Hot Start of GPS Receivers for LEO Microsatellites

Abstract

While spaceborne GPS receivers can in general be considered as a well established tracking tool for LEO (low Earth orbit) satellites, their use on micro- or nano-satellites may pose various problems from a systems engineering point of view. Representative examples include the mass budget, the lack of a suitable attitude stabilization system, antenna allocation problems, restricted command and telemetry links as well as limited onboard power resources. As a result of the latter constraints a discontinuous operation of the receiver is frequently required, which necessitates appropriate means for a hot (or at least warm) start of the receiver. To cope with these limitations, a signal acquisition aiding concept based on an analytical orbit model, which regularly calculates the approximate position and velocity of the receiver has been implemented inside the Orion GPS receiver. The receiver is based on the Mitel design information and the GP2000 chipsets. Data from the orbit model are used to compute the list of visible satellites and the expected Doppler shift for allocation and steering of the correlator signal tracking channels. In case of a temporary loss of track, the receiver is thus able to rapidly reacquire GPS signal and provide a navigation solution. Signal simulator tests for a LEO scenario demonstrated the ‘hot start ’ capability of the receiver with a time to first fix (TTFF) of better than 20 seconds. Preliminarily results from the flight experiment onboard the microsatellite PCsat (Prototype Communication Satellite) also demonstrated rapid signal acquisition performance and ‘warm start ’ capability of the receiver operating in LEO.