Your phone knows where you are. That is the Global Positioning System — a satellite system providing location and time information. GPS uses a constellation of about 30 satellites orbiting Earth. Each broadcasts a signal with its position and the time. Your receiver calculates distance from at least four satellites and triangulates your location.

The U.S. Department of Defense built GPS in the 1970s. It became fully operational in 1995. Other systems exist: Russia's GLONASS, Europe's Galileo, China's BeiDou. Together they are called GNSS, global navigation satellite systems. More satellites mean better accuracy.

Uses

• Navigation: cars, phones, ships, aircraft
• Mapping: surveying, cartography, GIS
• Timing: financial networks, power grids
• Tracking: fleet management, wildlife research
• Emergency: 911 location, search and rescue

GPS accuracy is typically a few meters. With correction signals, it can reach centimeters. But it does not work well indoors or underground. And it can be jammed or spoofed. For critical systems, backup timing sources are essential.