Satellites in GPS World

In GPS jargon, a satellite is the space segment. A constellation of 24 GPS satellites (21 operational and 3 spares) orbits about 12,000 miles above the Earth. The satellites zoom through the heavens at around 7,000 miles per hour. It takes about 12 hours for a satellite to completely orbit the Earth, passing over the exact same spot approximately every 24 hours. The satellites are positioned where a GPS receiver can receive signals from at least six of the satellites at any time, at any location on Earth —that is, if nothing obstructs the signals.
Three important pieces of hardware are on each GPS satellite:

• Computer: An onboard computer that controls its flight and other functions
• Atomic clock: Keeps accurate time within 3 nanoseconds (around 3-billionths of a second) A super-accurate clock is required for correctly measuring the distance between the satellite and a GPS receiver based on the amount of time it takes for a satellite signal to reach the Earth.
• Radio transmitter: Sends signals to Earth The solar-powered GPS satellites have a limited lifespan (around ten years). When they start to fail, spares are activated, or new satellites are sent into orbit to replace the old ones. This gives the government a chance to upgrade the GPS system by putting hardware with new features into space.

GPS satellites don’t just help you stay found. All GPS satellites, starting with NAVSTAR 8 (in 1980) carry NUDET sensors. No, this isn’t some high-tech, pornography-detection system. NUDET is an acronym for NUclear DETonation, and GPS satellites are equipped with sensors designed to detect nuclear-weapon explosions, assess the threat of nuclear attack, and help evaluate nuclear strike damage.