The GPS satellites

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 the Earth (if nothing obstructs the signals).
A satellite has three key pieces of hardware:

• Computer: This onboard computer controls its flight and other functions.
• Atomic clock: This keeps accurate time within three nanoseconds (around three-billionths of a second).
• Radio transmitter: This sends signals to Earth.

GPS satellites don’t just help you stay found. All GPS satellites since 1980 carry NUDET sensors. No, this isn’t some high-tech pornography-detection system. NUDET is an acronym for NUclear DETonation; GPS satellites have sensors to detect nuclear-weapon explosions, assess the threat of nuclear attack, and help evaluate nuclear strike damage. The solar-powered GPS satellites have a limited life span (around 10 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.