. Freymueller, J. and M. Golombek, Geometry and Treatment_of Fiducial Networks: Effect on GPS Baseline Precision in South America, Geophys. Res. Lett., Vol. 15, No. 13, p. 1467-1469, 1988. Hatch, R., The Synergism of GPS Code and Carrier Measurements, Proc. Third International Geodetic Symposium on Satellite Doppler Positioning, Vol. 3, p. 1213, Las Cruces, NM, Feb. 8-12, 1982. Herring, T., J. Davis,. I. Shapiro, Geodesy by Radio Interferometrv. the Application of Kalman Filtering to the Analysis of VLBI Data, J. Geophys. Res., to be submitted, 1989. Jannsen, M., A New Instrument for the Determination of Radio Path Delay Due to Atmospheric Water Vapor, IEEE Trans. on Geo. Science and Remote Sens., Vol. GE-23, No. 4, p. 485, 1985. Kaula, W., (Ed.), The Terrestrial Environment Solid-Earth and Ocean Physics: Application of Space and Astronomic Techniques, Report of a Study at Williamstown, Mass. to NASA, August 1969, sponsored by NASA and MIT. Ladd, J., Continuous Monitoring of Deformation with GPS, Proc. Deformation Measurements Workshop, MIT, Oct. 31-Nov. 1, 1986, p. 416-436, Cambridge, MA, 1987. Lichten, S., and J. Border, Strategies for High-Precision Global Positioning System Orbit Determination, J. of Geophys. Res., Vol. 92, No. B12, p. 12751-12762, 1987. Lichten, S., W. Bertiger, and U. Lindqwister, The Effect of Fiducial Network Strategy on High-AccuracY GPS Orbit and Baseline Determination, Proc. from Fifth International Geodetic Symp. on Satellite Positioning, March 13-17, Las Cruces, NM, in press, 1989. Lichten, S., and W. Bertiger, Demonstration of Sub-Meter GPS Orbit Determination and 1.5 Parts in 108 Three-Dimensional Baseline Accuracy, submitted to Bulletin Geodesique, in press, 1989. Lindqwister, U., G. _ Blewitt, W. Bertiger, Future of GPS Network Process~ng, EOS, Vol. 69, No. 44, p. 1152, 1988. Mader, J., W. Carter, B. Douglas, W. Krabill, Decimeter Level Aircraft Positioning with GPS Carrier-Phase Measurements, Bulletin Geodesique, in press, 1989. Melbourne, W., Series X Cycle Ambiguity Resolution Using Bandwidth Synthesis and "Angle SYnthesis " IOM #3300-82-119, Jet Propulsion Laboratory, 1982.

160 Melbourne, W., The Case for Ranging in GPS-Based Geodetic Systems, Proc. First International Symposium on Precise Positioning with the Global Positioning Systems, Positioning with GPS-1985, Vol. 1, p. 373, Rockville, MD., 1985. Melbourne, W., and E. S. Davis, GPS-Based Precision Orbit Determination: A TOPEX Plight Experiment, AAS/AIAA Astrodynamics Specialist Conference, Paper AAS 87-430, Kalispell, Montana, 1987 Melbourne, W., G. Blewitt, S. Lighten, R. Malla, R. Neilan, S. Wu, and B. Schutz, Establishing a Global GPS Tracking System for Fiducial and Ephemeris Production, Paper No. G21A-02, AGU Spring 1988, 1988. Melbourne, W., T. Yunck, L. Young, B. Hager, G. Lindal, C-H Liu, G. Born, GPS Geoscience Instrument for EOS and Space Station Global. Atmospheric Temperature Profiling Acoustic Gravity Wave analysis Ionospheric Tomography and Precise Tracking Using the Global Positioning System, Jet Propulsion Laboratory proposal, Investigation, Technical, Data and Management Plans, Vol. 1, July 1988. Mueller, I., S. Zerbine, J. Dickey, G. Freeman, C. Goad, W. Kaula, W. Melbourne, C. Reigber, R. Schutz, D. Turcotte, (Editors), The Interdisciplinary Role of Space Geodesy, Proc. of the International Workshop, Institute Nazionale di Geofisica in Erice, Sicily, Italy, July 1988, Springer-Velag, in press, 1989. Neilan, R., T. Dixon, J. Kellogg, T. Meehan, W. Melbourne, J. Scheid, J. Stowell, Operational Aspects of CASA UNO '88 - The First Large Scale International GPS Geodetic Network, IEEE Trans. on Instrumentation and Measurement, in press, April 1989. Remondi, B., Performing Centimeter-Level Surveys in Seconds with GPS Carrier Phase: Initial Results, NOAA Tech Memorandum NOS NGS-43 Rockville, MD, Oct. 1985. Rogers, A., The AccuracY of Position Determination by VLBI Expected bY the Year 2000, in Geodesy in the Year 2000, NRC Committee on Geodesy Nat. Acad. Press, Washington, DC, 1987. Shimada, S., S. Sekiguchi, T.-Eguchi, Y. Okada, and Y. Fujinawa, Fixed- Point GPS Simultaneous Baseline Determination Network in Kanto Tokai District Central Japan, AGU Chapman Conference on GPS: Measurements for Geodynamics, Sept. 19-23, 1988, Fort Lauderdale, FL, 1988. Sims, M, Phase Center Variation in the Geodetic TI4100 GPS Receiver SYstem's Conical Spiral Antenna, First International Symposium on Precise Positioning with the Global Positioning System, Positioning with GPS-1985, Vol. 1, p. 227, Rockville, MD, 1985.

161 Spiess, F. Seafloor Geodesy by the Year 2000, Nov. 1987. Thomas, J., Functional Description of Signal Processing in the Rogue GPS Receiver, JPL Publ. 88-15, 1988. Thomas, J. B. Rayhrer, L. Young, A Sampling Down-Converter for RF Signals, NASA New Tech. Report #NPO-17530, 1988. Tralli, D., T. Dixon, and S. Stephens, The Effect of Wet Tropospheric Path Delays on Estimation of Geodetic Baselines in the Gulf of California Using the Global Positioning System, J. Geophys. Res., 93, p. 6545-6557, 1988. Tranquilla, J., Multipath and Imaging Problems in GPS Receiver Antennas, Proceedings of the Fourth International Geodetic Symposium on Satellite Positioning, p . 557 - 571, 1986 . Treuhaft, R., G. Lanyi, The Effect of the Dynamic Wet Troposphere on Radio Interferometric Measurements, Radio Sci., Vol. 22, No. 2, pp. 251-265, 1987. Wu, S., and R. Malla, Determination of a Geocentric Coordinate Frame for GPS Measurements, AIAA/AAS Astrodynamics Conference, AIAA 88-4210- CP, Minneapolis, Minnesota, 1988 Wu, S., W. Melbourne, T. Yunck, Impact of Tracking Network Variation on GPS Orbit Determination, AIAA Paper #88-0573, AIAA 26th Aerospace Sciences Meeting, Reno, Nevada, 1988. Young, L., R. Neilan, F. Bletzacker, GPS Satellite Multipath:_ An Experimental Investigation, First International Symposium on Precise Positioning with the Global Positioning System: Positioning with GPS-1985, Vol. 1, p. 423, Rock~ille, MD., 1985, also Young, L., Private communication, 1988. Yunck, T., W. Melbourne, and C. Thornton, GPS-Based Satellite Tracking System for Precise Positioning, IEEE Trans. Geosci. and Remote Sens., GE-23(4), p. 450-457., 1985.

162 c' _ m > cn ~ 3 CD ~ 2 o z 1 6 5 > 4 co 3 C) 2 ~ 1 UJ o 5 4 3 - ~1 1 ~ O (9 -1 c ~ LLI -C ~3 4 -5 20 10 ~O > -10 -20 _ '`T _ -30 -,,,,, 1,,,,,1,,,,, 1,,,,, 1,,,,,1,,,,,~ 1983 1984 1985 1986 1987 1988 1989 DATE (yr) 6 5 4 )t _ ' 1 _ (a) (3 mm + 1 0 -8 O . t=~ , ~ ~ I , q 1, ~ l l l l ~l l I r_ -(b) _ o _, ~, ~ I Ol o; - 0 2 4 6 8 10 12 14 16 BASELINE LENGTH (x100 KM) - 1 , 1 , 1 , 1 _ _ (4 mm + 1 0 -8 L) _ _ ~ O O _ , 1 , 18 20 22 ' I ' ' ' ' 1 ' ' ' ' ' 1 ' ' ' ' ' 1 ' ' ' ' ' 1 ' ' 1 1 1 1 ' ' ' ' ' (c) OVRO130 ~MOJAVE LENGTH=245km {t ~ ~= GPS - ^= VLBI - I I I I I ~I I I I I I I I l I l L _l_l I _ - ' ' ' ' ' 1 ' ' ' ' ' 1 ' ' ' ' ' 1 ' ' ' ' ' 1 ' ' ' ' ' 1 1 1 1 ' ' 4 ~ 44 ~ 4 4 ~ ~ 4 4 4 ~ = GPS = Vl Rl _ Figure 1. Agreement Between \ILBI and GPS. a) and by Difference in Horizontal Components of Baselines Versus Baseline Length; Continental U.S. Campaigns of March and November, 1985, June, 1986, and January, 1988. c) and d) VLBI and GPS Length and Vertical Results Over a Five Year Period; OVRO/Mojave Baseline.

163 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' ' ' - (a) O - cn hi: - cn LU TV (b) 1 1 , , , , 1 6 ~ APRIL 4, 1985 ~ Act_ ~ VVV-' APRIL 3 1985 - ,_ , , 1 , , , , 1 , 5 +1 0 cn en 1 7 8 C) o 11 ID m -1 0 id UTC HOURS Figure 2. Carrier Multipath. a) Double Differenced ionosphere Corrected, Post-Fit, and 2 Minute Gaussian Smoothed Carrier Phase Residuals Using a Pair of Tl-4100 Receivers with the Standard Tl Antenna Observing SV 9 and SV 11 from Mojave and OVRO. Use of Ground Plane Could Reduce Multipath Variability Significantly. b) Cross - Correlation of April 3 and April 4 Residuals Versus Lag.

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~50 > :E 40 UJ cr: 30 lo: ~ E 20 O en LL ~ ~ * 10 c, ~ z co LL ~ ~ J I -1 0 (in ~ i O -20 o Lit -30 At a: -40 -50 Day 234 o co 1 0.0 us ~ 0.0 LL C-) -1 0.0 o 7 165 ROGUE P1 MULTIPATH AT O\/RO -- PRN 09 . . ~ (a) +/- 30 cm envelope -- "5/30'' CASA UNO Field Test January 26, 1988 _ 2 MINUTE AVERAGING \ 30 MINUTE SMOOTHING ~ / ~""'1 +/- 5 cm envelope -- "30/5" "Choke Ring" Configuration 8 9 10 11 12 TIME HOURS (UT) (b) P1 -4.091 L1 + 3.091 L2 ~ Day 234 ~1 RMS= 1.99 cm 1 w~ ,~ t~ ~ ~ At . . ~ 1 ~ V VI~.~T, y' P1 -4.091 L1 + 3.091 L2 Day 235 (offset 4 min) , 1 , 1 , 1 , ,1 ,, 1' .-,., . 120 150 180 0 30 60 90 TIME (min) it., difference J Figure 4. P-code pseudorange multipath obtained by a Rogue receiver operating with a Dorne & Margolin antenna coupled with a quarter wavelenth choke-ring backplane designed by JPL; a) Ordinate is the linear combination of Equations 1 and 2 that eliminates non-dispersive and ionospheric delays and should be a constant in absence of multipath and SNR errors. Variability is a strong measure of P-code multipath. b) P-code pseudorange multipath on two successive days; residuals are 2 minute averages.

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168 ~I ~1 1 ~ 80 c' - LL He 111 En LL o LL ' ' I I T r · r · 70 60 50 40 30 20 10 COVARIANCE ASSUMPTIONS \ \ 18 GPS CONSTELLATION 2 DEN BASELINES (3 cm) 3 NON-DSN SITES (1 0 cm) PSEUDORANGE (5 cm) CARRIER PHASE (0.5 cm) RANDOM-WALK ZENITH TROP \ \ \ \ _, ~ 5cm O I ~I ~I I I ~ I I I I L I I I I 0 6 12 18 24 30 TRACKING ARC LENGTH (hrs) Figure 7. Predicted Accuracy Versus Tracking Time for Recovery of Geocentric Offset in Reference Frame Using Full 21-Satellite Block 11 Constellation and a 12-Station Global Tracking Network. See [30] for Additional Covariance Assumptions

169 BROAD COVERAGE IMPROVES SHORT-TERM GPS ORBIT DETERMINAT10N SERVES AS AN INTERMEDIATE OBSERVING POINT BETWEEN WIDELY SEPARATED RECEIVERS EXTENDS VERTICAL VIEWING GEOMETRY REDUCES SENSITIVITY TO TROPOSPHERE , LEO/ / ~ J / / Figure 8. Utility of LEO in enhancing accuracies of baselines and GPS ephemerides using a global tracking network. GPS Or

170 Rogue GPS Receiver Architecture Signal Flow (L1-C/A only) ICY . B.P Fiber B.P liter _ >' DIGITAL BASE BAND DATA L-BAND RF data - _ DOWN- CONVERTER AC'C'=~.AOI~ 11 1~C~11~1 i ~: I'm 1 ~_ DSP BOARD === _ ~ ~ Cos 0 _ ~ ~ Sin0 ~ _ _ F Gen 1 1~l~ Satellite B Satellite A ~~ =~ ~ D , a, 1 11 ~ Hey. ~ 1 thy ~./A A. ~ IN 1/4 ACCUM. CH IP ~VMX BUS ~ | SATELLITE CPU BOARD (68020) _ ~ ~.j: ~_ ~ ~ ~ : . REAL TIME CLOCK BOARD . .. : NON-VOLATILE RAM BOARD . SERIA :'U ~ ~ ::~1 r:5 '~-D . ~ :::,:::.,.~7 ~ \/ME-BUS | HOST CPU BOAR[) (68020) TO USER Fig. 9a). Rogue GPS Receiver Architecture. Only the C/A signal processing is shown. In actuality there are five processing streams for C/A, L1, L2, Pi, and P2.

171 ~ROGUE TO TURBO ROGUE AFTER C)OWN-CONVERTER ~ _ _ _ ~11~11T DIGITAL SIGNAL PROCESOR _ _ ~ .' ~ _ _ ,___ ~ . ~ _ _ DIGITAL SIGNAL PROCESOR 11111 Weight : ~ 30 kg Size : - 0.16 m 3 Power :~ 250 W Cost : ~ $80 k it_ ~ IlIt =~ Ill! _ ..' .''.'2...2..."'. ...... ...... .... ...... ~ .... ...... ..... ...... . .... . . ·:: L I I . , TURBO-ROGUE Weight : ~1 0 kg Size :~0.02m3 Power :~25W Cost : ~ $15 k Fig. 9b). The Rogue to Turbo-Rogue Transition utilizing the DFE chip for RF down conversion to baseband digital sampling and the turbo-chip for baseband signal processing. ... ~

172 n 1 | PREAMP | TYPICAL ANALOG DOWN CONVERSION _ FILTER | NARROW | ~ FILTER ~ ~ it' - ., 1 T . ~-- -l SYNTH ES IZE R | ~ r - ~1 _ SAMPLER SYNTHESIZER 1 600 MHz BW FILTER 600 MHz BW@ LBAND __ ~- 20 MBS SAMPLED DATA TO BASEBAND PROCE' NOR ., ANALOG VS. DIGITAL APPROACH DIGITAL FRONT END _ . rim ~ | CHIP ~ FREQUENCY REFERENCE Figure 10. Functional Schematic of an Analog Front End for RF Down-Conversion to Baseband, A/D Conversion, and Digital Sampling. Digital Front End Eliminates Analog Components that are Expensive and Sources of Error. /

173 ANALOG RF IN - 0.1 TO 8000 MHz 600 MHz BANDWIDTH SAMPLE R A N D A D ONVERTER (ANALOG ! -:~ SAMPLE RATE SYNTH E SIZER (ANALOG AND DIGITAL) DIGITAL SINK X FILTER AND DATA RATE RE SUCTION _ (D IG ITAL) ~ :~'~2'~ PROD E f CONTROL . ~ ~(DIGITAL' ~ REFERENCE FREQUENCY TO 2C0 MHz SET-UP PRESENTLY (JAN 1 989) BEING DESIGNED AND ;6 SIMULATED a) . ~ . ~ ~ ................... ..... ~ . .......................... DIGITAL BASEBAND O 10 - 600 MEGA SAMPLES 4 TO 1 0 BIT QUADRATURE SAMPLES SELECTABLE COMPLETED DESIGN PRELIMINARY FA3RlCATION IN PROCESS - mom b) Figure 11. a) Functional Layout of Digital Front End Chip for Converting RF Analog to Baseband Digital Samples. by Photo of Completed Sections of the DEE Chip.

174 ~ -- ~.-~;~ Or, stat ~ ons `Th, ee Is ~' ~ e'-~`v' =~w,-`edi GAG - ~4 I ,- ~t ~ 0~ Add: AN OF ~ GLOE!~L | ~.~tF=rr,at~ ones Cavern ng 1_ IFS Tract..nc~ System arid C:ompe'- i In:: i ^1 Beat i u.ns--ALE. T .~01 } ousted p' ~.~-~ ~.se~ ~r;e ve~-t;~. ~ availabie pri or ~el a~ ~ ve vel c:c:it. es ~ T:e tfJ ' ,`wr ti a~ +~,~ 0 E,; i ~ i r;g t~ydr oger; ~:i E; i `~ ~ i r, ~ wa ~ er- `~ap c.r- ~ ad ~ omet er ~ lOue, at i or`a 1 ~d`~ant a.~e .~Jet wor ~ Er,L'=r,. emer, ~ Stat i c,-'s--EiF S or. l y c =~-~ i de t-`c~mocenec~`s ~ gI ot~1 di stri b~ on ~ Er~har, _e n7~tc`=i ~ ~ ,; ~ i ~ ~ y cs Geo~,~t ~ ~ ca1 ~tr-e. '~t! ~ c~ r~ed~nda~-,cy ~nd sy~tem ~ el ~=bil ity ~ F ~ ate tector'~ cs ~ '-, new areas Low Eart:~-, Ori--, tina Soacecraft 5F ~ Trac t: ~ ng lAet wor ~ Cer~ter Fur~ t i c~n: Oper at ~ c~ns ~' i mp ~ ementat ~ c:~r' Coo' d~ nat~ ooer-"ti o~-,s Data Col ~ ect ~ on ar,~ t.- ansmi ssi ur~-- GF=* l~eteorolcigiccil ~ W'vn' rloct`, rea1-t~rre E.~-~dcast . echemeri des, etc . -Lime ~,~-,i. {:or-~ng of r;etwort oerformar~ce Rea' Conf i c~urat ~ on a'-id f ac: ~ 1 i t ~ es ~c: ntr~-~l :~ _ =cista~ni;-,g er~g~neeri'~y "i~d m~i~atenance Ei-,at; l e per f or mance 2~nd co`~f i gur- at i c~n ~tandar d~ =vs~ e~, des;~ a', a'~d r esearc~i . _ ~ F:esoon~i ~e to Comoutatic:~r-~1 Ce;~ter . · , _ . Fr-act:, ca1 geodesy f User s ~i.S-'g product, of the Compc~tational Center- as they star~d) Cor~ti,-,~s t}per~t.~ '~~o .Res~,~.~1 ~ Nor,i tc~s-ed ~ays (~PlfAi Ne~ wol t-s ~1~;~:~> s~ ale' Re`erence f, ame ~i~ t~etwee7, pre-~,; i st: i ng netwc'~ t tJ ~F~/'~'LElfSLR ci te staL~ ~ i u blos-:i tor ~ rlg `~f ~fO3- ~r,i c: u~) i f ~ L:~ Def csr IrS~t] c:n s-IGar- ol =~i ~ ~:c;~r' ]~ ~ es ~ Fr-e- a.~d oost-se~ `_,nic: =~ - , r~ a~;-,itor ~:g 7~-~ - e~ser~-. Ji ~ ~ U~ i r~o ~'ract ~ cai ~rs ar:~ i, =;i"-~t,~ t]~r~~- ~ ^~:e t,-~e ~;-~Cte u~ , 0 Eng i nees~ i ng app 1 i Lat i on5 c;~ Seaf ~ oc:~r aead esy Dyn a~n i c ,oos ~ t i c~-' i ng 0 Frec~ si on na~i gat ~ or' u t~=pp] ~,~ . . . . .. .. . ~us~ on; User s: the Comouf 2~t i s~na1 ~er't er == t ~ ~ it~: `d _? '~ ~ ~;li; .~ ~v ~ ~r;;-y a:]j~-~. =~:~,~ w~ tt,~ pr~r~ct~ ::if th~ Corr,p,ut~t~c~r~ ~enter- i:~ic^~;,~ c,~ t.;~ictf;~t' ^~_;~ct(::v reci~, remer~-= G~ for~ resear- ct ar;,' .~= i ot; i,~-il =,1 '.=i ai~ ; ~ . ..t- o~;-` ~ -- clL~ ur-, I3^ agr-a~' to'~ ~r, vFt~ 8~tT=} t~c[;irig S~'j~ elrl

175 T NTEF<~4AT ~ f'~L'f' ~-F'D~SORED T ~ ~ t: ~ 1, `] ~ S ~ _ i ~ to I ~C3fl~l3Ll ~ It i ~l L~.r! ~ - ~ Boar ~ Fc7r~ 1 tat ' ·~. 'God Redip. __ ~|~IL`~I'L~.' orals-`' Reid er )~1~_~ t fit ~ U'~=l ~ ~I~l in en Limo ~ ~ <-a t ~ or ~ =~ 1 Herr i: e. - ~ or OF ~ ~ -~ h enter ~ ci e is-, Re'~rer.ce Frc'.~e ,_~-;~-~1 Fur,i-ti Brie: Off-' or,; d^~ quay itv .or,t:~;j' ~ ~L,; Or-, a,~d d~stributior~ c; F~rc-~mete. esti that i An-' E;x,;stem desi Or ar~'d Beset.- t:~ ~ Tr~~.i,~~ data ,ur-~du~ts +~r Netwc:'rf; ~e -`ter lo, 0 ~ U ~ ~ 3 So ~ c~t i c~n s ~4cc~ ~ate wFS ---te} 1 ~ te ept~em=r ~ des ,'~. ~! F'~m-~Ocrr' or better Tr~c'.ir~g stat ic~r-' loc^tior,~. s;ite ~p~cif i :: infor~n~tic~n lEr - c [; ~ ~ ~ ~ =t ~t i ;;:~, . ~ e l oc ~. t ~ e~ ' F: 1 ~ ~ ~ '~ ec t c:,n i Gewc=~-,tr i~ he, er£,~ e ~r a,~.e o Eartl-, orienta' ton (pol~r `~'uti or ~ 1}11-}17~ r a+~:, GI ;~1 so] ut ~ c,;-~s ~r~d f u1 ~ c~c,~ ar ~ ar-~e r ch ~ ved dat~ ~ Hi gl~' ctetcsl ~ ty ~Fa norroa, pc'~ r,ts ;a Raw tr-opospher e ca i br- at i on dat~ 0 Der ~ed tr-opc; ~,~-~er i~ path! de1 ~y c: c~rr ect ~ or~= IEF<S--I~-,1:ern~ti or~' Ea~-tl F:~ti oi-~ =~rvi ce Tt~e ~-,ct~f~r~s of ~ ts-^ct -~-~9 sy~a, or-~ani - at, sn d~ v~ ;de ~r-,to t:wo segfr,ente: t) Imp} - mentat~oF-, ar,~ operatiuri of `:~ata a.quiC=,~tir:'n s~=te'~, ) Data a~lysis a~id d~ssea;iriat~or, of so1 ~tior;s ar~d d~t~ pr-odc~cts to t'-'e internar iori~} ~`ser ce,in,~r.i<:: ~ ,fff/~;,~,~,~'i', f ',~'ff~'f 7,i'f7;,r'j~/,~;'~,'j~f,~,~,j',~f,-'Y' 'i~f~',~'J;',' " f~f~l~z' ~"7/ff~f/~ Jf~'ff//~r' _ H~ ;3~e,- F. e~ i 51 or' ~Ucer-= r-eq~^r-e itef~tio'~ or, +.t,~ rJr- CrC5~_~: "£~;tom" Gec3d==, ~ =, ~d Geopl~` v ~i =_ ~,~-`t~r,;..~, Ooer c`~51C] Rem<3t= Mo',itor ed -'-` mvs <~O~) ~d~4 w`: ~re`3i on~i s~ =1~) c~ '~L9G-- 'Ver-v ' C';[TG; =.~Se~< i~e GFS C~ ~ 1 =i t~ t: e. tt~ i c GF8= ~=,t~, `~t . ~rS ~-~-. ~t= r~ ~ C:~~'c3~oI'. ~ ~r ~ ~ t: c~s] i `~5 r ~ ~ s~ 1 ;, t ~ =, i } , ' ~ ~ ~ r O i. * ~ _ ~ v :i ,, i =; , . . . ~. . ,; ,~ !; ~ .s ~ 1 GOCI~ es ~ <, c'+ t ~i ~ ~c:~,p c~ ~ ~ t ~ or, ^,1 ,,er~ t. er ) a ~ e ~ 1 ~ t. e Ge;~ :i e=,; -- ( ~F ~-~ r ^c t; ~ r~c~ of ~ ~w-E^r t.~, u ~, ~, O~ ~ L t er-- ~ ~ or yeudes~ T~!r E X Eos~ Sp=~ ~ 2t ~ ~ ~ L.~. iD sra~'il v; B'robe _ t3 ~r~ =1 c ~ Sh ~ t 4 ~. ~ ~ L ~NI)S - T , .. . . . . __ . . . _. .. . . . . .. . .. . _ ... . . .. . . ... .. _ . ~ 1

176 1.0 4 0.01 9 4 2 0.001 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1, 1 1 6 t / / ~ , , 3 _ AN/ / _ 2 / / ~(n)/csk(n) 0.1 4: / =o 3t / / ~ Pr / / ~ 1 1/1 1 1 1 1 1 ~ 1 1 1 1 1 1, 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 10 20 30 40 50 60 70 1 000.0 100.0 10.0 AN (cm) 1.0 0.1 0.01 Figure 13. Predicted Gravity Recovery from GP-B Using a TOPEX/Poseidon Class GPS Flight Receiver in Conjunction with a GPS Global Tracking System. Mission Duration is 6 Months. aLn) is Based on Combined Tracking and A Priori Kaula Model Information. ok~n) is Based on Kaula Model.

176 1.0 4 0.01 9 4 2 0.001 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1, 1 1 6 t / / ~ , , 3 _ AN/ / _ 2 / / ~(n)/csk(n) 0.1 4: / =o 3t / / ~ Pr / / ~ 1 1/1 1 1 1 1 1 ~ 1 1 1 1 1 1, 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 10 20 30 40 50 60 70 1 000.0 100.0 10.0 AN (cm) 1.0 0.1 0.01 Figure 13. Predicted Gravity Recovery from GP-B Using a TOPEX/Poseidon Class GPS Flight Receiver in Conjunction with a GPS Global Tracking System. Mission Duration is 6 Months. aLn) is Based on Combined Tracking and A Priori Kaula Model Information. ok~n) is Based on Kaula Model.

Geodesy in the Year 2000 Get This Book
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Geodesy has undergone technological and theoretical changes of immense proportions since the launching of Sputnik. The accuracy of current satellite geodetic data has approached the centimeter level and will improve by one or two orders of magnitude over the next decade. This bodes well for the application of geodetic data to the solution of problems in solid earth, oceanic and atmospheric sciences. The report Geodesy in the Year 2000 addresses many areas of investigation that will benefit from this improvement in accuracy.

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