Radio Science Experiments with Onboard Arraying

The research described in this publication was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. © 2010 California Institute of Technology. Government sponsorship acknowledged. We propose a data processing technique applicable to interplanetary and near-Earth radio science experiments performed with one spacecraft but a network of N ground stations. Our technique relies on an onboard multichannel digital receiver, which separately distinguishes the N uplink signals from the N antennas, and further processes them on the spacecraft. The signals from each uplink element are distinguished by a small frequency offset so that their phases and amplitudes can be separately recorded onboard. By measuring, digitizing, time-tagging, and recording onboard the N uplink signals’ amplitudes and phases, it is then possible to (i) combine the N onboard phase measurements with those made on the ground to minimize or entirely remove the effects of common Doppler noise sources affecting radio science experiments, and (ii) coherently combine the N uplink signals to maximize the ratio between the received microwave power over the measurement thermal noise (i.e., the SNR). Our onboard arraying technique allows perfect cancellation of the frequency fluctuations due to the onboard frequency reference for synthesized two-way Doppler, and suppression by a factor of N of the atmospheric phase scintillation, the frequency fluctuations induced by mechanical vibration of the ground antennas, the ground microwave amplifiers, and the onboard and ground electronic frequency noises affecting the radio links. Depending on the number N of ground antennas used, the resulting improvement in Doppler sensitivity achievable with onboard arraying over that with a single antenna can be significant, and we derive an expression for the relative Doppler sensitivity enhancement that is independent of the particular radio science experiment considered.