Impact of GPS Receiver Tracking Loop Modifications on Precise Swarm Orbits

The European Space Agency (ESA) Swarm mission was launched on 22 November 2013 to study the dynamics of the Earth’s magnetic field and its interaction with the Earth system. The mission consists of three identical satellites flying in near polar orbits. Two satellites are flying almost side-by-side at an initial altitude of 480 km, while the third satellite was placed in a higher orbit at about 530 km altitude. The Swarm satellites are equipped with high-precision 8-channel, dual-frequency GPS receivers, which are used to compute Precise Science Orbits (PSOs). These PSOs nominally consist of a reduced-dynamic orbit for the geolocation of the onboard scientific instrument observations with highest accuracy, and a kinematic orbit for gravity field determination purposes. Independent Satellite Laser Ranging (SLR) validation shows that the reduced-dynamic Swarm PSOs have an accuracy of better than 2 cm, while the kinematic orbits have a slightly reduced accuracy of about 4–5 cm. Despite this good performance, the Swarm GPS receivers are shown to be susceptible to ionospheric scintillation. Generally, ionospheric scintillation is most intense in the equatorial and polar regions. When flying over these areas, the Swarm GPS receivers show a slightly degraded performance, resulting in occasional tracking losses, larger GPS carrier phase residuals and a reduced consistency between the kinematic and reduced-dynamic PSOs. For gravity fields determined from Swarm GPS data this can lead to severe systematic errors along the geomagnetic equator. In order to try to make the Swarm GPS receivers more robust for ionospheric scintillation, the GPS tracking loops have meanwhile been adjusted several times. The bandwidth of the L1 carrier loop has been increased from 10 to 15 Hz, while the L2 carrier loop bandwidth was increased from its original value of 0.25 Hz, to, respectively, 0.5 Hz, 0.75 Hz and 1.0 Hz. To assess which of these settings is optimal, an extensive analysis has been conducted. Because the different tracking loop modifications were first implemented on Swarm-C only, their impact can be assessed by a comparison with the close flying Swarm- A satellite. The assessment includes an analysis of the amount of collected GPS observations and their residual errors, the consistency between the kinematic and reduced-dynamic orbit solutions, as well as SLR validation. This analysis is performed using data collected during different seasons, to take differences in ionospheric scintillation conditions into account. Other low flying satellites with similar GPS receivers, 84 like e.g. the Sentinels, might also benefit from this analysis.