HR: 08:00h
AN: A51F-01 INVITED     [Abstracts]
TI: First LEO Satellite Constellation Based on GPS
AU: * Svehla, D
EM: svehla@bv.tum.de
AF: Institute of Astronomical and Physical Geodesy, Technical University of Munich, Arcisstrasse 21, Munich, 80333 Germany
AU: Rothacher, M
EM: rothacher@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg A 17, Potsdam, 14473 Germany
AB: In the beginning, an overview is given covering precise orbit determination (POD) of Low Earth Orbiting (LEO) satellites using GPS. Kinematic and reduced-dynamic POD approaches and results are presented for a single LEO satellite like CHAMP and for the first LEO formation flying in the case of the two GRACE satellites. It is shown that, when forming a GPS baseline in space between the two GRACE satellites, almost all ambiguities can be resolved to their integer values, leading to highly accurate orbit differences between the GRACE satellites on the two-millimeter level. The Taiwan-US mission FORMOSAT3/COSMIC will be the first mission at LEO altitudes with 6 satellites flying in a constellation making use of onboard GPS receivers. By forming GPS baselines between these 6 LEO satellites and by fixing ambiguities, a unique GPS network in space can be formed. Based on such a LEO network, orbits of the GPS satellites can be estimated simultaneously with the orbits of the LEO satellites without using measurements from the ground GPS network. However, as our simulation study shows, the orbit of one reference LEO satellite has to be kept fixed and assumed to be estimated, e.g., using SLR measurement. By adding GPS data from the ground IGS network, the strength of the whole system is considerably increased and the impact of a LEO constellation on the determination of global IGS parameters like Earth rotation, station coordinates, troposphere zenith delays and the orbits of the GPS satellites can be demonstrated. The simulation shows that FORMOSAT3/COSMIC satellites may have great impact on the IGS processing. Because SLR measurements are standard to externally validate satellite orbits and can easily indicate systematic errors or problems with attitude or GPS antenna characteristics, such satellites should be equipped with SLR reflectors. In addition, results obtained applying higher-order ionosphere corrections may be tested using LEO POD and may externally be validated by means of SLR measurements. The quality of radio occultation results depends critically on the quality of the orbits and fast near-real time POD. A reduced-dynamic POD approach based on Fourier series is presented. In contrast to the present POD approach based on the estimation of small velocity changes, it provides orbits with continuous first and second time derivatives (velocity and acceleration). This POD approach can be applied for both, near real-time and post-processed orbit determination and has the additional advantage that, when introducing a high-quality gravity field model (e.g. from GRACE), the non-conservative accelerations (air-drag, solar radiation) can be used to determine, e.g., air-densities or residual temporal gravity field variations. The FORMOSAT3/COSMIC mission poses a special challenge for POD due to the two sideways-pointing POD antennas. The GPS measurements from both GPS antennas have to be combined and processed together, as they are affected by different system delays. This problem may be critical when forming baselines between the LEO satellites and may degrade the accuracy of orbit determination. In this case the data pre-processing is also more difficult than for a zenith pointing GPS antenna. Using simulated GPS data, this problem is addressed and the impact on POD performance is presented.
DE: 1214 Geopotential theory and determination (0903)
DE: 1220 Atmosphere monitoring with geodetic techniques (6952)
DE: 1221 Lunar and planetary geodesy and gravity (5417, 5450, 5714, 5744, 6019, 6250)
DE: 1229 Reference systems
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005