Pinpoint precision

日期:2017-09-26 03:14:47 作者:万扃 阅读:

By Jon Copley NOT sure where you are? An improved version of the Global Positioning System of navigation satellites can tell you to within a centimetre. The system is being adapted to track fast-moving objects and could even lead to television viewers in different countries seeing different ads on the same Formula One racing car. A GPS receiver works out its position by converting the time that signals take to travel from several satellites into the receiver’s distance from each. Because the position of each satellite at any time is known precisely, these distances can then be used to calculate the receiver’s location. However, the timing information on which these calculations depend is coded as modulations of the basic carrier wave broadcast by each satellite, and to limit the value of GPS to potential enemies, the US Department of Defense introduces errors into this signal. While military GPS receivers give bearings accurate to within 20 metres, civilian receivers have an accuracy of only 100 metres. But enterprising researchers have realised that it is possible to bypass the deliberate errors and obtain bearings even more accurate than is possible with standard military GPS, by ignoring the timing signals and instead studying the phase of the carrier waves themselves. Paul Cross of University College London told the BA that by measuring differences in the phase of waves from each satellite with two or more receivers, it is possible to determine their relative positions with pinpoint accuracy. The trick, he says, is software that models how varying atmospheric conditions affect the carrier waves. Cross’s group has achieved an accuracy of 1 centimetre with receivers 1000 kilometres apart. This is good enough for GPS to replace the laser-based systems used to map out construction sites for buildings and roads. GPS receivers could monitor the precise locations of the shovels deployed by earth-moving equipment. Using phase-differential GPS to track fast-moving receivers poses particular problems, Cross told the BA, because nearby objects can reflect the signals, creating “ghost” signals similar to the double images seen when TV signals bounce off tall buildings. But Cross is working on algorithms that cancel out ghost signals. He already has an ambitious application in mind. If a GPS receiver were placed on a Formula One car, it could be used to track the car with a camera so that the position of the adverts emblazoned on the side is known precisely. Using digital image processing, the adverts could then be altered for different audiences. Japanese viewers might see an ad for whisky, for example,