Use of GNSS-Based Pseudo-Acceleration Measurements and Orbital Constraints to Initialize Spaceborne Receiver Orbit Solutions

Abstract

INTRODUCTIONThe problem of the time to first fix (TTFF) of a spaceborne global navigation satellite system (GNSS) receiver is crucial. The high Doppler range of data collected by a GNSS receiver on a low earth orbiter (LEO) makes it difficult to achieve the conditions classically required to achieve a fix (i.e., the presence of four simultaneous emitters), in particular when the number of receiver channels is small; when the configuration of the antenna is not optimal; or when the sensitivity of the receiver is poor, which is the case during cold starts. On a geostationary earth orbit (GEO) satellite, the problem of the TTFF is even more difficult, since the conditions of four simultaneous emitters are rarely met. This paper begins by presenting three detailed algorithms based on the three main configurations that have been studied: LEO orbit with two emitters, LEO orbit with three emitters, and GEO orbit with one emitter. Then, results of tests using actual TOPEX data are presented; these results are in the 100 km error range when two emitters are present and in the 10 km error range when three emitters are present. The GEO orbit algorithm has been tested with simulated data, collected during the qualification campaign of the Alcatel TOPSTAR 3000 GNSS receiver. The results are in the 100 km error range. In all of these configurations, the resultant position is precise enough to improve the TTFF, to reduce the acquisition thresholds of the receiver, or to initialize an orbital navigator. The enhancement of the TTFF is then reviewed, based on a characterization of the algorithms of the TOPSTAR 3000 receiver, on which theses techniques will be implemented in the near future. THE CLASSICAL POSITION FIXThe classical position fix is well known [1,2]: It requires the use of four or more simultaneous pseudorange measurements. The four unknown quantities are the three coordinates of the position of the receiver; b, the clock bias between GPS time and the time of the receiver.