Media propagation delay and delay-rate induced by the water vapor within the Earth's troposphere represent one of the main error sources for radiometric measurements in deep space. In preparation for the BepiColombo and JUICE missions, the European Space Agency has installed and operates the prototype of a tropospheric delay calibration system (TDCS) at the DSA-3 ground station located in Malargüe, Argentina.An initial characterization of the TDCS performance was realized using two-way Doppler measurements at X-band to perform the orbit determination of the Gaia spacecraft. This work will further characterize the system by analyzing two-way Doppler and range data at X- and Ka-band for 31 tracking passes of the BepiColombo spacecraft, which were recorded between March 2021 and February 2022 during the first two solar conjunction experiments. The performance exceeds the expectations based on the previous analysis, with a reduction of the Doppler noise of 51% on average and up to 73% when using the TDCS measurements in place of standard calibrations based on global navigation satellite system data. Furthermore, the campaign serves as validation of the TDCS operations during superior solar conjunctions, with most of the tracking passes at low elongation now satisfying the Mercury orbiter radioscience experiment requirements on two-way Doppler stability. These results, which are in line with those of similar instruments installed at other Deep Space Network antennas, are obtained using a commercial microwave radiometer with significantly lower installation and maintenance costs.

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)

Launched in October 2018, the ESA/JAXA BepiColombo mission is currently in cruise to reach Mercury in late 2025. The Mercury Orbiter Radioscience Experiment (MORE) is one of the 16 instruments hosted on board the spacecraft. Testing general relativity is among the primary objectives of MORE. Superior conjunction experiments (SCE) will be performed during the interplanetary trajectory, with the aim of obtaining an accurate estimate of the post-Newtonian parameter γ. This is allowed by MORE advanced radio tracking system which provides precise range and Doppler data almost at all solar elongation angles, thus enabling an accurate measure of the relativistic time delay and frequency shift undergone by the signal when the spacecraft is in a superior solar conjunction (SSC). The rst BepiColombo SCE will take place in March 2021, and others will follow during the cruise phase. The nal objective is to place new limits to the accuracy of the general relativity as a theory of gravity in the weak eld limit, improving previous result from the Cassini SCE (Bertotti et al. 2003), which was able to determine that γ-1=(2.1±2.3)×10-5. Because of the proximity to the Sun, the spacecraft will undergo severe solar radiation pressure acceleration, and the effect of the random uctuations of the solar irradiance may become a major concern. We address the problem of a realistic estimate of the outcome of the SCE of BepiColombo, by including the effects of solar irradiance random variations in the dynamical model. We analyzed the experiment under different assumptions on the ranging system performances, observation coverage and solar activity showing their impact on the attainable result. We propose a numerical method to mitigate the impact of the variable solar radiation pressure on the scienti c result. Our simulations show that, exploiting data from multiple SSCs, the accuracy obtainable in the relativistic time delay measurement is 13×10-6 for a strong solar activity, and 6×10-6 for weak irradiance uctuations. We found that the latter result can be obtained by the rst SSC alone if the plasma noise calibration works until the impact parameter reaches 6 solar radii.

Understanding the processes that led Venus to its current state and will drive its future evolution is one of the main challenges of the next generation of space missions. In this work we analyze the retrieval of the spin vector, the tidal response and the moment of inertia of Venus with VERITAS, a Discovery-class mission proposed to NASA. By simulating the joint analysis of Doppler tracking data and tie points provided by the onboard synthetic aperture radar, we show that VERITAS would provide strong constraints on the interior structure of the planet. In particular we show that VERITAS would provide accuracies in the estimates of the tidal Love number k2 to 3.9×10-4, its tidal phase lag to 0.04o, and the moment of inertia factor to 1.4×10-3 (0.4% of the expected value).