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Navigation - Small Satellites carrying GPS receivers
A space borne GPS receiver is different from a terrestrial receiver, as it travels at great velocity relative to the GPS constellation, and satellites tracked change constantly. As a consequence it has to cope with higher Doppler shifts and Doppler shift rates. In LEO this can be of the order of ±60kHz. Consequently, the receiver only has a short time available to perform a frequency/code search in order to lock onto the spread spectrum GPS signals. In order to predict GPS satellite visibility, the antenna geometry and orbital dynamics of the host satellite must be well known. For more precise applications, differential and double-differential filtering of GPS is proposed for formations of satellites. For greater precision, kinematic GPS employs the basic understanding of the dynamics to further improve accuracy. Orbit determination using GPS receivers on small satellites is now well demonstrated, and the next challenge now is to perform attitude determination on a small platform. Several small missions have recently been launched in order to demonstrate this. GPS can also be used in ionospheric research, and this is achieved by monitoring GPS signals as the satellite-to-satellite path skims the upper atmosphere. GPS can only be effectively used near the Earth's surface, but various experiments have demonstrated that sidelobe signals can be picked up above the GPS constellation at 20,000; e.g. from Geostationary heights.
The Trimble series 6 channel receivers are equipped with LEO firmware, to allow them to operate in the highly dynamic environment of space, with high velocity and high velocity derivatives. The Trimble Tans II employs a 68000 processor. The Quadrex employs antenna switching to commutate between four antennas. The Vector is a Quadrex which includes a coprocessor to permit basic attitude determination. Further information on space-borne GPS can be found at the Department of Geodesy and Geomatics Engineering, University of New Brunswick.
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Global Positioning System receivers provide highly accurate time and
position information, and use a constellation of 24 navigation satellites
maintained by the US DOD in order to accurately determine their own position.
Although designed for Terrestrial geolocation and navigation, the system can be employed in the highly
dynamic environment in Low Earth Orbit. Landsat was the first civil spacecraft to carry a
GPS receiver into orbit, but the receiver technology has advanced to a stage where GPS
receivers can now routinely be carried on smaller platforms. Since Selective
Availability has been switched off, accuracies in the order of 10m in all axis
are achievable for simple position fixes. 