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Satnav signals used to dowse for water
Technology Turn left here to find a good site for a well
MOISTURE METER The intensity of reflected GPS signals provides a measure of the amount of moisture in the soil. The reflected signal cannot be measured directly, so its intensity is assessed from the interference with signals direct from the satellite – taking into account the variation in the antenna’s sensitivity to microwaves from different directions
90˚ Direct GPS signals
60˚ 30˚
Plot of antenna sensitivity versus direction
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
‘Nanopaper’ reveals a heart of steel
CAN’T punch your way out of a paper bag? That wouldn’t be surprising if were made of a new kind of paper that’s stronger than cast iron. The super-strong paper could be used to make tough sticky tape or synthetic replacements for biological tissues. 26 | NewScientist | 14 June 2008
0˚ -30˚ -60˚
Reflected signal
1.5-5.0m
Receiving antenna MOIST GROUND
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. 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
Despite its exceptional strength, the paper is produced from cellulose, the same material as conventional paper. Cellulose fibres are the structural component of plants’ cell walls and the principal ingredient of wood. “Cellulose nanofibres are characterised by high strength and toughness,” says Lars Berglund of Sweden’s Royal Institute of Technology in Stockholm. These properties mean that cellulose is increasingly being used to make novel plastics, as well as paper, but it is mostly used as a cheap “filler”, extracted from wood pulp by bruteforce mechanical processes which tend to break the fibres. Berglund and his colleagues have come up with a gentler approach.
After breaking down the pulp with enzymes they fragmented it using a device that exposes the cellulose to strong shear forces, which separates it into its component fibres and leaves them suspended in water. When the team filtered out the water using vacuum filtration, the
“Despite its strength, it is made from the same stuff as conventional paper” fibres joined together in networks to form thin sheets of “nanopaper”. This turned out to have a tensile strength of 214 megapascals: more than 200 times the strength of ordinary paper, and almost as much as structural steel
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. Paul Marks ●
(Biomacromolecules, DOI: 10.1021/ bm800038n). The long, undamaged fibres that form the paper can slip and slide over each other while remaining strongly bound together, giving the nanopaper an effective mechanism for dissipating stresses. The individual cellulose fibres are also about 1000 times thinner than those in conventional paper, making any faults correspondingly smaller. “The material has very small defects compared with a conventional paper network,” Berglund says. “This shows quite clearly the potential for cellulose nanofibres to provide a basis for reinforcement,” says Stephen Eichhorn, a polymer researcher at the University of Manchester, UK. Jon Evans ● www.newscientist.com
MOISTURE METER The intensity of reflected GPS signals provides a measure of the amount of moisture in the soil. The reflected signal cannot be measured directly, so its intensity is assessed from the interference with signals direct from the satellite – taking into account the variation in the antenna’s sensitivity to microwaves from different directions
90˚ Direct GPS signals
60˚ 30˚
Plot of antenna sensitivity versus direction
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
‘Nanopaper’ reveals a heart of steel
CAN’T punch your way out of a paper bag? That wouldn’t be surprising if were made of a new kind of paper that’s stronger than cast iron. The super-strong paper could be used to make tough sticky tape or synthetic replacements for biological tissues. 26 | NewScientist | 14 June 2008
0˚ -30˚ -60˚
Reflected signal
1.5-5.0m
Receiving antenna MOIST GROUND
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. 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
Despite its exceptional strength, the paper is produced from cellulose, the same material as conventional paper. Cellulose fibres are the structural component of plants’ cell walls and the principal ingredient of wood. “Cellulose nanofibres are characterised by high strength and toughness,” says Lars Berglund of Sweden’s Royal Institute of Technology in Stockholm. These properties mean that cellulose is increasingly being used to make novel plastics, as well as paper, but it is mostly used as a cheap “filler”, extracted from wood pulp by bruteforce mechanical processes which tend to break the fibres. Berglund and his colleagues have come up with a gentler approach.
After breaking down the pulp with enzymes they fragmented it using a device that exposes the cellulose to strong shear forces, which separates it into its component fibres and leaves them suspended in water. When the team filtered out the water using vacuum filtration, the
“Despite its strength, it is made from the same stuff as conventional paper” fibres joined together in networks to form thin sheets of “nanopaper”. This turned out to have a tensile strength of 214 megapascals: more than 200 times the strength of ordinary paper, and almost as much as structural steel
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. Paul Marks ●
(Biomacromolecules, DOI: 10.1021/ bm800038n). The long, undamaged fibres that form the paper can slip and slide over each other while remaining strongly bound together, giving the nanopaper an effective mechanism for dissipating stresses. The individual cellulose fibres are also about 1000 times thinner than those in conventional paper, making any faults correspondingly smaller. “The material has very small defects compared with a conventional paper network,” Berglund says. “This shows quite clearly the potential for cellulose nanofibres to provide a basis for reinforcement,” says Stephen Eichhorn, a polymer researcher at the University of Manchester, UK. Jon Evans ● www.newscientist.com