Spacecraft radio occultations using multiple Doppler readouts

We present an innovative technique that eliminates the phase noise of the onboard frequency reference as the main limiting factor on uplink radio occultations, while allowing full utilization of the expected improvement in SNR over downlink experiments. This technique relies on conducting simultaneous uplink and downlink occultation measurements to synthesize a phase reference of quality determined by the stability of the atomic clock at the ground station, eliminating the frequency fluctuations of the onboard clock as the limiting factor. Atomic frequency standards typically have frequency stability that is about 2–3 orders of magnitude better than space-qualified state-of-the-art clocks. The quantum leap improvement in reference phase stability and, simultaneously, in measurement SNR opens the door to fundamentally new science capabilities for radio occultation observations of planetary rings and atmospheres. In particular, the improvement is expected to enhance sensitivity to tenuous target regions, to increase the dynamic range within opaque regions, and to improve the spatial resolution of mapping large- and small-scale target structures.