Science & Technology: Hide-and-Seek: For Nerds ; Geocachers Search for Buried 'Treasure' Using Global Positioning. Clint Witchalls Gets Digging
Witchalls, Clint, The Independent (London, England)
It's raining and I'm ferreting about in the shrubbery in a small church garden in the City of London. There is a row of people sitting under the eaves, eating their lunch, staring at me. 'What is this idiot doing?' they are no doubt asking themselves. Which is pretty much what I'm asking myself after half an hour of looking for an elusive Tupperware box with some tat in it, just so I can say I've tried geocaching.
You may well ask yourself, what the hell is geocaching? Well, it's the new sport for the cybergeek. It goes like this. You hide some 'treasure' " any old junk " in a public place. Take the co- ordinates of the place using a hand-held global-positioning system (GPS) device. Put the co-ordinates on to a website, for example www.geocaching.com, plus a cryptic clue, and let other people search for it. It's a hi-tech treasure hunt, except without any real treasure.
I borrowed a Garmin GPSmap 60C hand-held device for my adventure. These gizmos, which look like obese mobile phones, were designed for mountaineers, sailors and outdoorsy people, but, like all new technology, people find unexpected uses for them. Criminals are usually the first to find novel uses for gadgets, but in this case the nerds got there first.
GPS was initially developed by the American military in the late 1970s. Today, it consists of a constellation of 27 satellites (plus a few back- ups) that transmit high-frequency, low-powered radio signals. A GPS receiver scans for these signals and uses a mathematical system called 'trilateration' to work out your location. As the exact position of each satellite is known, as is the speed at which radio waves travel, the GPS receiver only has to work out how long it took for each of the three signals to reach you in order to calculate your exact longitude and latitude. Well, not 'exact', but they are accurate to within about three metres. With a fourth satellite signal, it is also possible for the GPS device to work out your elevation " useful if you're a mountaineer or skier.
Initially, hand-held GPS devices were expensive and, indeed, the model I borrowed retails for pounds 455. But there are many low- cost devices, such as the Magellan eXplorist and the Garmin eTrex Personal Navigator that can be bought for under pounds 100. The development of these low-cost hand- held devices has done a lot to fuel the interest in geocaching. A few mobile phones are also GPS enabled, and some video-game makers are working on hand-held GPS games. But none of these gizmos would be around today if it weren't for the Clinton administration deciding to unscramble the radio signal sent from military satellites, making GPS technology available to the hoi polloi.
Up until 2000, the US military satellite network worked on what was called 'selective availability'. Selective availability meant that the military could use GPS with an accuracy of 10 metres, but civilians could only get a location accurate to 100 metres " not much use if you're looking for a small plastic box in a church garden. …