Effects of Selective Availability (SA) and Antispoofing (AS)
Selective Availability (SA) denies precise positioning by corruption of the GPS signal structure. It is composed of two components: (1) corruption of broadcast navigation message; and (2) rapid "dithering," or oscillations of the frequency standards in the satellites. The first component of SA is of little consequence to scientific users because in precise millimeter applications, the orbits of the GPS satellites are computed from carrier measurements much more accurately than even the precise ephemeris available from the U.S. Department of Defense. Also, the dynamics of the GPS satellites are understood well enough that these orbits can be integrated forward in time by several days with accuracies better than the (uncorrupted) broadcast ephemeris, except when there are thruster firings on the GPS satellites.
The second SA component also currently has little effect when differential (either real-time or postprocessing) techniques are used (Feigl et al., 1991). Figure A.1 provides an example of the dithering of the GPS frequency standards obtained from analysis of the data collected with the global GPS tracking network. In this example, SA is turned off on the satellite at approximately 18:00 Universal Time Coordinated (UTC). The results of the dithering have been converted to meters of range error in this figure. Before SA is turned off, the root-mean-square (RMS) error in the range measurements is 22 m; when SA is turned off, the range error RMS drops to about 0.25 m. (These results are obtained from the analysis of carrier phase measurements, and therefore nearly all of the RMS is due to dithering and natural drifts in the satellite and ground station clocks.) While the effects of SA are almost totally eliminated in differential positioning, the rapid fluctuations in the satellite clocks complicate the process of removing slips in the number of carrier phase cycles accumulated by the receiver. In addition, SA makes it nearly impossible to use averaged phase measurements (normal points) with a much lower sampling
rate. In real-time navigation problems, SA limits the accuracy of navigation, although this is probably not a major problem for aircraft since the position of the aircraft can generally only be controlled to within about 100 m, the positional accuracy imposed by SA.
Antispoofing (AS) is meant to stop false signals from corrupting military receivers, but, as a consequence of the system used, AS denies access to the P-code. AS is implemented by modulating the P-code with an additional code (called the W-code), and because only the P-code (with the W-code modulation) is superimposed on the L2 frequency, some type of codeless tracking of L2 is required when AS is turned on. Figure A.2 shows the effect on two different receivers when AS is turned on at approximately 17:00 UTC. The degradation of the performance of the receiver in Figure A.2(a) is immediately obvious and severely limits the effectiveness of the automatic data processing systems. In Figure A.2(b), the degradation of the performance of the receiver is not as obvious, although careful analysis of these data indicates that the effectiveness of automatic data processing systems is also compromised when AS is turned on. (It is also interesting to note the general stability of the receiver shown in Figure A.2(b) is not as good as that for the receiver in Figure A.2(a) when AS is off, possibly indicating the manufacturer of receiver (b) has put most of the efforts into handling AS rather than developing a stable receiver for the periods when AS is off.) The prime consequence of AS is the loss of accuracy in both range and phase measurements. For aircraft applications, this loss of accuracy (about a thousandfold for L2 tracking) is particularly severe because of the dynamics of the aircraft and the usually high multipath environment on the aircraft. In particular, results of the quality shown in Figure A.2(b) can only be obtained by coherently averaging the L2 signal for about 1 s. This is an acceptable compromise for a static receiver, but in an aircraft it often leads to loss of lock on the L2 signal.
When a GPS receiver loses lock on a satellite, an arbitrary number of cycles of discontinuity in the carrier phase are introduced to the data. The postprocessing removal of cycle slips is complicated by the poorer-quality pseudorange data available under AS conditions. Further complications arise from the time it takes the receiver to reacquire lock on the satellite. Under AS conditions, reacquiring lock can take 5 to 10 seconds (of order 1 km of flight path). During this time, no L2-range data are available, resulting in gaps in the flight path when dual-frequency positions are
required (during conditions of significant ionospheric delay and when the aircraft is far from its ground tracking station). Improvements in current processing algorithms can improve the robustness of the analysis for interpolating across these times if the occurrences are infrequent enough. When losses of lock are too frequent under AS conditions, the utility of dual-frequency GPS will be totally lost and accurate aircraft positions will not be available.
In principle, the effects of AS on signal frequency (L1) observations should be minor, although the effect can depend on how the P-code is treated on the L1 frequency (e.g., some receivers remove the P-code from the L1 signal before cross-correlating with the C/A code; such techniques will not work correctly when AS is active). However, because the P-code is written with lower power than the C/A code and the codes decorrelate rapidly, the presence of the P-code on the L1 signal can be largely ignored in the extraction of the C/A code. When the P-code is ignored, AS has no effect. Some dual-frequency GPS receivers couple the L1 and L2 tracking, and these receivers can experience loss of lock on the L1 signal when there is a loss of lock on the L2 signal. Improvements to the tracking loops in these receivers could improve this characteristic.
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