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. Even though ESA's three-satellite low-earth orbit (LEO) mission Swarm isprimarily a magnetic field mission, it can also serve as a gravity fieldmission. Located in a near-polar orbit with initial altitudes of 480 kmfor Swarm A and Swarm C and 530 km for Swarm B and equipped withgeodetic-type dual frequency Global Positioning System (GPS) receivers, it issuitable for gravity field computation. Of course, the Swarm GPS-only gravityfields cannot compete with the gravity fields derived from the ultra-preciseGravity Recovery And Climate Experiment (GRACE) K-band measurements. But forvarious reasons like the end of the GRACE mission in October 2017, datagaps in the previous months due to battery aging, and the gap between GRACEand the recently launched GRACE Follow-On mission, Swarm gravity fieldsbecame important to maintain a continuous time series and to bridge the gapbetween the two dedicated gravity missions. By comparing the gravity fieldsderived from Swarm kinematic positions to the GRACE gravity fields,systematic errors have been observed in the Swarm results, especially aroundthe geomagnetic equator. These errors are already visible in the kinematicpositions as spikes up to a few centimeters, from where they propagate intothe gravity field solutions. We investigate these systematic errors by analyzing the geometry-free linearcombination of the GPS carrier-phase observations and its time derivativesusing a combination of a Gaussian filter and a Savitzky–Golay filter and theRate of Total Electron Content (TEC) Index (ROTI). Based on this, we presentdifferent weighting schemes and investigate their impact on the gravity fieldsolutions in order to assess the success of different mitigation strategies.We will show that a combination of a derivative-based weighting approach witha ROTI-based weighting approach is capable of reducing the geoid rms from21.6 to 12.0 mm for a heavily affected month and that almost 10 % morekinematic positions can be preserved compared to a derivative-basedscreening.

Mitigation of ionospheric signatures in Swarm GPS gravity field estimation using weighting strategies