Satellite navigation signals already help cruise missiles to find their targets and hungry motorists their way to the nearest fast-food joint. Now it turns out they can also be used to seek out a valuable resource: water.
The fleet of satellites making up 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. There has been speculation for a decade that it might be possible to exploit this effect, and now researchers at the University of Boulder in Colorado have shown how it 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 technique looks very promising,” says Larson.
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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 (see Diagram).
The approach could help fill a significant gap in techniques for measuring soil moisture. Right 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.
“One of the best things about this technique is that the GPS signals are free,” says Larson, whose team is now deploying soil sensors near Boulder to assess the effect of vegetation, temperature, snow and soil type on the GPS data. Meanwhile, 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.
