Satellite navigation. Field geologist checking his location with a handheld Garmin 45 global positioning system (GPS) receiver. The system uses signals sent by the network of 21 'Navstar' GPS satellites. Each satellite sends out a signal with its identity, position and the exact time from an atomic clock. The receiver reads these data and can instantly calculate its position anywhere in the world to within 100 meters. This is one of the new generation of GPS receivers which are portable and affordable. In addition to geologists GPS has benefits for hikers and climbers, sailing enthusiasts, private pilots and vehicle drivers.