Pieter Visser (Professor, Netherlands)

For the CryoSat-2 mission precision orbit determination (POD) is performed with the DORIS system and satellite laser ranging. The POD method requires that there are accurate models to describe the accelerations acting on the satellite. The importance of dynamic models increases when the tracking data is not continuously available; due to the 725 km altitude of CryoSat-2 gaps in the IDS tracking network happen more often than for other altimeter systems like the Sentinel-6. For this talk we updated the IDS beacon and SLR station coordinates to ITRF2020 preliminary version, we improved the editing procedures and we looked at the time variable gravity (TVG) modelling which depends on the availability of GRACE / GRACE-FO monthly series of gravity solutions that started in February 2002 well before CryoSat-2 was launched in April 2010. TVG solutions from GRACE are available until June 2017 when the mission came to an end; from this date on there is an outage until June 2018 when the GRACE-FO solutions become available. We will show that, despite the interruption and despite quality differences between GRACE and GRACE-FO, an equally realistic TVG model can be developed than by relying on the GRACE TVG modelling alone. The new TVG model strategy replaces the version developed from GRACE. Tracking residuals improve and our conclusion is that there is an optimal strategy for the POD of CryoSat-2.