New GNSS-derived precise orbits and thermosphere densities for the Swarm satellite constellation

Using the high-quality GNSS observations of the low Earth orbiting Swarm constellation, Delft University of Technology routinely delivers precise science orbits (PSO), aerodynamic accelerations, and thermosphere densities for all three satellites within the framework of the Swarm Data, Innovation, and Science Cluster. The PSO consist of a reduced-dynamic orbit to precisely geotag the onboard magnetic and electric field instrument observations and a kinematic orbit with covariance information to determine the large-scale time variable changes of Earth’s gravity field. The GNSS-derived densities can be used to improve thermosphere models and for studying the influence of solar and geomagnetic activity on the thermosphere. The aerodynamic accelerations are used to augment the higher-resolution accelerometer data, which are affected by accelerometer instrument issues. Due to these issues, the accelerometer-derived thermosphere densities are not continuously available for all satellites.

For both PSO and density products, the nominal processing strategy has recently been improved. The PSO processing strategy, which includes a realistic satellite panel model for solar and Earth radiation pressure modelling and integer ambiguity fixing, was updated with a new approach to reduce the impact of ionospheric scintillation-induced errors in the kinematic orbits. The previous procedure was not properly tuned for high solar activity conditions, resulting in many gaps in the kinematic orbits, with losses of up to 40% during periods with such conditions. With the new approach, considerably more kinematic orbit data are available.

For the GNSS-based thermosphere density retrieval, aerodynamic accelerations are estimated in a precise orbit determination using a Kalman filter approach and converted to densities using a high-fidelity satellite geometry model and gas-surface interaction modelling. To account for the large variations in the encountered aerodynamic signal by the Swarm satellites over the mission lifetime, a new approach was implemented that uses adaptive process noise settings for the estimated aerodynamic accelerations. These new settings lead to significantly improved densities during low-density signal conditions.

The new Swarm precise orbit products (version 0203) and thermosphere density products (version 0301) are available for users at the dedicated ESA Swarm website (https://swarm-diss.eo.esa.int). The Swarm densities are also available at our thermosphere density database (https://thermosphere.tudelft.nl).