AIUB BDSD 2021 Bern Data Science Day 23 April 2021 The Satellite Geodesy Team of AIUB Astronomical Institute, University of Bern, Switzerland Poster compiled by D. Arnold and M. Lasser, April 2021 Astronomical Institute, University of Bern daniel.arnold@aiub.unibe.ch Satellite orbit and gravity ﬁeld determination at AIUB LEO Satellites Plenty of satellites orbit Earth in Low Earth Orbit (LEO), i.e., at an al- titude of about 200 - 2’000 km. For many of them a very accurate and precise knowledge of their positions at any time is crucial. E.g., to reli- ably measure global and regional sea level by means of satellite altimetry (see Fig. 1), the LEO satellit orbits need to be know at a level of cm or even sub-cm accuracy. Figure 1: Principle of satellite altimetry. ©CLS/AVISO Often such LEO satellites carry GNSS (Global Navigation Satellite Sys- tems) receivers on board – most of them for GPS, but newer satellites also for other GNSS like the European Galileo. These GNSS measurements can be exploited to support a Precise Orbit Determination (POD) if • high-quality information about the GNSS satellites (in particular their orbits, atomic clock corrections and signal biases) are known • accurate information about LEO satellite geometry is available • substantial effort for the modeling of satellite dynamics and of the GNSS measurements (including signal propagation and effects in transmitter and receiver) is conducted. LEO POD at AIUB At the Astronomical Institute of the University of Bern (AIUB) the GNSS- based LEO POD is advanced since many years. The orbits of numerous different LEO satellites have been and are being computed using our in- house developed Bernese GNSS Software, which is used by more than 700 other institutions worldwide. For the GNSS orbits and clock correc- tions we use our high-quality Center For Orbit Determination in Europe (CODE) products. As an example, the AIUB is part of the Copernicus POD Quality Working Group, in the frame of which we perform POD for the ESA Sentinel-1, -2, -3, and -6 Earth observation satellite ﬂeet (see Fig. 2) for intercomparison and cross-validation (see Fig. 3). Figure 2: ESA’s Sentinel satellites. ©ESA For a typical 1-day reduced-dynamic (solution of a satellite equation of motion) and kinematic (purely geometric positioning) LEO POD with 10 s sampled GPS data from an 8-channel GPS receiver (e.g., Sentinel-1), we process about 250’000 GPS and 90’000 attitude measurements to estimate orbit and observation technique-speciﬁc parameters. The number of esti- mated parameter is in the range of 10’000 for the reduced-dynamic orbit and about 40’000 for the kinematic one. The processing time (including data pre-processing) amounts to 20-40 minutes and the maximum mem- ory consumption to about 150 MB. Processing GPS data at 1 Hz sampling and/or from GPS receivers with more channels increases these numbers. The POD is usually performed on UBELIX, for extended tests or repro- cessing purposes typically many (50-100) days are processed in parallel. Figure 3: Radial differences of numerous orbit solutions for Sentinel-3B w.r.t. a combined solution in the Copernicus Regular Service Review #19. AIUB is delivering dynamic (light green) and a reduced-dynamic (dark green) solutions. The former one belong to the highest- performing solutions within the Quality Working Group. Figure 4: The GRACE Follow-On mission to measure Earth’s time variable gravity ﬁeld. ©NASA From orbits to gravity ﬁeld The Earth’s gravity ﬁeld causes the main force acting on the satellite’s orbit and deﬁnes its shape. This process may be reversed, thus, precisely measuring the motion of an artiﬁcial satellite in LEO allows to deduce information about Earth’s gravity ﬁeld. The latter is constant neither in space nor in time, but changes due to all-present mass re-distributions above, on or below the Earth surface. In short time scales, e.g., monthly, mostly water re-distributes, which means one is able to measure through satell