Since its launch in 2002, the GRACE mission has provided time-variable gravity field solutions for more than 8 years. The monthly GRACE solutions clearly show the continental scale hydrological cycle, while the long-term time series reveal trends in deglaciation in Antarctica and Greenland, post glacial rebound in Canada as well as the mass signal of sea level rise. GRACE solutions are widely used in hydrology, oceanography, ice, atmosphere, solid earth and other related Earth science stu-dies.
In the preprocessing of GRACE gravity field solutions, high-frequency mass variations within the at-mosphere and ocean are removed from the measurements beforehand using geophysical models. This is done because they cannot be measured adequately from space due to undersampling. This process is called de-aliasing and is of great importance for future gravity missions.
Future gravity missions designed to determine the time-variable gravity field will use specific satellite constellations to extract signatures of global mass transport. In the present IGSSE project a closed-loop simulation is set up to study various constellations. Starting point for the simulation is the magnet-ic field mission Swarm, an ESA mission to be launched in 2012, consisting of three satellites. Since each of them is equipped with both GPS receivers and accelerometers, the constellation can be used for gravity field recovery as well. In this respect Swarm is comparable to three CHAMP satellites. In addition to absolute GPS positioning, one could also use relative measurements to increase the ob-servation accuracy.
A 24-month-simulation is carried out to recover the time-variable hydrology signal. By this simulation Swarm‘s potential for time-variable gravity field recovery is evaluated. It is shown that Swarm has the potential to recover the time variable hydrology signal to about d/o 6 of a spherical harmonic represen-tation of the Earth‘s gravity field, based on 2 years of data. In addition, the mission is of interest from a theoretical point of view, because it combines different types of inter-satellite baselines, i.e., cross-track (Swarm A-B) and radial/along-track (Swarm A-C). Therefore the Swarm constellation can be used as an example to evaluate characteristics of different baselines and combinations. In order to quantify the potential of these constellations for future mission scenarios, GRACE-type K-band inter-satellite links are tested as well. The tandem/pendulum constellation is shown to outperform GRACE. It gives smaller errors and isotropic error patterns.
Considering that GRACE may terminate in the time frame 2013 to 2015, Swarm may continue the time-series of gravity measurements until GRACE follow-on mission is in place. Given the fact that CHAMP has already terminated, Swarm could indeed be the only low Earth orbit mission contributing to temporal variations of the low degree spherical harmonics in the near future. Moreover, even a combination of Swarm and GRACE (if it still operates after the launch of Swarm) may help to reduce temporal/spatial aliasing in the low degree harmonics by providing a better sampling in space and time. Therefore, Swarm can be regarded as a welcome complementary mission to the dedicated gravity missions and will provide valuable information on both static and time-variable gravity fields in the near future.
Furthermore satellite observations can be combined with terrestrial data. This is important to expand the resolution of gravity field models, because gravity field models based purely on satellite data are restricted with regard to resolution. To combination of different data sources yields the best result in respect of accuracy as well as spectral and spatial resolution.
Literature: Olsen, N., Sabaka T., Gaya-Pique L., (2007) Study of an improved comprehensive magnetic field inversion analysis for Swarm (Fianal Report), pp. 14-23