GPS Networks

The Global Positioning System, or GPS, is a network of satellites that provides positioning, navigation, and timing information around the world. Its effectiveness depends on carefully designed orbits that keep satellites available to users across Earth's surface.

GPS satellites travel in medium Earth orbit at an altitude of about 12,550 miles (20,200 kilometers). Each circles Earth twice a day, completing one orbit in roughly 12 hours.

The system is built around a baseline constellation of at least 24 satellites arranged across multiple orbital planes, with additional satellites improving coverage and reliability.

Orbital Design

The GPS constellation is arranged so that receivers can generally detect signals from at least four satellites at the same time. This is necessary to calculate a three-dimensional position and correct for the receiver's clock error.

GPS satellites travel in nearly circular orbits inclined about 55 degrees to Earth's equator. The constellation uses six evenly spaced orbital planes to provide broad coverage across most of the planet, including high-latitude regions.

At this altitude, GPS satellites travel at roughly 8,700 miles per hour (14,000 kilometers per hour).

How GPS Works

Every GPS satellite carries highly accurate atomic clocks and broadcasts its position along with the time its signal was transmitted. A receiver measures how long each signal took to arrive and converts that interval into an estimated distance from the satellite.

By comparing signals from at least four satellites, the receiver can calculate its position and correct tiny errors in its own clock. Because GPS signals travel at the speed of light, even a very small timing error can shift the calculated location.

Ground-control stations continuously monitor the satellites, update their orbital information, and help maintain the accuracy of their clocks.

Relativity and Orbital Maintenance

GPS satellites require occasional maneuvers to maintain their assigned orbital positions. Their paths can gradually change because of small gravitational influences, solar radiation pressure, and other long-term effects.

GPS also depends on corrections predicted by Einstein's theories of relativity. The satellites' high speed makes their clocks run slightly slower than clocks on Earth, while their weaker gravitational environment makes them run faster. The combined effect is carefully calculated so positioning and timing remain accurate.

Applications Beyond Navigation

GPS helps guide cars, aircraft, ships, smartphones, and emergency services. Its precise timing signals are also important to power grids, financial networks, telecommunications systems, and scientific instruments.

Scientists use GPS to measure tectonic-plate motion, monitor earthquakes, track changes in sea level, and study Earth's atmosphere. Modern devices often combine GPS with other global navigation satellite systems to improve coverage and accuracy.