Mitigation of ionospheric signatures in Swarm GPS gravity field estimation using weighting strategies
Abstract. Even though ESA's three-satellite mission Swarm is primarily a magnetic field mission, it became more and more important as gravity field mission. Located in a low earth orbit with altitudes of 460 km for Swarm A and Swarm C and 530 km for Swarm B, after the commissioning phase, and equipped with geodetic-type dual frequency GPS receivers, it is suitable for gravity field computation. Of course the Swarm GPS-only gravity fields are not as good as the gravity fields derived from the ultra precise GRACE K-Band measurements, but due to the end of the GRACE mission in October 2017, data gaps in the previous months, and the gap between GRACE and the recently launched GRACE Follow-On mission, Swarm gravity fields became important to maintain a continuous time series and bridge the gap. By validating the Swarm gravity fields to the GRACE gravity fields, systematic errors have been observed, especially around the geomagnetic equator. These errors are already visible in the kinematic positioning from where they propagate into the gravity field solutions. We investigate these systematic errors by analyzing the geometry-free linear combination of the GPS carrier phase observations. Based on this we present different weighting schemes and investigate their impact on the gravity field solutions in order to assess the success of different mitigation strategies.