Adopting the empirical CODE orbit model to Galileo satellites

Sidorov, Dmitry; Dach, Rolf; Polle, Bernard; Prange, Lars; Jäggi, Adrian (2020). Adopting the empirical CODE orbit model to Galileo satellites. Advances in space research, 66(12), pp. 2799-2811. Elsevier 10.1016/j.asr.2020.05.028

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In 2012 the Center for Orbit Determination in Europe (CODE) joined the Multi-GNSS-EXtension project (MGEX) of the International GNSS Service (IGS). Since the end of 2013 the CODE MGEX contributions were based on combined solutions of five already established and emerging GNSS: GPS, GLONASS, Galileo, BeiDou and QZSS. This undertaking was made possible thanks to the continuous development of new models and approaches and their introduction in our processing schemes in order to ensure the delivery of products of the highest quality.

The European Galileo system with a total number of active satellites reaching the level of a nominal constellation is currently complete. Because of their relatively high area-to-mass ratio the Galileo spacecraft are more sensitive to non-gravitational forces than other GNSS satellites. The introduction of the extended empirical CODE orbit model (ECOM2) to the CODE MGEX solutions in early 2015 resulted in a significant improvement of the Galileo products. The use of the Galileo satellites metadata, which were made publicly available in the course of 2016 and 2017, has further enhanced the quality of the produced solutions. However, they still show significant degradations during eclipse seasons in particular for long-arc solutions (e.g., over three days), which are similarly observed in solutions of other IGS MGEX analysis centers to different extents. In particular this is reflected in elevated orbit misclosures, deterioration of the estimated satellite clock corrections and excessive satellite laser ranging (SLR) residuals during these periods. Since the ECOM2 parameters are designed to absorb the effect of solar radiation pressure, they are switched off during eclipses. Hence, there is no empirical force parameter left that can absorb any unmodelled perturbations (e.g., due to thermal radiation (TR)) during an eclipse period.

In this study we take advantage of the satellites’ metadata to address the Galileo TR-induced accelerations and, therefore, to advance our orbit model further. By adjusting existing and introducing additional empirical parameters, as well as adopting a priori accelerations to account for unmodelled perturbations we achieve a significant improvement of our solutions. In particular, the use of a once-per-revolution sine term in the ECOM direction (satellite-Sun), activation of the constant term in the ECOM direction (along the solar panels) during the Earth’s shadow transitions as well as the use of a priori acceleration in the body-fixed direction substantially improve our solutions. The introduced modifications to the orbit model allow for an efficient adjustment of satellite velocity along the orbit that is necessary due to the apparent presence of unmodelled thermal radiation. The refinements lead to a substantial reduction of orbit misclosures and improvement of the radial orbital component accuracy during eclipse seasons up to 14% w.r.t. the solutions using ECOM2.

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