Comparison of plasma calibration techniques to enhance radiometric observables performance: BepiColombo MORE test case

Radio science experiments on board a spacecraft aim at investigating different fields of physics, including the study of gravity field and geodesy of a planetary body, test of general relativity and atmospheric studies. In this work we focus on improving the quality of radiometric observables by analyzing different techniques to calibrate the dispersive noise. The primary source of the dispersive medium for interplanetary missions is the solar corona plasma, with the Earth ionosphere contributing to a lesser extent. In order to isolate the dispersive contribution, a multifrequency radio system is generally adopted. In the state-of-the-art calibration scheme (Bertotti, et al., 1993) three 2-way links are established. The system of equations of the calibration scheme is composed of three equations function of three observables and three unknowns, namely the non-dispersive observable and the uplink and downlink plasma contribution. This study is performed with the BepiColombo Mercury Orbiter Radio science Experiment (MORE) data collected during superior solar conjunction experiments. The MORE radio-tracking system simultaneously supports coherent radio links in X/X, X/Ka, and Ka/Ka bands. Not all spacecraft are equipped with a complete radio link due to the required hardware complexity; hence, this work aims at characterizing the performances of alternative calibration schemes. The incomplete dual link calibration consists of only two observables with three unknowns; thus, it is not possible to determine both the uplink and downlink plasma contribution. However, a linear combination of the observables can be used to compensate for one of the two plasma contribution (uplink or downlink). This calibration schemes can be applied to the different link pairs, i.e., X/X + Ka/Ka, X/Ka + Ka/Ka, and X/X + X/Ka. Moreover, the single link Differenced Range Versus Integrated Doppler (DRVID) technique is implemented (MacDoran, 1970) and the results are compared with the ones obtained from triple and dual link configurations. The accuracy of the various calibration techniques is evaluated for both Doppler and range data by comparing the residuals’ standard deviation, root mean square, mean value, and Allan deviation or Power Spectral Density.