How GPS Works, And How It Got Better Than The Designers Ever Imagined

Summary

Scott Manley explains how GPS works and why it became more accurate and widely used than its original designers expected.

Origins and adoption

GPS grew out of U.S. military navigation projects in the 1970s. After Korean Air Lines Flight 007 was shot down following a navigation error, the U.S. government committed to making GPS available for civilian use. During the 1991 Gulf War, military personnel also found GPS receivers useful for navigating featureless desert terrain, accelerating military adoption.

How GPS determines a receiver’s position

GPS satellites broadcast precisely timed signals and information about their orbits. A receiver estimates its distance from each satellite based on signal arrival times. It needs signals from at least four satellites to determine its three-dimensional position and correct for its own clock error.

Satellites use code-division multiple access (CDMA), which lets them transmit on the same frequency while receivers distinguish their signals using satellite-specific codes. The civilian signal’s repeating code helps receivers acquire and track satellites, while accompanying data carries timing, orbital information, and correction parameters.

Obtaining the full set of orbit information can take about 12.5 minutes, though a receiver can save data to speed up later starts.

Sources of error and accuracy improvements

The ionosphere delays radio signals, sometimes introducing errors of tens of meters. Using signals on two frequencies, such as L1 and L2, helps measure and correct this delay. GPS accuracy is also weaker vertically than horizontally because satellites are visible across the sky but not below the receiver.

The military-designed system initially aimed to limit civilian accuracy. Selective Availability deliberately introduced timing errors, typically causing civilian errors of roughly 50–100 meters. Other government agencies developed differential GPS, in which fixed receivers at known locations calculate errors and broadcast corrections to nearby users. In May 2000, the U.S. turned off Selective Availability, greatly improving civilian accuracy.

Aviation augmentation and modern use

The FAA’s WAAS (Wide Area Augmentation System) broadcasts corrections and warnings about faulty satellite data through geostationary communications satellites. Europe’s EGNOS provides a similar service, and compatible aircraft equipment can use both.

Local augmentation systems can provide more precise guidance near an airport. They can support approaches to multiple runways without requiring a separate traditional instrument-landing installation for each one, and can enable approaches at airfields that previously lacked them.

Newer GPS satellites added civilian signals on L1 and L2, as well as the aviation-oriented L5 signal, which offers higher power and improved resistance to interference. GPS now supports aircraft navigation and approaches, terrain displays, and ADS-B aircraft position reporting. Manley notes that augmentation can provide around meter-level accuracy in favorable conditions—far better than the original civilian expectations.

Other satellite navigation systems

GPS is one of several global navigation satellite systems. The video also names Russia’s GLONASS, Europe’s Galileo, and China’s BeiDou. Receivers can often support multiple systems through software and signal-format compatibility.

Content type and source

• Content type: Explanatory overview; not a product review or step-by-step tutorial.
• Main speaker/source: Scott Manley