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Space Blog

Retired GIOVE-A could help GPS reach even higher heights

In June 2012, the SSTL-built GIOVE-A satellite was retired after an impressive 6 years of service broadcasting prototype Galileo signals.
But that wasn’t the end for the plucky satellite. Not content with paving the way for European GNSS, engineers at SSTL- with the support of the European Space Agency- included an experimental receiver called SGR-GEO on GIOVE-A to look into extending the reach of the existing US GNSS system- GPS- in space.

Although GPS is commonplace on satellites in Low Earth Orbit (LEO) to determine orbit position, velocity and time, there are technical challenges preventing the service reaching satellites further away from Earth. The problem is, spacecraft higher than LEO are often also orbiting higher than the GPS constellation. This means that they can only receive a few signals that “spill over” from the far side of the Earth, so the GPS signals are weak and a position fix can’t always be attained.

The SGR-GEO was specially developed to be able to receive these weak signals. It has a high-gain antenna and a precise oven-controlled clock that helped it to achieve a position fix at 23,300km altitude; the first time a civilian satellite has achieved a position fix outside the GPS orbit. The receiver will also demonstrate longer integration tracking loop algorithms to allow reception of weak signals, and an orbit estimator intended to allow a near continuous position output throughout the orbit.
Apart from the spill-over signals, the SGR-GEO receiver on GIOVE-A is also acquiring side lobes from the GPS transmit antennas at higher signal strengths than expected. These signals of opportunity help to improve the availability of the useable measurements for navigation.

A first look at some of the results is given in the figure above. This shows a view of GPS signals tracked by the receiver over several days using standard tracking loops (software controlled delay-lock loops). The majority of the signals are in the first ring surrounding the Earth – these are signals that have travelled past the Earth from the main lobe of the transmit antenna. Outside that ring, there are some more signals – these are emitted by the first side-lobe of the GPS transmit antennas. Beyond that, there are five signals that have been acquired from the second sidelobe, and are spread out as far as 45 degrees away from the Earth.

When using the weak signal tracking loops, it is expected that even more signals will be acquired from first and second sidelobes, increasing the availability of GPS signals and improving the positioning geometry to give a better position accuracy.

The first position fix above GPS orbit is potentially an exciting problem-solver. It is allowing SSTL to collect data from the SGR-GEO receiver that could help GPS extend its reach further than LEO to Geostationary (GEO) and beyond - all through the use of more sensitive and smart receivers.





12 December 20120 Comments1 Comment

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