Kinematic orbit positioning applying the raw observation approach to observe time variable gravity

Observing temporal changes of gravity has become a vital source of information about changes in the system Earth. Currently these variations are observed by the satellite mission Gravity Recovery and Climate Experiment (GRACE). Besides this mission no other technique is capable of providing the same resolution, both in space and time. Although a follow-on mission is under preparation, it is likely that the highly valuable observational record might be interrupted. Hence, there is a great interest in developing an additional opportunity to observe variations in the Earth’s gravity field. One possible method to observe the gravity field is based on precise positions of low Earth orbiting satellites. The position of the satellite can be observed by an on-board Global Navigation Satellite System (GNSS) receiver. It provides measurements which can be used to compute a kinematic orbit without introducing a priori information. Subsequently the positions can be used to estimate the Earth’s gravity field. The approach, denoted as Satellite-to-Satellite Tracking in high-low mode (SST-hl), is well known and widely used. However, the accuracy of the derived gravity field solutions depends on the quality of the introduced orbit positions. Current state-of-the-art orbits are only capable of resolving the largest gravity variations. Available orbit estimates are degraded by systematic effects or the measurement noise exceeds the amplitude of the sought for signals. The goal of this work was to develop a new method for kinematic orbit determination based on raw GNSS measurements. The essence of the proposed raw observation approach is to leave the GNSS measurements unchanged and process all observables jointly in an iterative least-squares adjustment. Systematic effects are either corrected or set up as additional parameters. The combination of different observation types necessitates a realistic weighting scheme in combination with a flexible outlier detection. The validation of computed kinematic orbits revealed that the raw observation approach is capable of producing orbit positions with the same or better accuracy than existing methods. Estimated satellite positions were then used to generate monthly gravity field solutions. Investigations based on a 13 year time series, including data from 15 satellites, showed that it is possible to observe gravity changes. An analysis of major river basins revealed that mass variations can be detected for areas larger than 500 000 km2 , if the amplitude of the signal is sufficiently large. In view of the continuously increasing number of satellites equipped with GNSS receivers and the ongoing evolution of GNSS in general, the results obtained in this work suggest that SST-hl could be a true alternative or at least a supplement to other technologies.