Orbit and Gravity Field Solutions from Swarm GPS Observations - First Result

Abstract:

Although ESA’s Earth Explorer Mission Swarm is primarily dedicated to measure the Earth’s magnetic field, it may also serve as a gravity field mission. Equipped with GPS receivers, accelerometers, star-tracker assemblies and laser retro-reflectors, the three Swarm satellites are potentially capable to be used as a high-low satellite-to-satellite tracking (hl-SST) observing system, following the missions CHAMP (first single-satellite hl-SST mission), GRACE (twin-satellite mission with additional ultra-precise low-low SST and GOCE (single-satellite mission additionally equipped with a gradiometer). GRACE, dedicated to measure the time-variability of the gravity field, is the only mission still in orbit, but its lifetime will likely end before launch of its follow-on mission GRACE-FO in August 2017 primarily due to aging of the onboard batteries after meanwhile more than 12 years of operation. Swarm is probably a good candidate to provide time-variable gravity field solutions and to close a potential gap between GRACE and GRACE-FO. Consisting of three satellites, Swarm also offers to use inter-satellite GPS-derived baselines as additional observations. However, as of today it is not clear if such information will substantially improve the gravity field solutions. Nevertheless, the properties of the Swarm constellation with two lower satellites flying in a pendulum-like orbit and a slightly differently inclined third satellite at higher altitude still represent a unique observing system raising expectations at least compared to CHAMP derived time-variable gravity field solutions. Whatever processing method will be applied for Swarm gravity field recovery, its success strongly depends on the quality of the Swarm Level 1b data as well as the quality of the derived Swarm orbits. With first Level 1b data sets available since mid of May 2014 (excluding accelerometer data), first results for Swarm orbits and baselines, as well as Swarm gravity field solutions are presented here. The latter are also compared to GRACE GPS hl-SST solutions based on the same amount of data and processing methods.