Orbital Design Considerations for Multiple Spacecraft Engaged in Mutual Radio Occultations

F. N. Bauregger, G. L. Tyler (Stanford University)

The use of two or more mutually occulting spacecraft in orbit around a planet yields more frequent, widely distributed radio occultations than can be obtained with spacecraft-Earth geometries. The relative shape and orientation of the satellites' orbits control the geometry of the occultations. Thus, the important factors related to the occultation geometry may be varied by modifying the satellite orbital elements. By examining the geometry and tabulating various geometrical factors pertaining to recurring mutual occultations, upper and lower bounds are established on these parameters, and the "quality" of the resulting occultations may be gauged quantitatively.

Occultation ray periapsis loci with high vertical and low horizontal velocities typically provide the most accurate and numerically stable measure of a planetary atmosphere. A ray periapsis with a low horizontal velocity also has the advantage that it samples the smallest possible atmospheric volume. Such a condition may be achieved by placing low altitude, mutually occulting satellites in the same orbital plane. Moreover, if the satellites move in opposite directions, a large number of distributed, very high quality occultations will occur daily. There are trade-offs between optimal geometries and ease of instrumentation. For example, Doppler frequency rates are highest for oppositely moving low orbiters, which in turn requires greater instrumental capability; more moderate conditions can be achieved by increasing the height of one of the satellites. Simple rules govern the number and distribution of the mutual occultation points. Arbitrary orbital geometries typically provide only mediocre observational conditions.