SCIENCE: YOU ARE HERE

AT FIRST glance, Gwyn Robert's sheep look no different from the thousands of others grazing on the upland hills of the Lake District. The only clue that there is something special about them is the small green pack that every one is carrying on its back. Inside each of these little pouches is a tiny receiver that links these half a dozen sheep in remote Cumbria to American military satel-lites orbiting 20,000km out in space. And every time a sheep rambles over to munch a new area of grass, the equipment in its pouches picks up a new signal from the satellites.

Monitoring the movements and eating habits of sheep in the Lake District and North Wales was not one of the uses envisaged by the designers of the multibillion-dollar satellite system which was developed by the US military 20 years ago. But then the US Department of Defense's Global Positioning System (GPS) is now being used for a variety of unlikely tasks: by farmers to spot weed-ridden fields, by weather forecasters to check the moisture content in the atmosphere, by transport companies to trace delivery trucks, and by biologists to track migrating turtles. In Oklahoma, scientists have "barcoded" trees so that they can be monitored by GPS, and plans for a satellite-based navigation system for blind people are in the pipeline.

Advanced equipment now means that the satellites 20,000km away can be used to give a remarkable accuracy of better than one centimetre, which is far greater than their original designers ever intended.

The concept of GPS began to emerge after the launch of the first Soviet satellites four decades ago. It was then realised that the radio transmissions from a satellite in a known and fixed orbit could be used as a navigational aid. In the mid-Seventies, the United States began to design and develop a satellite system that would give its military personnel the ability to determine their location instantly, anywhere in the world. The system they came up with was GPS, based on a network of 24 satellites (three of them acting as spares) which orbit the earth, sending out high-frequency radio waves.

The principle was simple: each satellite continuously broadcasts a signal indicating the time, using an on-board atomic clock, and its precise position. Back on earth, portable receivers note the time when the signal was sent and when it arrived, and from that they work out the distance back to the satellite. By picking up the signals from four satellites at the same time the receiver can, by using a form of triangulation, fix the user's longitude, latitude and height anywhere on or above the Earth.

"It was built as a US military system and is a weapons-aiming system and is really there to give the US more bangs for their bucks by putt- ing missiles and bombs onto their targets much more precisely. So instead of sending 10 missiles, you send one and it lands right down the chimney pot," says Professor David Last, who heads the radio navigation group at the University of Wales, Bangor.

But having designed and built the system, the US military then faced something of a conundrum. It wanted GPS to be generally available for civilian use, not least because of the need to claw back to the economy some of the costs of building the system, but at the same time they didn't want an enemy to be able to make use of it.

The solution they arrived at, known as selective availability, was a sophisticated system where the signal timing was corrupted. Knowing the exact time the signal leaves each satellite is crucial, so the designers installed a system where the timing of the on-board atomic clocks is slightly altered, according to a secret programme.

Thus only the military, with equipment that knows the code and therefore corrects the clock errors, has access to the most accurate data. The result was that while the military could get navigation fixes to within 15 metres, the best everyone else had was an accuracy of 100 metres. "They were trying to do two things at once - to promote it for civilian use, because US industry dominates the GPS business, and at the same time hang onto it because of its strategic advantages. They have got around that by selective availability, which means they bugger it up for the civilian users so as to deny them the full accuracy. We now understand, however, that selective availability will disappear by 2006," says Professor Last.

But because the concept of GPS is such an attractive one, it was not long before civilian scientists and engineers started looking for ways of getting around the timing errors and eventually devised differential GPS. What they did was to measure the signals from several satellites from a fixed point on the ground. By knowing the exact location of the receiver and the satellites, they could calculate the real distance to the orbiting craft and compare it with the data being sent from the satellite. The difference between the two figures represented the coded clock error.

"The errors are the same for all receivers over large areas. So you measure the errors, moment by moment, broadcast the correction, and the other people around apply the corrections to cancel their errors. If I tell you that everywhere in Britain is 50 metres to the east, that's all you need. You subtract 50 metres from your reading and off you go," says Professor Last.

These constantly changing clock errors are now decoded by various providers, including commercial companies, which broadcast or sell the errors to users. Ironically, one of the biggest providers of the decoded data is the US government itself, in the shape of the Federal Aviation Authorities and the Coast Guard.

Differential GPS meant that civilian users had access to the even greater accuracy hitherto only available to the military. Civilian scientists also succeeded in improving the accuracy of the system and have brought it down to less than 1cm, which has led to a huge increase in civilian use. Sales of GPS technology are projected to reach $8bn by the year 2000.

In Cumbria, Dr Roberts, manager of the GPS project at IDB, a hi-tech company owned by the University of Wales, Bangor, has been using the backpacks on sheep to see if it is feasible to track them. "There was concern that there were high spots of post-Chernobyl radiation in some areas where the sheep were grazing. Some were going out clean and coming back affected, while others weren't. We wanted to see where the hot areas were, and we have been using GPS for this. We've finished the first part of the programme by proving it is feasible, and are now waiting to see whether there will be any funding available for the main project," he says.

At Oklahoma State University, Professor Paul Hsu is using GPS to monitor trees. He barcodes them, fixes their position via GPS, and then keeps check on their condition. If a hurricane or twister takes out one of his trees, its demise is recorded and replacement can begin immediately. "We get an accuracy of one to 10ft, but it is getting better. The only problem is where you get dense foliage, which interrupts the signals," he says.

At the Massachusetts Institute of Technology, a team is working with GPS to create a system to guide blind people. With an accuracy of 2 or 3mm, a blind person would be guided around obstacles by audible cues broadcast from a backpack containing equipment that interprets the signals from the GPS into a map.

Farmers are also using GPS to help them to distribute fertiliser more effectively. By pinpointing precisely where less productive areas of land are, it can be used to program automated tractors to deliver fertiliser where it is most needed.

At the University of Wales, Cardiff, they have gone one step further, with the development of Robo-Shep, a robot based on a Kawasaki Quad bike, which is operated remotely by the farmer. It will be equipped with a range of visual sensors, a GPS navigation system, and a video link back to the farmer. "The aim is to use it in the field via remote control as a robotic shepherd. The farmer will be able to specify the area and movements of the robot and GPS will help it follow those instruction," says Professor Duc Pham of UWC.

GPS has also, of course, exercised the military applications it was designed for 20 years ago. It was first put to combat use was in the Gulf War where it was crucial for desert navigation, from aiding the SAS in seeking out Scud missile sites to helping chuck wagons deliver food to troops. "GPS was a godsend for ground troops, especially in sandstorms. Tank crews and drivers of all sorts of vehicles swore by the system. Meal trucks were equipped with GPS receivers to enable drivers to find and feed soldiers in front line units among the dunes," says a US Airforce report.

There was, however, one problem in the Gulf. Not enough encoded receivers were available to the Coalition forces, so the coding that GPS designers had so cleverly built in had to be switched off, which meant that anyone with a commercially available receiver, including Iraqi forces, was able to use the system. So useful was GPS, and such a potential life-saver, that with the coding off, many US troops (and maybe a few Iraqis, too) telephoned America with their credit card numbers to get commercial receivers by mail order to augment the inadequate official supplies. !