Precise science orbits for the Swarm satellite constellation

Swarm, the 4th Earth Explorer mission by the European Space Agency (ESA), was launched in November 2013 to study the dynamics of the Earth's magnetic field and its interaction with the Earth system. To better distinguish between the different sources of magnetism, the mission consists of three identical satellites, flying in carefully selected near-polar orbits. Two satellites will fly side-by-side at an initial altitude of 460 km, and slowly descend to around 300 km during the mission. The third satellite remains in a higher orbit of about 510 km altitude. To geolocate the Swarm observations, each of these satellites is equipped with a 8-channel, dual-frequency GPS receiver for precise orbit determination. On-board laser retro-reflectors provide the opportunity to validate the orbits computed from the GPS observations. The computation of precise orbits for the Swarm satellites is part of the activities performed in the Swarm Satellite Constellation Application and Research Facility (SCARF), supported by ESA. This consortium of several research institutes is responsible for providing the Swarm Level 2 products, including precise science orbits. A brief overview is given of the adopted orbit determination strategy, together with a preliminary assessment of the Swarm GPS receiver performance. Precise orbit determination results for the first months of data are presented, including a validation based on satellite laser ranging measurements.