The Global Positioning System or GPS uses a group of at least 24 satellites orbiting approximately 12 600 miles or 20 200 kilometers above the earth. The vehicle is equipped with a receiving antenna and computer system. The GPS receiver on the vehicle has to locate four or more of these satellites, determine the distance to each, and use this information to establish it's own location. This operation is based on a mathematical principle called “trilateration”.
“Trilateration” in three-dimensional space is quite complex. For ease of understanding, the term that is generally used in the automotive industry to describe how the GPS positioning system operates, is “Triangulation”.
“Triangulation” is the process of finding the position of an unknown point based on forming a triangle with two known points.
For simplicity, the dimensions used to determine these points are commonly known as Latitude, Longitude and Elevation
If a group of three satellites are taken to form a triangle, 12 600 miles or 20 200 kilometers above the earth, the shape and size of this triangle will never change.
Equally any fixed point on the surface of the Earth will triangulate with the satellites. The numbers of fixed points are infinite.
What the GPS does to work, is form many triangles with different pairs of satellites.
Each satellite has an atomic clock onboard and regularly transmits a unique radio frequency signal simultaneously with all other Global Positioning Satellites. The RF signals travel out across space in all directions.
The speed at which an RF signal travels in space is approximately 186,000 miles or 300,000 kilometers per second, the speed of light in a vacuum.
Each of these transmitted signals will reach the GPS antenna of the vehicle. The time taken for the RF signal to travel from each satellite and arrive at the vehicle is dependant on the distance each satellite is from the vehicle. The greater the distance the longer the time taken.
The vehicle’s onboard GPS system needs to know three things to determine the location of the vehicle.
The time it takes for the signal to travel from the satellites to the vehicle.
The location of each satellite
And accurate time.
Given these facts, enough information is available to form a three dimensional figure of a pyramid with a triangle base.
The base of the pyramid is formed by the location of the satellites and the apex of the pyramid is the location of the vehicle on the Earth, a point derived from triangulation of the known points of the base.
The GPS equipment knows that all of the apexes (the position of the GPS) of each triangle must be in the same literal position.
Adding more satellites makes this more accurate because it gives the GPS more triangles to work with.
Satellites are positioned so that every location of the earth’s surface has access to four satellites 24hrs a day.
Satellites rely on the use of solar energy for their operating power and to maintain position. As the earth and moon move through their orbits, varying gravitational forces are placed on the satellites. The system allows for accurate positioning ensuring the satellites do not move from their correct location.
Inaccuracies caused by weather conditions do occur as the RF signal moves into the earth’s atmosphere it travels through air, with varying levels of density. This slows the speed at which the RF signals travel.
Inaccuracies also occur when RF signals are reflected off objects such as large buildings. Solar radiation can also cause inaccuracies.
