The effect of colored noise on the VERITAS gravity science experiment

The gravity science experiment of NASA’s VERITAS mission will provide a uniform and high-resolution mapping of Venus’s gravity field, resulting in much stronger constraints on the interior structure of the planet. The radio tracking relies on a multi-frequency system in X and Ka band (7.2-8.4 GHz and 34.0-32.3 GHz) able to provide Doppler measurements with an accuracy, in terms of radial velocities, of about 18 μm/s at 10 s integration of time, that is 2-10 times better than for a standard X-uplink, X/Ka downlink. In addition to the plasma noise suppression provided by the Ka band link, the multifrequency configuration will enable an additional 75% removal of the plasma noise, important when the Sun-Probe-Earth angle is smaller than 15-20°. We performed numerical simulations of the radio science experiment using JPL’s orbit determination software MONTE. In particular, we accurately model the Doppler data noise by considering the contributions coming from the Earth troposphere and interplanetary plasma (variable over time), as well as constant effects related to the ground station and to on-board components. To quantify the effects of the colored noise on the parameters estimation, the precise Allan deviation and power spectral density of each contributor are considered to produce the correlation matrix relative to the colored noise for each tracking data arc. The recovery of the gravity field via spherical harmonic coefficients up to degree and order 220 will be compared for the cases of white noise (a standard assumption) and of colored noise. We assess how the ability of VERITAS to retrieve the Stokes coefficients and additional parameters related to the precession rate and tidal response of the planet is affected by this more realistic assumption for the noise sources on Doppler measurements.