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Session 54 - Ground and Instrumentation Techniques and Catalogs.
Display session, Wednesday, June 12
Tripp Commons,
Tropospheric phase scintillation degrades the coherence of a radio link and thus introduces noise in interferometer observations and spacecraft Doppler tracking experiments. High-quality Doppler data were taken in March-April 1993 with the Mars Observer spacecraft when it was in interplanetary cruise (sun-earth-spacecraft angle \approx100 degrees; earth-spacecraft distance \approx500 light seconds). The radio wave phase residuals from these tracks can be used to study the statistics of the tropospheric scintillation and to assess its importance in precision tracking. Here I present temporal radio wave phase structure functions, \langle \mid \phi(t) - \phi(t + \tau) \mid ^2 \rangle, for X-band data taken at the three NASA/JPL Deep Space Network Tracking complexes. The observed structure functions are approximately powerlaw, D(\tau) = const \tau^\alpha. I characterize the structure functions by their levels at \tau = 100 seconds and their powerlaw indices, \alpha. The powerlaw indices varied between 0.67 and 1.6, averaging 1.2. Substantial variation in the structure function level was observed, with a histogram of level showing many relatively low values and fewer relatively large levels. There were small systematic variations in the levels between the tracking sites, with Australia having larger levels in this sample. I compare these observations with interferometric (i.e., spatial) measurements and discuss some implications for spacecraft tracking, particularly as these observations refine the noise model for low-frequency gravitational wave searches.
