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CHAMP and GRACE in Tandem: POD with GPS and K-Band Measurements

The BlackJack GPS receiver on-board the CHAMP satellite allowed for the first time to compute highly accurate LEO kinematic orbits and the development of new methods for gravity field determination based on kinematic POD. Several groups estimated Earth gravity field coefficients and studied their temporal variations using these CHAMP kinematic positions and making use of the energy balance approach or boundary value method rather than classical numerical integration. The validation of gravity field models computed in such a way showed that CHAMP kinematic positions contain high-resolution gravity information and that the accuracy of the derived gravity models is comparable to that of official GRACE models. Here we present POD for CHAMP and GRACE for a period of two years and four months, respectively. The main focus will be on the double-difference GRACE baseline in space and the study of zero- and double-difference, kinematic and reduced-dynamic POD approaches for the GRACE formation flying. We compare ambiguity resolution methods like Melbourne-Wübbena wide-laning with narrow-lane bootstrapping and the quasi-ionospherefree (QIF) approach for the GRACE baseline and demonstrate that 95%-100% of the ambiguities of can be resolved. The ambiguity-free and ambiguity-fixed results for kinematic and reduced-dynamic POD are compared with the K-band measurements (KBR) . The best POD approaches show an agreement on the level of 3 mm with KBR, which is an improvement of a factor of about 7 compared to the zero-difference and ambiguity-free cases. Finally, a combination of the ambiguity-resolved GPS phase measurements with the KBR phase measurements is performed and the role of a constellation of LEO satellites, forming a LEO network in space, is studied from the point of view of future gravity missions.

GPS; LEO; precise kinematic orbite determination; gravity field

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