engleski

Kinematic and Dynamic Precise Orbit Determination of Low Earth Orbiters: Importance of the GPS Receiver Performance

Thanks to the high performance of the BlackJack GPS receiver flying on CHAMP, kinematic precise orbit determination (POD) has turned out to be a new method in orbit determination for Low Earth Orbiters (LEO). Kinematic POD is based on GPS measurement and does not make use of information on force models. Therefore it is independent of orbit design (e.g. satellite altitude). From that point of view, kinematic orbits are very well suited for the Earth observation satellites at very low altitudes, where air-drag and gravity become more difficult to model. With highly accurate CHAMP kinematic positions, interesting new methods are being developed nowadays to e.g. determine gravity field parameters, validate dynamical models, or derive atmosphere density information. In this paper we would like to show the latest results in orbit determination for today's geodetic missions like CHAMP, SAC-C, and JASON-1 and point out the importance of GPS receiver performance for kinematic and reduced-dynamic POD as well as for gravity field recovery from SST data. Receiver performance is especially critical for a satellite mission as complex as GOCE, flying at very low altitude, where accurate orbit determination is much more challenging. Since the accuracy of kinematic POD mainly depends on the quality of phase GPS measurements, this kinematic method in combination with a reduced-dynamic approach is very well-suited to validate the performance of the spaceborne GPS receiver. We would like to show the large differences in quality of the estimated kinematic orbits among today's geodetic missions. The quality differs considerably because of L2-ramps in phase measurements, large data gaps, phase breaks, multipath, number of GPS satellites tracked, GPS antenna design and attitude characteristics. As an example, JASON-1 kinematic orbits are typically less accurate than those of CHAMP. To a great extent, this also stands for the reduced-dynamic (GPS-derived) orbits of these two satellites, although the forces acting on a satellite are much easier to model for the higher altitude of JASON-1. This apparent paradox in orbit determination is due to the performances of the GPS receiver. Problems similar to those of JASON-1 GPS receiver can also be identified in the early GPS data of the CHAMP mission. Under these circumstances, kinematic POD is very inaccurate as seen from SLR validation and the GPS data are not really suited for gravity field determination. The last two years of CHAMP GPS data allows for kinematic and reduced-dynamic orbit determination on the level of 1-3 cm.

New method of orbit determination of Low Earth Orbiters

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