A Demonstration of the Time-Delay Mechanical Noise Cancellation (TDMC)
Technique with Cassini Doppler Data
Abstract
Radio science experiments for planetary geodesy or tests or relativistic
gravity involve precise measurements of the spacecraft range rate
enabled by two-way microwave links. Since the uncertainty on the
estimated parameters depend almost linearly on the noise in the radio
link, finding ways to reduce disturbances is essential for best
scientific results. Tropospheric and antenna mechanical noises, among
the leading noise sources in two-way Ka-band radio links, could be
reduced using a suitable combination of Doppler data collected at the
two-way antenna and at an additional, smaller and stiffer, three-way
antenna [1]. The Time-Delay Noise Cancellation technique (TDMC) can
provide significant reduction of the measurement noise if the
listen-only antenna is located in a site with particularly favorable
tropospheric conditions. This noise-reducing technique has only been
tested by artificially producing a large mechanical noise event at the
two-way antenna and using a similar three-way antenna to cancel the
disturbance. We report on a practical demonstration of the capabilities
of the TDMC technique applied to Doppler data from Cassini spanning 2004
through the last Titan gravity flyby in 2016. The tracking configuration
in those passes was not tailored for the use of the TDMC, therefore the
technique proves to be effective only with favorable noise conditions.
Nonetheless, for those passes where tropospheric or antenna mechanical
noises were relevant, we find substantial noise reduction. For example,
Doppler data from the Titan-122 gravity flyby processed with the TDMC
show about a factor-of-three noise reduction (at 60-s integration time)
with respect to the two-way link. These results suggest that the choice
of the three-way antenna and the scheduling of the tracking passes are
crucial parameters that should be considered to fully exploit the
improvement in accuracy provided by the TDMC technique. [1]
Armstrong, J. W. et al., Radio Science 43 RS3010 (2008)