Radio propagation experiments in the outer solar system with Voyager

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.