An experimental GPS receiver, built by Surrey Satellite Technology Ltd (SSTL), has successfully achieved a GPS position fix at 23,300 km altitude – the first position fix above the GPS constellation on a civilian satellite. The SGR-GEO receiver is collecting data that could help SSTL to develop a receiver to navigate spacecraft in geostationary orbit (GEO) or even in deep space.
GPS is routinely used on low earth orbit satellites to provide the orbital position and offer a source of time to the satellite. Spacecraft in orbits higher than the 20,000 km of the GPS constellation, however, can only receive a few of the signals that "spill over" from the far side of the earth, meaning that the signals are much weaker and a position fix cannot always be secured.
Achieving Position Fix from High Orbit
With the support of the European Space Agency and the ARTES 4 program, SSTL included the SGR-GEO receiver on the GIOVE-A satellite to prove that a receiver could achieve a position fix from a higher orbit. The SGR-GEO is adapted from SSTL's SGR range of receivers and incorporates a high-gain antenna and a precise oven-controlled clock. It will demonstrate special algorithms to allow reception of weak signals and an orbit estimator intended to allow a near continuous position fix throughout orbit.
"The results from the SGR-GEO receiver are really encouraging," says Martin Unwin, principal GNSS engineer at SSTL. "We're getting higher signal strengths than anticipated and also acquiring side lobes from the GPS transmit antennas, which improves the availability of the usable signals for navigation. With the success of the SGR-GEO receiver, GPS, in combination with Galileo and Glonass, could soon be helping navigate spacecraft much further away from earth."
The experimental GPS receiver onboard GIOVE-A has been inactive for 6 years while the satellite has been used for its primary purpose of transmitting prototype Galileo signals. GIOVE-A's retirement in June 2012 has allowed the commissioning of the experiment; it is now providing valuable data to SSTL and the European Space Agency in support of the future use of spaceborne GNSS receivers at GEO altitudes. Engineers at SSTL will continue operations, testing out, tuning, and improving the receiver software onboard GIOVE-A to achieve the best possible performance.