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The Multi-talented GPS Receiver

As well as providing end-to-end missions for our customers, Surrey subsystems can also be found in a broad range of international third-party missions.
Some of our most popular, and flexible, subsystems are our SGR range of GPS receivers. GPS receivers have been used on satellites since the late 1990s to provide accurate position, velocity, and time—in much the same way that a TomTom or Garmin is used in an automobile.

Our SGR-10 model, for example, supplies a position that is accurate to 10 meters (as good as 1 meter after processing) and a velocity to 15 cm/s. A series of accurate position, velocity, and time measurements is exactly what is needed to determine a satellite’s orbit, allowing prediction of its motion into the future. The timing from a GPS receiver is accurate to better than 1 microsecond, enabling synchronization of clocks on the satellite with the ground. Additionally, Surrey’s receivers are capable of more than you might think. Their flexibility allows standard models to be adapted to suit many varied requirements and they’ve been used for a broad spectrum of different applications.

receiving GPS signals outside of GPS orbitHigh-Altitude GPS
Last November, the SGR-GEO—an experimental receiver built by Surrey—achieved the first public domain GPS position fix above the GPS constellation. To do this, Surrey GNSS experts adapted an existing SGR receiver model for high orbit with a high-gain antenna to receive weak signals, and orbit estimation algorithms that offer a near continuous position fix. The SGR-GEO receiver is currently collecting in-orbit data to help Surrey develop a receiver to extend the GPS reach above low Earth orbit satellites and provide navigation and timing to spacecraft in geostationary orbit or even deep space. Deep-space timekeeping, of course, is an area of great interest as NASA aims to improve spacecraft navigation and timing with the Deep Space Atomic Clock (DSAC) demonstration mission in 2015.

Remote Sensing
SGR-ReSIWe can also use GPS receivers for something far closer to home: remote sensing. Because GPS transmits signals globally and continually over both land and sea, we can capture the signals reflected off the ocean and use them to measure ocean roughness, from which wind speed can be determined. Surrey’s UK-DMC-1 satellite, launched in 2003, carried a pioneering experiment to measure reflected GPS signals with illuminating results. Now, NASA’s Cyclone Global Navigation Satellite System (CYGNSS) mission aims to use reflected GPS signals to improve extreme weather prediction. Expected to launch in 2016, it will fly a delay Doppler mapping instrument that will utilize a Surrey receiver model called the SGR-ReSI to detect the GNSS signals reflected from the ocean surface.

Time-Stamping Payload Data
GPS receivers provide a reliable time reference and time-stamping for remote sensing data captured in orbit. The need for synchronization is immediately apparent when satellites operate together in constellation, for example in the DMC constellation built by Surrey. Satellites in low Earth orbit travel at 7.5 kilometers per second. An error of 1/10 second in the timing of an image results in an error of 750 meters on the ground. Reliable time referencing from satellite GPS receivers ensures that scheduling of image acquisition is highly accurate.

The tiny SGR-05U

Tiny Satellites

Cubesats as small as 5 kg have made use of Surrey’s miniaturized GPS receiver, the SGR-05U. Although it lacks some of the features of its bigger siblings, it has just what is needed to find the satellite’s position, velocity, and time—taking less than a watt and weighing less than 50 g.

Attitude Determination
By using multiple antennas, GPS receivers can measure the “carrier phase difference” of GPS signals received by the different antennas and use this to determine the attitude of the satellite (the orientation of the satellite in relation to Earth). Surrey’s experiment on the Topsat-1 orbiting satellite is one of the few that showed GPS attitude determination in orbit, and Surrey’s SGR-20 and its next-generation receivers support up to 4 separate GPS antennas to allow this capability.

Surrey and its subsystem customers continue to exploit the flexible SGR receiver in new and novel applications. The next-generation of GPS receivers is heralded by the SGR-ReSI which is capable of using dual frequency GPS signals (L1 and L2C) for remote sensing. The SGR-Axio is on the horizon; this will be compatible with multiple GNSS signals, including GPS, as well as Galileo, Glonass, and Beidou. These new capabilities will lead to even more applications in remote sensing, attitude, timing, and navigation.


25 January 20130 Comments1 Comment

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