Global Positioning System

Overview

The Global Positioning System, usually called GPS (the US military refers to it as NAVSTAR), is a satellite navigation system used for determining one's precise location and providing a highly accurate time reference almost anywhere on Earth or in Earth orbit. It uses an intermediate circular orbit (ICO) satellite constellation of at least 24 satellites.

Over fifty GPS satellites such as this NAVSTAR on the right have been launched since 1978.

The GPS system was designed by and is controlled by the United States Department of Defense and can be used by anyone, free of charge. The GPS system is divided into three segments: space, control, and user. The space segment is comprised of the GPS satellite constellation. The control segment is comprised of ground stations around the world that are responsible for monitoring the flight paths of the GPS satellites, synchronizing the satellites' onboard atomic clocks, and uploading almanac data for transmission by the satellites. The user segment is comprised of GPS receivers used for both military and civilian applications. A GPS receiver (GPSR) decodes time signal transmissions from multiple satellites and calculates its position by trilateration.

(Trilateration is a method of determining the relative position of objects using the geometry of triangles in a similar fashion as triangulation. Unlike triangulation, which uses both distances and angles to calculate the subject's location, trilateration uses the known distances between two or more reference points, and the measured distance between the subject and each reference point. To accurately determine the relative location of a point on a 2D plane using trilateration alone, at least 3 reference points are needed.)

The cost of maintaining the system is approximately US$400 million per year, including the replacement of aging satellites. The first of 24 satellites that form the current GPS constellation (Block II) was placed into orbit on February 14, 1989. The 50th GPS satellite since the beginning in 1978 was launched March 21, 2004 aboard a Delta II rocket.

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