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GPS echoes help monitor Earth
01 May 2004
From New Scientist Print Edition.
Anil Ananthaswamy

Enlarge Remote sensing with GPS

The Earth is bathed with signals from the network of Global Positioning System satellites, but few realise that the GPS signals that are reflected back into space contain a wealth of information about the planet. Now scientists are proposing a network of inexpensive satellites that could use these echoes to monitor some of Earth's vital signs.

Using only reflected GPS signals, the new satellites could measure soil moisture content, ocean salinity and currents, and ocean surface wind speeds. They could also help reduce the amount of expensive equipment needed on other remote-sensing satellites, such as those that measure sea surface height - a key factor in our understanding of climate change.

GPS signals contain the satellite's identity and position, and the precise time the signal was sent. A GPS receiver on the ground uses signals from at least four satellites to calculate its distance from each - so it can determine its position in three dimensions.

Over the past few years researchers have begun studying the reflected signals using GPS receivers mounted on aircraft (New Scientist, 17 October 1998, p 7). By comparing the direct signal with the echo, researchers have managed to tease out three useful pieces of information: the delay of the reflected signal (which reveals how far it has travelled), the power of the echo and how strongly it has been distorted.

Aerospace engineers George Born and Penina Axelrad at the University of Colorado in Boulder have used this information to measure the moisture content of soil. They found that the wetter the soil, the stronger the reflected signal. The state of the ocean's surface also modifies the signal in a characteristic way. "The signal power drops as a function of the amount of waves on the surface, which is proportional to the wind velocity," says Born. In tests, their airborne instruments were able to measure wind speeds on the ocean surface to an accuracy of 2 metres per second.

What's more, the reflected GPS signals also pick up information about the electron activity in the ionosphere. Electrons here delay the passage of radio waves. This is a nuisance for remote-sensing satellites that bounce signals off the Earth's surface, because the electron content of the ionosphere varies with location and can change in a matter of hours, making the signal delays unpredictable.

To get around this ionospheric interference, satellites such as NASA's Jason-1, which measures ocean surface height down to a few centimetres, beam down two sets of radar signals at different frequencies. Since each signal is delayed differently by the interference, the satellite can calculate the error introduced by the ionosphere and correct for it. But it is more expensive to fly the equipment needed to generate two radar signals.

Each GPS satellite is already beaming down two signals at 1.58 and 1.23 gigahertz - one for civilians and one for the military - so it should be possible to use the reflections of these two signals to continuously calculate the electron content of the ionosphere. Born says that this would be considerably cheaper than today's dedicated satellites that use radar to do the same. "It's totally passive. You don't radiate any power," he says. It would also mean that in future, remote-sensing satellites that are sensitive to ionospheric delay need only carry one set of radar equipment, correcting for delays using the GPS reflection data.

There are already 28 active GPS satellites, and with Europe planning its own global navigation system, called Galileo, there will soon be even more reflected signals. "Those new signals will make this approach much, much stronger," says Axelrad.

Axelrad and Born will propose in July's edition of the journal Acta Astronautica that an inexpensive network of just four low-orbit satellites could map the electron content of the ionosphere. And the European Space Agency is interested in the idea for future remote-sensing missions, says Manuel Martin-Neira of ESA's European Space Research and Technology Centre in Noordwijk, Netherlands. "We believe in the technique," he says.

From issue 2445 of New Scientist magazine, 01 May 2004, page 23

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