Using GPS signals to find water

28 July 2008, 4:59pm

Satellite navigation signals can be used to seek out a valuable resource - water. The satellites comprising the Global positioning System (GPS) bathes the Earth in weak microwave signals. When these signals bounce off the ground, their characteristics change in ways that convey information about the soil's moisture content. Researchers at the University of Boulder, Colorado, have shown how it might be possible to exploit this effect can be done.

The team, led by aerospace engineer Kristine Larson and geologist Eric Small, estimated soil moisture levels using data from a permanent GPS station in Tashkent, Uzbekistan, which is normally used for seismic monitoring. The GPS-derived estimates correlated well with data recorded by nearby sensors, capturing changes in moisture content as the soil dried out after rain (GPS solutions, vol 12, p 173).

The method works because the more moisture soil contains, the more effectively it reflects radio waves. GPS receivers cannot measure the strength of the reflected signal directly, but interference between the reflected microwaves and the signal coming straight from the satellite shows up as fluctuations in the signal-to-noise ratio recorded by the receiver. The effect of the direct signal can be subtracted out to produce a measure of the strength of the reflected signal. The receiver's sensitivity depends on the angle at which the signal arrives, so this has to be taken into account as well.

The approach could help fill a significant gap in techniques for measuring soil moisture. Now there is no way to measure changes on scales ranging from about 100 metres up to a few kilometres - the size of towns and villages. Ground probes typically have a resolution of less than a metre, while satellites using radar take readings on areas between 40 and 60 kilometres across. A GPS receiver can check out an area up to 40 metres in diameter, so a network of such receivers could help locate the best places to sink wells or plant crops. Because fluctuations in soil moisture reveal a lot about an area's hydrology ¬such as how full groundwater reservoirs are, or how likely an area is to flood - the GPS-based technique could also help researchers assess the effects of climate change.

Clovis de Matos, a Paris-based earth scientist with the European Space Agency, says ESA is looking into using the forthcoming Galileo positioning system to "monitor the physical and chemical states" of the atmosphere and the Earth's surface.

Source: Paul Marks in New Scientist magazine, issue 2660

New Scientist full article

Geo: International