Radio Science Techniques for Deep Space Exploration

2 Planetary Atmospheres, Rings, and Surfaces

2.1 Overview of Radio Occultations

Astronomers have used the occultation method of atmospheric research for over a century since planets naturally pass in front of stars, or occult the stars. First used with visible light, the occultation method has been extended to other wavelengths over time. The development of airborne instruments allowed observation of infrared frequencies blocked by water vapor in the atmosphere. Stellar occultations were brought to the forefront with the 1977 discovery of the rings of Uranus. Since then, the technique has been used to probe the atmospheres of planetary bodies (Elliot and Olkin 1996).

Similarly, since the early robotic exploration of the Solar System, the technique of spacecraft radio occultation (RO) utilized radio signals transmitted from spacecraft, not natural radiation. The resulting measurement investigated the effects of refraction, absorption, and scattering on the amplitude and phase of radio signals by planetary ionospheres, neutral atmospheres, plasma tori, and rings.

As illustrated in Figure 2.1, as a spacecraft travels behind a planet and the line-of-sight to Earth propagates through the planet’s atmosphere, the radio link transmitted by the spacecraft and received by an Earth station is perturbed in phase due to atmospheric refraction and sometimes in amplitude due to absorption caused by the constituents of the intervening plasma and neutral gas. RO studies flourished when the ...