Clementine Bistatic Radar Experiment

Description

The Clementine Bistatic Radar Experiment made use of the radio transmitting equipment aboard Clementine to search the Moon's polar regions for evidence of ice in permanently shadowed craters. The basic method of bistatic radar involves a spacecraft transmitting a radio signal at a point on the target body. Reflections of these signals from the target are received on Earth. Properties of the received reflections can be interpreted to give information on the target surface.

In this experiment the Clementine spacecraft transmitted an unmodulated S-band right-circularly polarized signal through the 1.1 meter high-gain antenna. The signal had a frequency of 2.273 GHz (13.19 cm wavelength) and net power of about 6 watts. The high gain antenna has a half-power half-beamwidth of 4.0 degrees. Reflections were received on Earth by the 70 meter antennae of the Deep Space Network (DSN) at Goldstone (United States), Madrid (Spain) and Canberra (Australia). During these passes, the lunar axial tilt toward Earth at the DSN stations was 4.5 to 5.5 degrees. The experiment can be done when the target and DSN receiver are coplaner with the spacecraft orbital plane. The bistatic radar experiment was performed for the lunar south pole during orbits 234, 235, 236, and 237 on 9 and 10 April 1994 (received at Goldstone, Madrid, and Canberra). Observations of the north pole were made on orbits 299, 301, and 302 on 23 and 24 April 1994 (received at Madrid and Canberra). Systematic errors made the results from orbits 236, 237, and 299 unusable.

The properties of the reflected radio signal which are of particular interest in the search for water ice and other frozen volatiles are the echo magnitude and the sense of polarization. The echo magnitude, or amplitude of reflectivity, is higher for frozen volatiles than for typical silicate rocks. Total internal reflection, characteristic of volatile ices, tends to preserve the original sense of circular polarization, in this case right-circular polarization, or RCP, in the reflected signal. There is also a detection, known as a coherent backscatter opposition effect, which can be made as the spacecraft-target-receiver angle approaches zero, which may indicate scattering centers embedded in ice. Results of the experiment showed a significant RCP to LCP (left-circular polarization) ratio enhancement on orbit 234 over the south pole and a coherent backscatter opposition effect, possibly indicative of volatile ices (presumably well-mixed with soil and dust) in the permanently shadowed craters in this region. However, other scattering mechanisms, such as roughness, can not be ruled out as the cause of these results. The other orbits did not show statistically significant RCP/LCP enhancements.