Time-variable gravity field from satellite constellations using the energy integral

Abstract

The magnetic field mission Swarm, expected to be launched in 2012, comprises a constellation of three satellites. As all of them are equipped with GPS receivers and accelerometers, they can be used for gravity field recovery. We study the capability of a Swarm-like constellation for (time-variable) gravity field recovery and compare it with a gravity field tandem mission of GRACE-type. Due to the lower accuracy of the GPS measurements compared with GRACE low—low satellite-to-satellite tracking (SST), the accuracy of a Swarm derived gravity field cannot compete with the state-of-the-art GRACE models. However, unlike the GRACE mission, Swarm allows for the derivation of GPS-baselines between the satellites in directions other than purely along-track. This makes Swarm an interesting case for studying the error structure of gravity field models derived from various constellation geometries. Therefore, one focus of this study is the general analysis of different baseline constellations independent of the observation accuracy, that is, we do not restrict ourselves to just the actual Swarm case of pure GPS-baselines. To make the results comparable to GRACE, we explicitly study the error behaviour of the different Swarm baseline geometries assuming GRACE-type K-band ranging (KBR) links. This gives an indication of candidate constellations for future missions; at least in regards to the general error structure. To study the potential of Swarm for recovering time-variable components of the gravity field, we have set up a 2-yr simulation to recover annual and semi-annual components of continental hydrology. We show that Swarm has the potential to recover the annual signal up to spherical harmonic degree 6. This is of interest should there be a gap between the end of the GRACE mission and the launch of a follow-on mission. All our simulations make use of the energy integral method. This method is usually (implicitly) formulated for static potential fields. Therefore, it is necessary for our study to investigate the properties of this method when applied to the analysis of time-variable fields.