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)

The recently selected VERITAS and EnVision missions will fly with X and Ka Band telecommunications channels, permitting dual band radio occultations of the Venus atmosphere. While signal absorption measurements during S and X Band radio occultations of Venus in the past have been used to retrieve vertical abundances of H2SO4 vapor as a function of latitude, Ka Band links have yet to be employed to sound the neutral atmosphere. Laboratory measurements and propagation models of the Venus atmosphere suggest that H2SO4 cloud aerosols/vapor and SO2 absorb radio signals differently between X and Ka bands, permitting inversion of their abundance profiles down to the attenuation limit near 45 km. Such measurements would be of great value to the study of Venus atmospheric chemistry and dynamics. While the bulk abundance of SO2 at the cloud base has been inferred from microwave/infrared radiometry and from X Band occultations, Ka Band measurements could be used to derive vertically resolved profiles at the deepest altitudes yet, spanning a region where the abundance of SO2 changes by several orders of magnitude. Vertical profiles of lower cloud bulk density could also be achieved at higher resolution than any prior remote measurements. This presentation will discuss approaches to retrieving H2SO4 vapor/aerosol, and SO2 abundances using dual X/Ka Band radio occultations of the Venus neutral atmosphere. H2SO4 vapor can be retrieved with very high accuracy, surpassing that of prior single frequency occultations. Due to the relatively low (high) X (Ka) Band opacity of both SO2 and H2SO4 aerosols, retrievals of these species from dual band occultations are highly degenerate. To improve accuracy, we find that it is necessary to incorporate the results of chemical and dynamical modeling as prior information. At lower latitudes and in regions of high abundance, preliminary results suggest retrievals of SO2 profiles can be accomplished within 15-20% uncertainty. This error increases at higher latitudes, where models of cloud bulk density span a wider range of predictions. We compare the effects of various assumptions on the accuracy of the resulting retrievals, and discuss prospects for coupling a chemical/dynamics model to the retrieval process.