Mars Express tracking and orbit determination trials with Chinese VLBI network

With strong support from European Space Agency (ESA), Shanghai Astronomical Observatory (SHAO) organized a tracking and orbit determination trails using Chinese VLBI Network (CVN) to track Mars Express, the first Mars probe launched by ESA. Using a high-resolution VLBI software correlator and Doppler measurement system developed in-house, two sets of tracking data, VLBI and Doppler, were acquired. The trials represent the first successful foray held in China to track a probe about 360 million kilometers away from the Earth. The tracking data are analyzed using a Mars satellite orbit determination software system developed at SHAO. The results show that the accuracy of 5 s integrated three-way-Doppler data is about 0.3 mm/s, or roughly the same accuracy as ESA’s tracking data. Position discrepancies between the Doppler-based orbit solution of 8 h arc-length (about 1 orbital revolution) and ESA’s reconstructed orbit are of the order of several hundred meters. In preparing for the Russia-China co-sponsored Mars exploration mission Phobos-Grunt-YingHuo, simulations were carried out to evaluate the achievable orbital accuracy levels and the contributions of VLBI and Doppler data respectively. Results show that Doppler data provide better orbit accuracy, so that for VLBI to be able to provide kilometer level orbit solutions, the accuracy of VLBI measurement needs to be improved by at least one order of magnitude.