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Using Sat Nav signals to measure winds and waves

Data from an instrument on-board TechDemoSat-1 demos sea-state info
Here at SSTL we’re excited about early results from an instrument flying on-board TechDemoSat-1, launched earlier this year. It’s called the SGR-ReSI – full name the Space GNSS Receiver Remote Sensing Instrument – and it was developed by our GNSS experts, led by Dr Martin Unwin, Principal Engineer at SSTL.

The SGR-ReSI has been designed to collect the signals from GPS and other navigation satellites after they have been reflected off the ocean surface and process them on board the satellite into images called Delay Doppler Maps, from which ocean roughness and wind speed measurements at the sea surface can be interpreted.

The technique works in a similar way to existing scatterometric radar from satellites, however it eliminates the need for a transmitter and can process up to four reflections from different GPS satellites simultaneously, presenting an opportunity for collecting more data, more frequently, and over a greater area at a low-cost.

“This is a complementary technology to the well-proven space-based scatterometers.” said Dr Martin Unwin. “GNSS Reflectometry uses a longer wavelength and measures the slope of the swell at the sea surface. It only needs a relatively small, low power and low cost modified GPS receiver so it’s a practical payload that can be flown on multiple satellites. Measurements from many satellites will increase the available temporal and spatial resolution over today’s state of the art, and in future could lead to better global weather forecasting”.

So how does it work?

Here’s a map showing the SGR-ReSI targeting four potential reflected signals from GPS satellites around the Bay of Alaska.

The position of the TechDemoSat-1 satellite in orbit overhead is marked by the white cross, and the targeted GPS reflections are marked as yellow points

The SGR-ReSI captures the reflected GPS signals and processes the data on board the satellite to be downlinked to a ground station on the next pass and translated into Delay Doppler Maps.

Three Delay Doppler Maps taken simultaneously over the Gulf of Alaska from GPS satellites numbers 12, 25 and 2. The fourth reflection from GPS satellite 31 was too weak to be picked up as it was outside the view of the SGR-ReSI antenna.

SSTL has been working with the National Oceanography Centre in Southampton  who have translated data from the Delay Doppler Maps above into an interpretation of wind speed measurements at the sea surface, as shown in the diagram below.

Interpretation of Delay Doppler Maps from GPS satellites numbers 12, 25 and 2 into Wind Speeds and compared to Eutemsat’s ASCAT scatterometry measurements.

What could the data be used for?

The maritime industry depends on wave height and wind speed information for optimum ship routing, oil and gas rig operations and undersea cable laying, all of which aim to avoid high ocean swells, storms and extreme conditions which will either slow progress or at worst, cause damage to vessels and infrastructure. This information is routinely verified in coastal waters where instruments on buoys provide regular and true reference data, however over the open ocean the industry mainly relies on information based on forecasts and predictions.

And this is why the results coming down from the SGR-ReSI are so exciting – by flying the instrument on a constellation of small satellites GNSS Reflectometry data could be used to map all of the Earth’s ocean surface with refreshed data every couple of hours. This would not only be of enormous benefit to the maritime industry, it also offers improvements to weather services and climate research. And there’s more. The SGR-ReSI can pick up GPS reflections not only off the ocean, but also off land, snow and ice, opening up other potential new opportunities for remote sensing – for example, measuring the edges and possibly the thickness of sea ice, snow depth, soil moisture levels and the classification of vegetative foliage. It’s a clever, innovative, and low cost solution to providing one of the world’s missing Big Data sets, and we’re anticipating great strides ahead for sea-state intelligence.

What’s next for the SGR-ReSI?

SSTL, with support from the European Space Agency, is now working on preparing the ground processing and web interface that will allow users access to the measurements over the internet with a short delay.

We’re also planning upgrades to the SGR-ReSI will allow it to track Europe’s new GALILEO satellite signals, Russia’s GLONASS and China’s Beidou satellite signals, yielding additional measurements on the ocean surface.

And on the horizon, NASA’s 8-satellite “CYGNSS” mission, due for launch in 2016 will use SSTL’s SGR-ReSI to target tropical cyclones and hurricanes.

We’ll keep you posted on developments – in the meantime, if you’re going to be at the International Astronautical Congress in Toronto next week then you can find out more by coming to see a paper we are presenting on the SGR-ReSI – Session B4, 6A at 13.30 on 3rd October in Room 714A.

SSTL received funding to support the development of the SGR-ReSI and ground processing from the UK CEOI, SEEDA, Innovate UK, and the European Space Agency.

A video is available on YouTube at  which shows the processing and application of the SGR-ReSI data.

Notes on the video:
The video shows in about 20 times real time speed the motion of the TechDemoSat-1 satellite over an orbit, indicated by a white cross on the world map. The specular reflections targeted by the SGR-ReSI are shown by yellow spots, and the measurement tracks are shown in yellow. The four Delay Doppler Map channels from the SGR-ReSI are shown at the top right. The spreading horseshoe shape is caused by reflections being received away from the specular point, and a rougher ocean causes more spreading. When reflections are received from over land and over ice, there is much less spreading. The red band on the map indicates the collection of “raw” unprocessed data, which takes a few minutes to transfer before the processed Delay Doppler Maps resume.






25 September 20140 Comments1 Comment

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