Error budget for the VERITAS gravity science investigation

Veritas is a NASA mission to be launched towards Venus in 2028 to investigate the planet's ongoing and past geological and geophysical processes, and to shed light on how the planet lost its potential as a habitable planet. Accurate radio science data will allow the estimation of several physical parameters related to the planet's crustal and interior structure: the high-degree harmonic expansion of the gravity field (small spatial scale), pole position and precession, tidal Love numbers, moment of inertia, and more. A realistic knowledge of the radio science data noise is crucial to perform accurate and realistic numerical simulations of the gravity experiment and help planning of the measurements. We developed an accurate model to reproduce range and Doppler noise budget, considering several sources related to the media crossed by the radio link (plasma, Earth's ionosphere, and troposphere) and instrumentations (frequency and timing system, Rx/Tx chain, ground and SC antennas, the onboard IDST, TWTA). For all contributions to the Doppler error budget, except solar plasma and tropospheric noise, the Allan deviations have been modeled by power laws in the time domain. Tropospheric noise required different handling because of its significant seasonal dependence. Interplanetary and coronal plasma turbulence is the most complex contribution to model, also because of its dependance on the solar cycle and its intrinsic non-stationarity. At the Ka-band frequencies (32-34 GHz) of the VERITAS radio link (uplink and downlink), its contribution to the noise budget is relatively small, but in proximity of superior solar conjunctions, it may become the dominant noise source. For range measurements, the largest noise contribution comes from thermal noise, but systematic effects due to propagation media and uncalibrated delays of the ground and onboard instrumentation may play a major role in driving the final accuracy of the system.