Global Positioning System (GPS)

GPS satellite (MEO satellite that circle the earth twice a day) continually transmits messages containing the time the message was sent. These signals travel at the speed of light through outer space, and slightly slower through the atmosphere. The receiver uses the arrival time of each signal to calculate its distance from satellite. This information is then used to estimate the position of the GPS receiver as the intersection of sphere surfaces (use triangulation concept).

The earth has three dimensions; normally we only need three satellites to cover the entire earth surface. But in GPS, it is enough only to calculate 2D position (latitude and longitude) and track movement, because three satellites usually intersect in two points.

For 3D position, four or more satellites are needed. The fourth satellite is needed to determine which intersect is the GPS receiver position. Also, it is use to correct the GPS receiver clock. Once the user position has been determined, the GPS unit can calculate other information, such as speed, track, trip distance, distance to destination, sunrise and sunset time and a lot more.

GPS position determination is based on a concept termed time of arrival ranging. There are two type of GPS position determination. Lower precision application such as navigation use code phase pseudo range. While in geodetic surveying, carrier phase pseudo range are used as it allows high precision.