Performance of various satellite gravimetry concepts for monitoring mass transport in the Earth's system

R. Klees, J. de Teixeira da Encarnacao, P. Ditmar, and B.Gunter
Delft University of Technology, Faculty of Aerospace Engineering, Delft Institute of Earth Observation and Space Systems (DEOS)

The Gravity Recovery and Climate Experiment (GRACE) satellite mission, which has been acquiring data since 2002, demonstrated that satellite gravimetry is a powerful tool to monitor mass transport in the Earth's system. Models based on GRACE data are extensively used in various Earth sciences, including hydrology, climatology, oceanography, physics of the solid Earth, and others. However, the GRACE mission is characterized by some intrinsic limitations. First of all, errors in the models based on GRACE data show a strongly anisotropic pattern. This can be explained by the fact that the main observation technique of the GRACE mission - K-band ranging - delivers data, which can be interpreted as measurements of gravitation difference between the locations of the two satellites forming the GRACE mission. Since the satellites follow each other in a nearly polar orbit, they are located most of the time at nearly the same meridian. Consequently, the collected observations describe North-South variations of the gravitational field (and mass transport) much better than East-West variations. Secondly, the GRACE satellites re-visit each geographical location, roughly speaking, only once per month. This does not allow rapid mass transport processes to be monitored and, even worse, results in temporal aliasing, i.e., an unpredictable and destructive propagation of high-periodic signals into the monthly gravity field models.

A number of alternative satellite gravimetry concepts can be designed to overcome the limitations of the GRACE mission. Some of them have been introduced already a number of years ago. For instance, the so-called "pendulum" configuration allows information about East-West spatial variations to be collected. Alternatively, the "cartwheel" configuration implies that both along-track and vertical gravitation differences are measured.

We propose two novel satellite gravimetry concepts. Firstly, we consider a pair of satellites separated by a vertical offset and connected by a tether. Secondly, we discuss a constellation of multiple cheap satellites equipped with high-quality GPS receivers, so that a spatially isotropic "snapshot" of the Earth's gravitation and mass distribution can be generated as frequently as once in a few days or even hours.

The performance of various satellite gravimetry concepts and their synergetic effects are analyzed by mean of numerical simulations. In this way, we are able to draw conclusions regarding strong and weak points of each of these concepts.