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1. Introduction

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:

Measurement of Doppler fluctuations to determine the level and spectral index of turbulence and possible wave activity in the Titan atmosphere, and
Measurement of Doppler and signal level modulation to monitor probe descent dynamics, including its rotation rate and phase, parachute swing, and post-impact status.

Achieving the first of these objectives, DWE will contribute, along with probe accelerometry, and radio occultation measurements, to the assessment of atmospheric turbulence associated with vertical wave propagation, the buoyantly driven surface layer or possible methane moist convection. The second objective represents a potential DWE ``service'' to other probe instruments to assist in the interpretation of their data. The known probe-orbiter geometry enables a determination of directions on Titan from the phase of the Doppler/amplitude modulation. The passively controlled probe spin rate will also be determined to a high degree of accuracy from the same data. These precise measurements of velocity will be integrated to reconstruct the descent flight path, thereby providing the most accurate determination of the Probe's impact coordinates on Titan.

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.

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