GOCE SSTI L2 tracking losses and their impact on POD performance

The state-of-the-art GOCE Satellite-to-Satellite Tracking Instrument (SSTI) delivers high-quality GPS data with an almost continuous 1Hz data rate, which allows for very Precise Orbit Determination (POD). Despite this good performance, the GPS receiver shows occasional unexpected L2 tracking losses, which mainly occur close to the geomagnetic poles and, to a lesser extent, also along the geomagnetic equator. The number of unexpected L2 tracking losses varies in time and shows some correlation with solar activity. Less than 3% of the observation data is affected by these losses. Therefore, the effect on the POD remains limited. However, systematic effects might be present, as the quality of the GOCE orbits is slightly reduced over the polar regions. The striking correlation between the global distribution of ionospheric irregularities and L2 losses suggests scintillation effects might be present. Analysis of the time derivative of the geometryfree combination of GPS phase observations shows that unexpected L2 losses occur during times of rapid ionospheric fluctuations. GPS satellites in cross-track direction are most affected by L2 losses.