S. P. Yoon

An integer ambiguity resolution algorithm for real-time GPS attitude determination

The problem of integer ambiguity resolution must be solved to fully exploit the information contained in the GPS carrier phase measurements for real-time attitude determination. A new algorithm for integer ambiguity resolution that uses single-difference smoothed pseudorange measurements is proposed that has several advantages over the conventional algorithms such as the search methods and the motion-based algorithms. These advantages include minimal computational effort at each epoch and an analytically derived equation which calculates the expected time to ambiguity resolution. The validity and the performance of the new algorithm are investigated through simulations although processing real data will be necessary to verify the usability of the proposed algorithm in real situations.

Radiation Pressure Modeling for ICESat Precision Orbit Determination

A macro-model was developed for modeling radiation forces in ICESat POD. It consists of a six-sided box and two flat plates, representing the body and the solar panels. The optical properties assigned to each of these surfaces should yield radiation-induced forces that match those experienced in orbit. Prior to the launch, these forces were simulated using a micro-model developed by Ball Aerospace. After generating force histories with the micromodel over the full range of orbit and satellite orientations, a least-squares fit was performed to determine the macro-model optical properties. In this study, the performance of the macro-model was evaluated.

Phase Center Variation Modeling of ICESat GPS Antenna for Precision Orbit Determination

For precision orbit determination (POD), the precise location of the phase center of the on-board GPS antenna must be modeled. The phase center of a GPS antenna varies as a function of the azimuth and elevation angle of incoming signal. There are two methods to model the phase center variation. One is to measure the phase center variation with ground test in anechoic chamber. The other is to calibrate the phase center location based on onorbit measured GPS data. There are again two method to calibrate the phase center location using the on-orbit data. One is called direct method and the other method is called residual method. In the research described in this paper, the residual method was applied to ICESat precision orbit determination. Since ICESat POD at Center for Space Research uses doubledifferencing approach, the resulting residuals were converted into zero-differenced residuals. The quality of the resulting orbit solution was assessed by measurement residual RMS, SLR residual RMS and orbit overlap analysis. Orbit solution with phase center variation model developed in this research showed noticeable improvement.

MULTIPATH DETECTION AND MITIGATION FOR CHAMP PRECISION ORBIT DETERMINATION

As the orbit accuracy from Precision Orbit Determination (POD) using GPS carrier phase measurements approaches to a few centimeter level, the multipath noise could become one of the primary error sources. Typical magnitude of multipath noise in carrier phase is on the order of centimeters1. However, by employing simple solutions such as using chokering antenna, or installing the antenna at the end of an extended boom, the level of multipath noise reduces to a few millimeters.2 For example, the GPS antenna for TOPEX/Poseidon is located on top of a boom, which is longer than a few meters, to avoid signals reflected from the spacecraft main body and the solar panel. The GPS antenna for CHAMP has a choke-ring as shown later.

Effect of center of mass location error on ICESat precision orbit determination

he Ice, Cloud and land Elevation Satellite (ICESat) was launched in January 2003 carrying a laser altimeter designed to detect changes in the polar ice sheets. Like radar altimeters, POD is an essential component to create surface elevations from altimeter measurements. POD team at Center for Space Research (CSR) has generated POD solutions over the periods when the Geoscience Laser Altimeter System (GLAS) was operated. Table 1 show the eighteen GLAS campaigns (L1a and L1b are one campaign with different attitude mode) that were completed by the October 11 th , 2009, totaling about 600 days. ICESat POD solutions are generated by ingesting GPS data into the CSR’s POD system, Multi-Satellite Orbit Determination Program (MSODP) using 30 hour estimation arcs from which the central 24 hours are reported as the POD ephemeris product. This ephemeris product has been used to generate SLR residuals to estimate the accuracy of the ephemeris product. The statistics of the SLR residuals and a simulation study show that the POD product has a radial orbit accuracy of less than 1.5 cm 1 , which exceeds the POD requirement of 5 cm in radial component. Ball Aerospace and Technologies Corporation (BATC) measured the position of COM of the combined ICESat bus and GLAS instrument in the Satellite Coordinate System (SCS) prior to launch with an accuracy of ±5 mm in all three axis in SCS 2 , where +xsc is in the zenith direction, +ysc axis is along the solar panel axis, and +zsc axis completes the right hand system (see Fig. 1). After launch, the COM moves slowly, mainly along the +zsc axis, as the orbit maintenance maneuvers use propellant. The Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder (LASP) updates the COM position in the SCS using models developed by BATC following each maneuver. Since the POD solutions are generated by using GPS double-differenced carrier phase measurements, MSODP models the position of the GPS receiver phase center with respect to COM using the updated COM position. To reduce the impact of the COM position error and the GPS receiver phase center location error on the radial component of ICESat orbit, a COM offset parameter in +xsc axis (radial component) is estimated for the POD solutions (see Fig. 2). In this paper, COM offset correction parameter in the cross-track component (along the +ysc axis for “airplanemode’, and along the +zsc axis for “sailboat-mode”) is introduced, and the impact of this new parameter on the POD solutions is investigated. COM offset correction parameter in the along-track component cannot be estimated due to