Spacecraft-to-ground bistatic radar provides a straightforward method for surveying planetary surfaces on scales of importance to landers and rovers. Centimeter wavelengths, currently in use for deep-space telecommunications, interact with surface structure of similar to somewhat larger scales. For the quasi-specular component of scattering and for surfaces uniformly illuminated by monochromatic signals from an orbiting or flyby vehicle, the echo Doppler dispersion is proportional to the root mean square (rms) surface slope. When the specular condition occurs within 10°-20° of the Brewster angle, the surface dielectric constant can be derived from relative echo power measured simultaneously in orthogonal polarizations and the Fresnel reflection laws. Cross spectra, computed from outputs of the orthogonally polarized receivers, may be used to calculate a complete description of the polarization properties of the scattered fields. Application to planetary studies requires accurate amplitude and phase calibration of the polarization channels, including correction for any leakage between the two receiving paths, such as from imperfectly isolated antenna feeds. We illustrate these techniques using Mars Express results from “Stealth” (Medusae Fossae), a region on Mars that has not previously been detected by Earth-based radar, and from a long profile including Acidalia Planitia. Single-location Stealth observations support previous conclusions that the surface is rough and porous (dielectric constant ≈ 1.4). But the longest experiment (in which the specular point was followed for an hour) yields relatively high dielectric constants (2.8), suggesting that the model is incomplete. The surface of Acidalia Planitia has low dielectric constants ( ≈2.6) over 60-90 W at 50 N and higher values (≈3.6) as the specular point moves south and crosses the equator.

Valid application of the Kirchhoff approximation (KA) for scattering from rough surfaces requires that the surface radius of curvature exceed approximately the electromagnetic wavelength /spl lambda/. Fractal surface models have characteristic features on arbitrarily small scales, thereby posing problems in application of the electromagnetic boundary conditions in general as well as in the evaluation of surface radius of curvature pertinent to KA. Experiments and numerical simulations show variations in scattering behavior that are consistent with scattering from progressively smoother surfaces with increasing wavelength, demonstrating surface smoothing effects in the wave-surface interaction. We hypothesize control of KA scattering from fractal surfaces by an effective average radius of curvatureas a function of the smallest lateral scale /spl Delta/x contributing to scattering at /spl lambda/. Solution of=/spl lambda/ for /spl lambda/ is one possible method for approximating the limit of KA validity, assuming that /spl Delta/x[/spl lambda/] is known. Investigation of the validity of KA for the calculation of scattering from perfectly conducting Weierstrass-Mandelbrot and fractional Brownian process fractal surface models shows that for both models the region of applicability of KA grows with increases in /spl lambda/ and the Hurst exponent H controlling large-scale roughness. Numerical simulations using the method of moments demonstrate the dependence of /spl Delta/x on /spl lambda/ and the surface parameters.

Microwave telecommunications transmissions from the two Voyager spacecraft are being used to make detailed studies of planetary atmospheres, ionospheres, rings, and magnetic fields in the outer solar system. Coherently related sinusoidal signals, at wavelengths of 3.6 and 13 cm, transmitted from Voyager but received and analyzed on Earth serve as an active probe of planetary environs. Such studies have been carried out during the spacecraft encounters of Jupiter, Saturn, and Uranus; observations of Neptune's system are planned for August, 1989. The required occultation geometries are obtained either as by-products of the gravity-assist trajectories employed to travel among the planets or, in some cases, by design. Both spacecraft and ground systems are specifically modified and improved to support radio investigations. In particular, the use of hydrogen maser frequency standards on the ground in conjunction with thermally controlled, radiation-hardened quartz oscillators on the spacecraft guarantees long coherence intervals and allows application of new signal processing techniques. These advances lead to spatial resolutions in the media of interest well below the Fresnel zone scale, to current limits of a few tens of meters in the rings of Uranus. The present theory and data reduction methods permit detailed studies of atmospheric structure and scintillation parameters, the radial structure and particle size distribution of planetary rings, and the magnetic control of small-scale ionospheric irregularities. Atmospheric measurements are made at pressures from a few tenths of a millibar to a few bars, extinction profiles of planetary rings have been made to slant optical depths of 8 to 10, and alignments of magnetic fields within ionized regions have been determined to within a few degrees. Fundamental information concerning planetary composition, evolution, and dynamics is obtained from these measurements. Future improvements in radio systems through the use of powerful (> 100 000-W) ground transmitters in conjunction with high-performance receiving and digital signal processing units aboard spacecraft would permit improvements in dynamic range and sensitivity of such propagation experiments of 20 to 40 dB.