How Does a GPS Navigation System Work?

Global Positioning System Satellite Transmissions

Global Positioning System, or GPS, navigation gets its transmission from GPS satellites. These systems act only as receivers; they cannot transmit signals. They require an unobstructed view of the sky to receive signals, so they are more functional outdoors. While on the road, motorists may be in areas where tall trees stand close to each other, creating a canopy that blocks the view of the sky, or you may travel to a city crowded with tall buildings. Under these conditions, the GPS navigation has trouble receiving transmissions. Each GPS satellite transmits data containing the time, precise orbital information, the general system health and rough orbits of every GPS satellites. The receive, in turn, interprets this data.

GPS Satellites

Approximately 24 satellites operate simultaneously to form a constellation that orbits around the Earth every 12 hours. Ground stations can track each satellite's location. All GPS satellites synchronize with each other so they can repeat the signals and transmit them at the same time. Transmitted signals arrive at the GPS receiver at slightly different times because some satellites are farther from Earth than others. To calculate the distance of the GPS satellites, you must determine how long it takes the signals to reach the receiver. The GPS receiver estimates the distance to at least four GPS satellites by calculating their positions in three dimensions.

Finding the Position of the GPS

The GPS receiver calculates the location of the satellites from the information included in the satellite transmission. The GPS receiver estimates the distance of the satellite residing at the center of the imaginary sphere within the satellite constellation (see the accompanying image). There is an imaginary sphere for every satellite. You can locate the receiver depending the satellite's location at the point where the imaginary spheres intersect. This position of the GPS receiver is closest to the surface of the sphere corresponding to the fourth satellite.

GPS Mapping

GPS navigation manufacturers use surveyors and mapmakers to ensure precision positioning. These professionals map out the locations of common facilities, such as the nearest gas stations, restaurants, hospitals and other landmarks. Some GPS models may display icons for these landmarks for easier identification. If you must find any of these locations immediately, zoom in on the icons that represent them on the navigation screen, and then find their location using the GPS navigation. The surveyors and mapmakers assign codes to identify roads, streams or other objects. They then compare and analyze the data using computer programs such as Geographic Information Systems, or GIS.

GPS Limitations

A GPS cannot detect road conditions, such as construction work or detours for drivers. It also cannot detect traffic congestion and accidents. It may not detect new communities or newly developed streets and avenues. You must update the software annually.

Resources

• GPS Satellite Constellation