GPS is best understood as a timing system that enables location. Satellites broadcast precisely timed signals and orbital information. A receiver compares when several signals arrived and solves for its position and its own clock error.
Distance from signal travel time
Radio signals travel at approximately the speed of light. A tiny timing error becomes a large distance error, so satellite clocks and signal processing must be extremely precise. The receiver estimates a “pseudorange” to each satellite rather than directly measuring physical distance.
Why several satellites are needed
Three-dimensional position has three unknown coordinates, but the receiver clock adds another unknown. Signals from at least four suitably placed satellites allow the receiver to solve for position and clock offset. More satellites usually improve geometry and error checking.
Trilateration
The receiver does not measure angles to the satellites. It finds the point consistent with several distance-like measurements, a process called trilateration. Satellite geometry matters: measurements from satellites clustered in one part of the sky are less informative than measurements spread across the sky.
Error sources
Atmospheric delay, reflected signals, obstruction, orbital-data error and receiver noise affect accuracy. Buildings and terrain can block or reflect signals. A phone may combine satellite positioning with cell, Wi-Fi and motion-sensor data to improve speed or continuity.
Augmentation and corrections
Differential and augmentation systems compare known reference locations with satellite results and distribute corrections. Survey and precision applications may use carrier-phase techniques and local or network reference services.
Global navigation systems
GPS is the United States system, but receivers may also use Galileo, GLONASS, BeiDou and regional systems. Multi-constellation reception can improve availability and geometry.
Infrastructure dependency
Satellite positioning supports transport, surveying, communications, finance and power-system timing. It depends on satellites, ground control, spectrum protection and receiver software. Disruption can therefore affect services that appear unrelated to navigation.
Position, navigation and timing
Many systems use the timing output even when they do not display a map. Telecommunications networks, financial systems and electric grids may use satellite-derived time to coordinate events. This creates a dependency that should be recognized in continuity planning.
Indoor and urban limitations
Satellite signals are weak by the time they reach Earth. Roofs, underground spaces and dense buildings can block them. Reflections may cause the receiver to measure a longer apparent path. Devices often switch among satellite, inertial, cellular and Wi-Fi information, but each source has its own uncertainty.
Spoofing, interference and sanity checks
Radio interference can reduce availability, while deceptive signals can produce a plausible but incorrect result. High-reliability users compare GPS with independent sensors, maps, clocks or operational limits. A position should not be trusted solely because the receiver reports many decimal places.
Receiver output and maps
A receiver estimates coordinates; a mapping application then places them on a road or property model. Errors can come from either stage. The map may be outdated or the nearest-road algorithm may choose the wrong route even when the coordinate estimate is reasonable.