Analysis and Quality Assessment of LEO GPS Data for Geophysical and Ionospheric Applications

During the last few years, an ever-increasing fleet of Low Earth Orbiting (LEO) satellites for scientific purposes became operative. Most of these satellites carry dual-frequency Global Positioning System (GPS) receivers. The highly accurate dual-frequency observations allow mitigating the ionospheric signal contribution to estimate precise orbits and eventually the earth's gravity field. However, when comparing the obtained GPS only gravity fields derived from Swarm to gravity field solutions obtained by the dedicated gravity field mission GRACE, systematic band-shaped differences are visible in the vicinity of the geomagnetic equator. In this work, an empirical approach for the appropriate weighting of GPS observations is derived to mitigate these ionospheric artifacts. The cause of the artifacts is further analyzed by investigating the loop filter implementation. A tracking loop-specific transfer function is derived and used to invert the loop filter response to derive corrections for the GPS phase observations. Both methods are evaluated to achieve the best possible Swarm GPS only gravity field. Vice versa, the collected GPS observations from the LEO precise orbit determination antenna can also be used to gain insight into the topside ionosphere and plasmasphere. A three-dimensional model approach is developed using a fleet of LEO satellites to estimate a model of the electron density distribution between LEO and GPS satellites. Both aspects represent possibilities of using GPS/GNSS on-board of LEO satellites for geophysical applications

Assessment of individual and combined gravity field solutions from Swarm GPS data and mitigation of systematic errors

Although ESA's Earth Explorer Mission Swarm is primarily dedicated to measure the Earth's magnetic field, it currently also serves as a gravity field mission. Equipped with GPS receivers, accelerometers, star-tracker assemblies and laser retro-reflectors, the three Swarm satellites provide the necessary instrumentation to be used as a high-low satellite-to-satellite tracking (hl-SST) observing system. Since GRACE-B, one of the two satellites of the GRACE mission dedicated to measure the time-variability of the Earth?s gravity field, has been decommissioned on 12 Oct 2017, Swarm is now in the focus to close the gap between the GRACE and its Follow-On (GRACE-FO) mission. In this presentation we will report the current state of Swarm gravity field determination as performed at various analysis centers. Besides the individual contributions we will also present combined Swarm solutions computed by the scientific combination service established in the frame of the European Gravity Service for Improved Emergency Management (EGSIEM) initiative. Besides this, we will assess the various efforts made to mitigate systematic errors in the Swarm Level-1B GPS data related to ionospheric activity that are affecting the Swarm gravity field solutions