B03 - Kinematic GNSS positioning of low Earth orbiters

However, the current accuracy and reliability of GNSS-determined LEO orbits is restricted by weak geometry due to the epoch-wise estimation of receiver clocks, the large number of ambiguities to be set-up, and due to remaining systematic like antenna phase center variations (PCV) or near-field multipath. Thus, new concepts and perspectives for precise and reliable LEO positioning should be developed. To this end we can benefit from new technologies currently available, like e.g., ultra-stable clocks, inter-satellite laser links and future receiver types tracking multi-frequency multi-GNSS.

The project will focus on two approaches to improve the orbit quality for future missions: (i) Taking advantage of new ultra-stable space-qualified clocks, concepts of clock modeling will be investigated, i.e. the receiver clock error is modeled by a low order polynomial rather than estimated every epoch. This will yield a strengthening of the geometry and an improvement in both accuracy and integrity. In addition, multipath and PCV can be better detected and separated from other error sources. (ii) The second approach is the development of the so-called virtual receiver, i.e. the observations from multiple antenna/receivers will be combined into one spacecraft solution. The observations can either be collected by multiple optimally oriented antennas at one spacecraft or different antennas separated over multiple spacecrafts if the inter-satellite link is given. In such a way, more GNSS observations can be tracked, the common field of view is significantly enlarged, yielding a better integrity and better ambiguity resolution thanks to longer visible GNSS satellite arcs. The concepts will be developed in close interaction with the projects B02 – B06 and C01. Simulation studies as well as real GNSS data analysis of current and upcoming gravity field missions will be used to demonstrate and validate the concepts.