Feasibility of an innovative technique for noise reduction in spacecraft Doppler tracking

Precise measurements of spacecraft range-rate enabled by two-way microwave links are used in radio science experiments for planetary geodesy. The final accuracies in the gravity field recovery depend almost linearly on the Doppler noise in the link. In this work, we present results of simulations carried out to evaluate the improvement attainable in Doppler measurements using an innovative noise-cancellation technique proposed by Armstrong et al. [1], using two mission profiles: a representative low-altitude Venus orbiter and the BepiColombo spacecraft. The Time-Delay Mechanical Noise Cancellation (TDMC) technique involves a combination of Doppler measurements collected (at different times) at the two-way antenna and at an additional, smaller and stiffer, receive-only antenna that should be located in a site with favorable tropospheric conditions. This configuration could reduce the leading noise sources in a Ka-band two-way link, such as tropospheric and antenna mechanical noises. We considered a two-way link either from NASA's DSS 25 (in Goldstone, CA) or from ESA's DSA-3 (in Malargüe, Argentina) antennas. Moreover, we selected the 12-m Atacama Pathfinder EXperiment (APEX) in Chajnantor (Chile) as the three-way antenna and developed its noise model according to atmospheric data and mechanical stability specifications available in literature. For an 8-hour Venus orbiter tracking pass in Chajnantor's winter/nighttime conditions, the Allan deviation of fractional frequency fluctuations Δf/f
₀
(a measure of the link's frequency stability, thus accuracy) of the simulated TDMC observable at 10-s integration time is 4.5×10
^-14
, to be compared to 1.5×10
^-13
for the two-way link. For BepiColombo, we obtained 1×10
^-13
and 2.6×10
^-13
, respectively for the TDMC and two-way links. If successfully implemented, the use of this noise-reducing technique could be valuable for planetary geodesy missions, where the accuracy in the estimation of high-order gravity harmonic coefficients is limited by tropospheric and antenna mechanical noises, difficult to reduce at the short integration times of interest. The improved orbit determination could be also beneficial whenever high navigation accuracies are required.