One of many important applications of Global Navigation Satellite Systems (GNSS) is the precise orbit determination (POD) of Low Earth Orbiting (LEO) satellites, i.e., satellites flying at low altitudes of roughly 200-2000 km. Many of these satellites carry on-board GNSS (up to now mainly GPS) receivers which allow the continuous tracking of GPS satellites and the absolute positioning of the GPS antenna with cm accuracy. Such accuracies are mandatory, e.g., for altimetry missions, where radial orbit errors directly degrade the height measurements.
We distinguish between three different types of orbits:
Kinematic LEO orbit positions can serve as pseudo-observations for a subsequent gravity field determination. At the AIUB this is realized in the framework of the Celestial Mechanics Approach (CMA) as a common orbit and gravity field estimation. Not only the parameters of a (reduced-)dynamic orbit, but also the gravity field part of the underlying force models (e.g., the coefficients of a spherical-harmonic expansion of the gravity field) are estimated simultaneously. There are a number of dedicated gravity missions the data of which were and are successfully processed at the AIUB to produce high-quality gravity fields:
The satellite-based determination of the global gravity field of the Earth (or other celestial bodies) has become an invaluable tool for geophysical research and the monitoring of the complex system Earth.