Measurement of a zonal wind profile on Titan by Doppler tracking of the Cassini entry probe

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As the Cassini probe enters and descends through the Titan atmosphere, the winds will cause perturbations to the probe local horizontal velocity resulting in an anomalous drift in the probe location and a measurable shift in the frequency of the probe telemetry due to the Doppler effect. By using an iterative algorithm in which the time variation of the probe telemetry frequency is monitored throughout the descent and the probe trajectory is updated to reflect the effect of wind on the probe location, a highly accurate relative wind profile can be recovered. Coupled with a single wind velocity measured at a known altitude by independent means, an absolute profile results. The accuracy with which the wind profile can be recovered is primarily limited by the a posteriori uncertainties in the probe and orbiter trajectories and oscillator frequencies. An unmodeled drift in oscillator frequency will cause a time‐varying frequency that is inseparable from Doppler effects. Similarly unmodeled changes in the probe‐orbiter relative geometry will lead to Doppler frequency contributions that are indistinguishable from the wind signature.