The use of dispersion to eliminate multipath in planetary radio occultations


  • Thomas A. Croft


During planetary atmospheric and ionospheric study through radio occultation of spacecraft, multipath propagation often occurs at several altitudes in the medium. Although the signals are then simultaneous in time, they are separate in frequency. It is shown that dispersion can be used to separate such signals because they are inherently single-valued with respect to a family of curves having dT/dF = λD/V2 (where F = frequency, T = time, V = velocity normal to the limb, λ = wavelength, and D = distance from the spacecraft to the limb plane). The concept underlying this corrective measure is much like that of an FM-CW radar wherein frequency separation is converted to time separation. Application of the recommended dispersion may eliminate a major form of multipath and permit signal detection by receivers employing phase-lock loops that depend for their action on the existence of only one frequency. This should facilitate the processing of open-loop recorded data and it might permit the closed-loop reception of multipath signals without loss of information, making possible the observation of radio occultations with signals from Earth to spacecraft. Heretofore, this approach has not shown promise in multipath situations. No specific processes or designs are given here, but there is a practical discussion concerning the degree of accuracy that must be provided in flight and during postflight analysis.