MODEL RELEASED: Satellite navigation. A walker checking his location with a hand-held satellite navigation receiver. The system uses signals sent by the network of 21 'Navstar' Global Positioning System (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 metres. This is one of the new generation of GPS receivers which are portable and affordable to the general public. GPS has benefits to a wide variety of users, including hikers and climbers, sailing enthusiasts, private pilots and vehicle drivers.