Precise science orbits and thermospheric densities for the Swarm mission

The European Space Agency (ESA) Swarm mission was launched in November 2013 and consists of three identical satellites flying in near-polar low Earth orbits to study the dynamics of the Earth’s magnetic field. In the Swarm Data, Innovation, and Science Cluster framework, precise science orbits (PSO) are computed for the Swarm satellites from onboard GPS observations. These PSO consist of a reduced-dynamic orbit to precisely geotag the magnetic and electric field observations, and a kinematic solution with covariance information, which can be used for determining large-scale time variable changes of Earth’s gravity field. In addition, high-resolution thermospheric densities are computed from onboard accelerometer data. Due to accelerometer instrument issues, these data are currently only available for Swarm-C and the early mission phase of Swarm-A. Therefore, also GPS-derived thermospheric densities are computed, which have a lower temporal resolution but are available for all Swarm satellites during the entire mission. The Swarm density data can be used to study the influence of solar and geomagnetic activity on the thermosphere.

We will present the current status of the processing strategy used to derive the Swarm PSO and thermospheric densities and show recent results. For the PSO, our processing strategy includes a realistic satellite panel model for solar and Earth radiation pressure modelling, integer ambiguity fixing, and a screening procedure to reduce the impact of ionospheric scintillation-induced errors. Validation by independent Satellite Laser Ranging data shows the Swarm PSO have high accuracy, with an RMS of the laser residuals of about 1 cm for the reduced-dynamic orbits, and slightly higher values for the kinematic orbits. For the thermospheric densities, our processing strategy includes a high-fidelity satellite geometry model and the SPARTA gas-dynamics simulator for gas-surface interaction modelling. Comparisons between Swarm densities and NRLMSIS model densities show noticeable scaling differences, indicating the Swarm densities' potential to contribute to thermosphere model improvement. The accuracy of the Swarm densities is dependent on the aerodynamic signal size. For low solar activity, the error in the radiation pressure modelling becomes significant, especially for the higher-flying Swarm-B satellite.

The Swarm precise orbit and thermospheric density products are available for users at the dedicated ESA Swarm website (ftp://swarm-diss.eo.esa.int). The Swarm densities are also available at our thermospheric density database (http://thermosphere.tudelft.nl). This database also includes thermospheric densities for the CHAMP, GRACE, GOCE, and GRACE-FO satellites. For future work, it is planned to further improve the Swarm densities, especially for low solar activity conditions, by including a more sophisticated radiation pressure modelling of the Swarm satellites.