The Doppler Wind Experiment (DWE) is a high-precision tracking investigation to determine the direction and magnitudes of the winds in the Titan atmosphere. The prime science objective, a height profile of the wind velocity, will be derived from the Doppler shift of the Probe Relay Link (PRL) signal from the Huygens Probe to the Cassini Orbiter. After correcting for all known Doppler shifts due to orbit and propagation effects, the wind-induced motion of the Probe will be determined to an accuracy better than 1 m/s, commencing with parachute deployment at an altitude of ca. 160 km down to impact on the Titan surface.
In addition to the measurements of drift motions due to winds,
DWE is capable of achieving two secondary scientific objectives:
The most severe constraints on the accuracy of the DWE wind measurement are trajectory errors and instability of the probe oscillator used to generate the PRL signal. Present assessments of these limitations indicate that a zonal wind height profile can be recovered with a mean error less than m/s. This can be achieved only with a sufficiently stable PRL signal over the duration of the descent ( Hz at S-band) in order to exclude contamination of the measurement by oscillator drift. The frequency stability of the transmitter is guaranteed by using an ultrastable oscillator (USO) to generate the PRL carrier signal. In addition to this transmitter USO (TUSO), it is necessary to incorporate an additional unit into the receiver (RUSO) of the Probe Support Avionics (PSA) on the Cassini Orbiter, where the frequency measurement is recorded.
Current theories of Titan's winds and some comments on DWE synergism with other ground-based and Cassini/Huygens investigations are presented in the next section. Following this we describe the methodology of the investigation and outline the recovery algorithm for retrieving the zonal wind profile from the PRL frequency measurements. The report concludes with a description of the DWE-instrumentation, the TUSO/RUSO ultrastable oscillators, and their associated development program.