Abstract
Satellite remote sensing systems require accurate, autonomous and real-time orbit determinations (RTOD) for geo-referencing. Onboard Global Positioning System (GPS) has widely been used to undertake such tasks. In this paper, a novel RTOD method achieving decimeter precision using GPS carrier phases, required by China’s HY2A and ZY3 missions, is presented. A key to the algorithm success is the introduction of a new parameter, termed pseudo-ambiguity. This parameter combines the phase ambiguity, the orbit, and clock offset errors of the GPS broadcast ephemeris together to absorb a large part of the combined error. Based on the analysis of the characteristics of the orbit and clock offset errors, the pseudo-ambiguity can be modeled as a random walk, and estimated in an extended Kalman filter. Experiments of processing real data from HY2A and ZY3, simulating onboard operational scenarios of these two missions, are performed using the developed software SATODS. Results have demonstrated that the position and velocity accuracy (3D RMS) of 0.2–0.4 m and 0.2–0.4 mm/s, respectively, are achieved using dual-frequency carrier phases for HY2A, and slightly worse results for ZY3. These results show it is feasible to obtain orbit accuracy at decimeter level of 3–5 dm for position and 0.3–0.5 mm/s for velocity with this RTOD method.
Highlights
The dynamic changes of Earth environments, and natural disasters, such as earthquakes and forest fires, have been monitored in real-time with spaceborne Earth observation systems (EOS) in the LEO region
Accurate, autonomous and real-time orbit determination (RTOD) using global, abundant, and low-cost Global Positioning System (GPS) measurements onboard LEO satellites has been widely applied to many LEO missions [1,2,3,4,5,6,7,8,9,10,11,12]
“Phoenix-XNS” was equipped on X-SAT, a mini-satellite developed by the Satellite Engineering Centre of the Nanyang Technological University at Singapore in 2004 [8], as well as PROBA-2, another mini-satellite developed by the European Space Agency (ESA) in 2010 [9]
Summary
The dynamic changes of Earth environments, and natural disasters, such as earthquakes and forest fires, have been monitored in real-time with spaceborne Earth observation systems (EOS) in the LEO region. GSFC, JPL, and DLR’s algorithm and software seem to use only pseudo-range data They employ the extend Kalman filter (EKF) to estimate the LEO satellite’s orbit parameters with simplified dynamic models, and the errors of the determined position and velocity are usually at 1.0 m and 1.0 mm/s level, respectively, due to the use of the GPS broadcast ephemeris and large pseudo-range noises [13,14,15]. For the HY2A and ZY3 satellites, high-precision real-time navigation is essential for their scientific applications, and the real-time orbits at decimeter level or better are required These two missions are equipped with dual-frequency GPS receivers developed by the China Academy of Space Technology (CAST).
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