A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum (2015) / Chapter Skim
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6 Planetary Radar Astronomy
6 Planetary Radar Astronomy
Pages 122-146
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From page 122... ...
Finally, the sections "Future Science Drivers and Technical Requirements" and "Frequency Assignment Requirements" look to the future, discuss plans for new radar systems, and speculate on possible requirements for additional spectrum assignments. The rapid development of radar during World War II opened the possibility of utilizing this new technology to learn more about our solar system by detecting radar echoes from large and small solar system bodies.
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From page 123... ...
CURRENT RADAR ASTRONOMY SYSTEMS The sensitivity of a planetary radar system is proportional to the average transmitter power and the effective area of the transmitting and receiving antennas, which are the same for a monostatic system but may be different for a bistatic system (Figure 6.2) , and inversely proportional to λ3/2, where λ is the operating wavelength, and to the system temperature, which characterizes the noise contributions from the receiver, Earth's atmosphere, our galaxy, and the cosmic microwave background.
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From page 124... ...
, and • Polarization state. The time origin allows precision measurement of the distance to the target body, while the frequency of the echo relative to the transmitted frequency, the
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From page 125... ...
Measuring the echo power as a function of time delay and Doppler shift relative to the sub-Earth location on the target body provides an image of echo power versus these two coordinates, which can be converted into a more normal latitude-longitude image if the size, shape, and spin vector of the body are known. However, as is clear from Figure 6.3, two locations symmetrically placed north and south of the equator have the same time delay and Doppler shift relative to the radar and the sum of the echo powers from these locations is imaged, leading to what is known as the N-S ambiguity problem (Figure 6.4)
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From page 126... ...
Goldstone, California DSS-14 GBT, Green Bank, West Virginia Arecibo, Puerto Rico VLA, Socorro, New Mexico 10 VLBA sites Goldstone, California 7.190 80 0.08 (CW) Goldstone, California DSS-13 GBT, Green Bank, West Virginia Arecibo, Puerto Rico NOTE: Close asteroid observations require bistatic operation.
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From page 127... ...
, and the lines of constant Doppler 6-3b.eps shift intersect these concentric delay circles at two locations. Measuring the radar echo power as a function of time delay and Doppler shift relative to the sub-Earth location provides an image in time delay and Doppler shift coordinates.
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From page 128... ...
Initial maps of the planet's surface had resolutions of about 15 km covering 40 percent of the planet's surface, allowing the first large-scale look at the surface of our nearest planetary neighbor and the first indication that its surface is very young compared to those of Mars, Mercury, and the Moon, about 700 million years. The Galilean satellites of Jupiter were detected in the 1970s, resulting in the discovery of the unusual radio wavelength scattering properties of the low-temperature water ice surfaces of Europa, Ganymede, and Callisto.
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From page 129... ...
However, radar's sensitivity to centimeter-scale and larger surface roughness, its ability to penetrate dry surfaces, and its unique ability to detect water ice has kept it relevant to investigations of the Moon and terrestrial planets as well. Furthermore, its ability to make precision measurements of rotation vectors via speckle interferometry is providing information about the interior structure of planets and satellites and potentially giving insight into phenomena such as the transfer of angular momentum between the atmosphere and the solid body of Venus.
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From page 130... ...
Because of the very small scale of the speckles, the time at which correlation occurs gives the orientation of the projected rotation axis to high accuracy and the time delay gives the rotation period with, in some instances, an accuracy of 1 part in 105. Multiple observations at different orbital orientations for Earth and the target body allow the full rotation vector to be determined.
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From page 131... ...
Figure 6.5 shows the expected interval between impacts of NEAs on Earth as a function of their size. However, there have been recent suggestions based on analysis of the Chelyabinsk event and the number of small impact events in the interval 1984-2013 that the number of NEAs between 10 and 50 m in size may be considerably greater than previously thought, increasing the risk from the airbursts of these objects.2 In the 1990s, as awareness increased of the potential hazard from NEAs, P
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From page 132... ...
No other technique short of a spacecraft flyby or orbiter can achieve the 4 m to 15 m image resolution on NEAs offered by the Goldstone and Arecibo radars. Figure 6.6 shows a series of Arecibo radar images, in delay and Doppler coordinates, of the NEA 1999 JM8 at different orientations as viewed from Earth.
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SOURCE: Cour tesy of Michael C Nolan, Arecibo Observatory.
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,5 and one of radar astronomy's major achievements was the confirmation that 5 J.D. Giorgini, S.J.
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Golevka's distance and velocity during the last measurement could only be accounted for if the Yarkovsky effect was real. Apart from its effect on NEA orbits, the confirmation of the Yarkovsky effect had profound implications for our understanding of how small bodies in the main asteroid belt between Mars and Jupiter are translated into the inner solar system.
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From page 136... ...
Radar speckle interferometric measurements of Mercury's spin vector revealed the presence of a liquid outer core, a major advance in our understanding of this planet (Figure 6.10)
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From page 137... ...
Figure 6.12 shows an Arecibo radar image of Mare Serenitatis on the Moon made at 70 cm wavelength, where the penetration depth is 5 to 10 m, compared with an optical image. Extensive subsurface structure is apparent in the radar image, sug
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From page 138... ...
radar that was on the Lunar Reconnaissance Orbiter. Moon sensitivity is not an issue, but the range resolution of the radar images is limited by the bandwidth of the transmitter system, in this case about 200 m, due to the small bandwidth of the Arecibo 70 cm wavelength transmitter.
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From page 139... ...
Problems related to the N-S range-Doppler ambiguity causes ghost images in the opposite hemisphere, especially for radar bright features such as Alpha Region. The black bar is the location of the Doppler equator, which is perpendicular to the apparent rotation axis as viewed from the radar.
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A lunar recon naissance orbiter wide-angle camera mosaic optical view of the same region. There is little evidence of the structures apparent in the radar image.
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From page 141... ...
In the direction orthogonal to range, the azimuth or Doppler direction, the Doppler broadening of the radar echo from a rotating NEA is linearly dependent on the transmitter frequency. This means that achieving the spectral resolution corresponding to the size of the desired resolution cell is easier at higher frequencies.
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From page 142... ...
Utilizing the 100 m GBT in West Virginia for echo reception would increase the sensitivity of the Goldstone radar by a factor of two to three, and utilizing Arecibo for reception for those objects that are within Arecibo sky coverage would increase sensitivity by a factor of about four. The possibility of an 8.6 GHz radar system on the 100 m GBT is being dis cussed.
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From page 143... ...
(MW) Receive Location Green Bank Telescope, 8.6 100 0.5 GBT, West Virginia West Virginia VLA, New Mexico Goldstone, California Arecibo, Puerto Rico 4.6 50 0.5 Arecibo, Puerto Rico GBT, West Virginia Goldstone DSS-14, 8.60 120 1.0 Goldstone, California California GBT, West Virginia Arecibo, Puerto Rico the Goldstone radar and would be more available since the 70 m Goldstone antenna is a prime component of NASA's Deep Space Network (DSN)
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From page 144... ...
Radar speckle interferometric measurements of Europa and Ganymede, two of the icy Galilean satellites of Jupiter, are providing measurements of their obliquities, the angles between the rotation axis and the plane of their orbits about Jupiter. These measurements have the potential to reveal the presence of subsurface oceans and to generally help in constraining interior models of these satellites.11 The sen sitivity of these measurements is about 3,000 times smaller than for the equivalent measurements on Mercury, so any increase in sensitivity of the Goldstone radar system or the installation of an equivalent system on the GBT would help resolve some of the most important issues regarding these satellites.
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From page 145... ...
For the two transmitting stations used for planetary radar astronomy at NASA's Goldstone site, there are NTIA assignments of 200 MHz centered on 8.60 GHz at the 70 m DSS-14 antenna, and an 80 MHz allocation centered on 7.210 GHz at the 34 m DSS-13 antenna. A transmitting capability at NSF's GBT in West Virginia could be based on the redesigned 8.60 GHz klystron if a NTIA frequency assignment of ~120 MHz bandwidth centered on 8.600 GHz is requested and approved, or it could utilize the currently available 8.56 GHz klystrons.
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From page 146... ...
FINDINGS AND RECOMMENDATION Finding 6.1: Active remote sensing through planetary radar astronomy continues to make important contributions to our understanding of the solar system, planning for space missions to extraterrestrial objects, and in particular for the tracking and characterization of NEAs that may pose a threat to society. Finding 6.2: Radio frequency interference has not been a significant impediment to planetary radar astronomy observations to date.
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