Physics
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....3081s&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #3081
Physics
Scientific paper
In bistatic radar experiments, the transmitter and receiver are spatially separated. When the transmitter orbits a distant planet, its antenna can direct relatively small amounts of power toward the remote surface, probing both its physical structure (roughness) and electrical properties (dielectric constant). Several bistatic radar experiments are planned as part of Mars Express Radio Science (MaRS)investigations. Stations on Earth will receive the surface echoes at both 3.6 and 13.1 cm wavelengths. Similar experiments have been conducted previously using Viking and Mars Global Surveyor spacecraft. In quasi-specular experiments, the spacecraft antenna is aimed toward the point on the mean planetary surface which would give mirror-like (specular) reflection toward Earth. Because the path length via the specular point changes at a different rate than the direct-to-Earth path, the surface echo may be distinguished at the receiver by its Doppler shift. When the surface is rough, many reflections contribute to the total echo, each with a slightly different Doppler; dispersion of the composite echo is proportional to the rms roughness of the surface in the vicinity of the specular point. To first order, the reflected power in the echo is proportional to the Fresnel reflection coefficient of the dielectric material making up the top few centimeters of surface. Volcanic plains and the "Stealth" region west of Tharsis Montes will be studied using Mars Express bistatic radar. Icy surfaces sometimes display anomalously high radar backscatter; Europa, Ganymede, and Callisto are distinctive in this way. The south residual polar cap on Mars is also radar bright; but the north appears not -- or, at least, less so. Bistatic radar experiments conducted in either a backscatter geometry (as the spacecraft antenna sweeps across the surface in the anti-Earth direction) or in spotlight mode (where the antenna's boresight is fixed on a single surface point, which also passes through the backscatter condition) may be helpful in understanding the nature of icy surfaces on Mars.
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