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GIOVE-A (2012)

First GPS position fix at 23,300km altitude, using GPS signals outside a GPS constellation – GPS experiment on GIOVE-A (2012)

SSTL’s experimental GPS receiver on board GIOVE-A successfully achieved a GPS position fix at 23,300km altitude – the first position fix above the GPS constellation on a civilian satellite. 
GPS is routinely used on Low Earth Orbit (LEO) 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.
With the support of the European Space Agency (ESA) 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 data could help SSTL to develop a receiver to navigate spacecraft in Geostationary orbit (GEO) or even in deep space.

GIOVE-A credit ESA