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Sat nav signals enlisted to measure ocean wind and waves

(Jan 2009) It’s rarely a problem to tell how rough the sea is when you’re afloat on it. But gauging conditions from a distance and across a wider area has always proved much harder.

Now scientists have pioneered a way to use signals from satellites in navigation systems like Global Positioning System (GPS) or Galileo to measure the intensity and direction of ocean wind and waves from space.

GPS signals are found constantly everywhere in the world, and if properly interpreted could dramatically improve our ability to monitor the oceans, providing a large amount of data on conditions at sea to marine scientists and meteorologists. This would help improve advance warning of storms and weather forecasts.

Specialised satellites can already provide data on wind speed and direction, but the global coverage is daily at best. Taking advantage of GPS signals could give scientists access to far more measurements closer to real-time.

The researchers hail from the National Oceanography Centre, Southampton (NOCS), private company Surrey Satellite Technology Ltd and the University of Sannio in Italy.

Surrey Satellite Technology developed a small, lightweight instrument that can be installed on a satellite in low Earth orbit to measure the signals bouncing off the planet from the network of GPS satellites orbiting far above. The researchers’ findings appear in Geophysical Research Letters.

‘This is a great achievement as it demonstrates the capability of this low-cost technology to provide ocean roughness data’, says Dr Christine Gommenginger, a specialist in exploiting satellite data for oceanography who supervised the project at NOCS, adding that this information is expected to complement rather than replace the data gained from dedicated Earth observation satellites.

The technique involves detecting signals from global navigation satellites after they are reflected from the ocean’s surface. The idea, known as Global Navigation Satellite System-Reflectometry, or GNSS-R, was first proposed in 1993 but its spaceborne implementation is only now becoming a reality.
Satellites of opportunity

‘The GNSS-R instrument doesn’t need to generate its own sounding signals; it can therefore be very small and has low power requirements, so it could piggy-back on any satellite,’ adds Gommenginger.

‘In the future we would like to be able to put this kind of Earth observation payload on commercial satellites, such as telecommunication satellites, taking advantage of these as platforms of opportunity in space in the same way as in oceanography we now gather data with instruments on ships of opportunity.’

One such opportunity could have been on the Iridium NEXT constellation of telecommunications satellites, which was seeking Earth Observation instruments to include in their payload, but the timescale proved too short to make this a reality.

Work is now underway to build the next generation of GNSS-R receivers with improved performance in a project funded by the Centre for Earth Observation Instrumentation led by SSTL. The team hope that such a GNSS-R receiver will be included in the payload of the follow-on to the European Space Agency’s Soil Moisture and Ocean Salinity (SMOS) mission – SMOSOps.

Researchers first proved the concept from space in 2005, but this early work gave information only on ocean roughness; the new work establishes for the first time that reflected navigation signals can also provide information about the direction of roughness.

Navigation satellites orbit around 20,000 kilometres above the ground. For this research their signals bounced off the sea surface and were collected by a receiver on Surrey Satellite Technology’s UK-DMC satellite, which orbits at just 680 kilometres. The UK-DMC satellite was part of the SSTL Disaster Monitoring Constellation, which main function is global imaging primarily for disaster monitoring purposes.

Just one second’s worth of GNSS-R data gives the scientists the information needed to build a picture of conditions at the sea surface. As well as directional wind and wave information, the reflected signals could also be mined for information about the mean sea level to perform ocean altimetry.

The researchers compared the satellite results with model predictions and against in situ wave data from the US National Data Buoy Center. Earlier research had collected reflected navigation satellite signals over the Mediterranean using a receiver on an aircraft, but the technique needed to be demonstrated from satellites which make it possible to achieve global coverage and does not depend on a time-limited aircraft mission to take measurements.