GPS determines location by measuring the apparent travel time of radio signals transmitted by satellites whose positions are known. Each GPS satellite broadcasts a precisely timed signal together with information that allows a receiver to determine the satellite's position at the time of transmission. The receiver generates or recognises the corresponding timing code and compares the received signal with the expected signal sequence.
The difference provides an estimate of how long the signal took to travel from the satellite to the receiver. Because electromagnetic signals travel at approximately the speed of light, the measured travel time can be converted into an estimated distance. This estimated distance is called a pseudorange because it contains errors and because the receiver clock is not perfectly synchronised with the satellite's atomic clocks.
The receiver combines pseudorange measurements from several satellites and solves a set of equations for its three dimensional position and its clock offset. The resulting coordinates can then be expressed as latitude, longitude and altitude within the coordinate reference framework used by the GPS system.
The geometry behind GPS positioning can be understood using the concept of ranging. If a receiver knows that it is a particular distance from one satellite, it must lie somewhere on an imaginary sphere centred on that satellite. A second satellite produces a second sphere, and the receiver must lie where the relevant surfaces intersect. A third measurement further constrains the possible locations.
In an idealised two dimensional explanation, three ranges can determine a position, but actual GPS positioning normally involves three spatial coordinates plus an unknown receiver clock error. This creates four unknown quantities: horizontal position, vertical position and receiver clock offset. Consequently, measurements from at least four satellites are generally required to solve the basic three dimensional navigation problem. In practice, receivers usually track more than four satellites when available. Extra observations make the solution more robust and allow mathematical estimation techniques to reduce the influence of measurement noise and individual errors.
Satellite position is not assumed to be fixed. GPS satellites move rapidly around the Earth, so the receiver must determine where each satellite was at the precise time its signal was transmitted. The navigation message provides orbital information that allows the receiver to calculate satellite position. More precise systems also account for satellite clock behaviour and relativistic effects. Relativity is relevant because satellite clocks experience differences in elapsed time associated with their motion and their position in Earth's gravitational field.
The GPS system incorporates corrections so that satellite timing remains sufficiently consistent for positioning. Without accurate timing, small errors would produce large range errors because radio signals travel extremely quickly. The receiver therefore combines orbital calculations and precise timing information before solving the positioning equations. The resulting position is an estimate derived from multiple physical measurements rather than a location simply sent directly from a satellite.
The receiver clock is one of the most important reasons why four satellites are normally needed. GPS satellites contain extremely accurate atomic clocks, but it would be impractical for ordinary receivers to contain clocks with equivalent stability. The receiver's internal clock can therefore differ slightly from GPS system time. A clock error affects every measured signal travel time and consequently every pseudorange. The receiver treats the clock offset as another unknown and estimates it alongside its position.
With four or more satellites, the system can solve for latitude related position, longitude related position, altitude and clock offset. With additional satellites, the receiver can use redundant observations and statistical estimation methods to obtain a more reliable solution. This design is one of the central technical features of GPS because it allows relatively inexpensive receivers to achieve useful positioning without requiring an atomic clock.
The calculated position is affected by several sources of error. The ionosphere can change the propagation speed of radio signals, producing delays that vary with frequency and atmospheric conditions. The troposphere also introduces propagation delays. Satellite orbit information and satellite clock errors can contribute additional uncertainty. Multipath occurs when a signal reflects from buildings or other surfaces and reaches the receiver along a longer route. Receiver noise and antenna characteristics also influence measurements.
Satellite geometry is commonly described using dilution of precision measures. When satellites are distributed widely across the receiver's sky, geometric conditions are generally more favourable for calculating position. When satellites appear clustered together, the same measurement errors can produce greater positional uncertainty. A receiver therefore considers not only how many satellites it can observe but also where those satellites are located in the sky. Modern receivers can use multiple frequencies, multiple satellite systems and correction services to reduce several of these error sources.
High precision GPS positioning uses techniques that go beyond the basic standalone pseudorange solution. Differential GPS uses measurements from a reference receiver at a known location to estimate errors that can then be applied to another receiver in the same general area. Real Time Kinematic positioning, commonly known as RTK, uses carrier phase measurements and correction information to achieve very high positional accuracy under suitable conditions.
Precise Point Positioning uses precise satellite orbit and clock information to improve positioning without necessarily requiring a nearby traditional base station, although convergence and other conditions affect performance. Carrier phase measurements can be much more precise than ordinary code based measurements because the wavelength of the carrier signal is far shorter than the uncertainty associated with code timing. Resolving the integer number of carrier cycles between satellite and receiver is technically challenging but enables centimetre level positioning in suitable systems. These techniques are widely used in surveying, construction, precision agriculture and scientific geodesy.
GPS coordinates are normally expressed relative to a geodetic reference system. Latitude and longitude are angular coordinates describing position on an Earth model, while height can be defined in different ways. GPS fundamentally provides a position relative to the reference ellipsoid associated with its geodetic system, whereas many maps and engineering applications require height above a physical approximation of mean sea level called an orthometric height. The difference between ellipsoidal height and orthometric height is related to the geoid.
This distinction explains why a GPS receiver's displayed altitude may differ from an elevation value shown on a conventional topographic map. Coordinate transformations can also be required when GPS coordinates are used with local or national mapping systems. Thus, obtaining a numerical coordinate is only part of determining a location. Correct interpretation requires knowing the reference frame, coordinate system, units and meaning of the height value.
Once the receiver calculates its position, software can convert the coordinates into useful geographic information. A navigation application can place the position on a digital map, identify a nearby road through spatial databases and calculate a route. A reverse geocoding service can convert the coordinates into an address or place description. GIS software can display the coordinate alongside geographic layers and perform spatial analysis. GPS therefore commonly forms one stage in a larger geospatial workflow.
Its fundamental role is precise measurement of a receiver's position and time using signals from satellites. The accuracy and usefulness of that measurement depend on satellite geometry, signal quality, atmospheric conditions, receiver technology, correction methods and the coordinate reference system. Understanding these factors is essential when interpreting a GPS location as a measurement rather than assuming that every displayed coordinate represents an exact physical point without uncertainty.
