Global Positioning

September 1998

Satellites and Global Positioning

The Global Positional System (GPS) is a U.S. space-based technology which provides free, worldwide, full-time electronic services:

	Positioning - GPS receiver determines the latitude, longitude and altitude of the current location of the receiver's antenna.
	Navigation - GPS receiver guides a user to a location or along a route.

The U.S. GPS system is designed for 24 solar-powered satellites, traveling in such a manner that several are "visible" from any point on the earth at any time. The first satellite was launched in February 1978. By mid-1994, the constellation was complete. There are currently 27 GPS satellites in orbit, including three "spares".

The satellites orbit the earth at an altitude of 10, 900 nautical miles. This is below the height of geo-synchronous satellites, so each GPS spacecraft that is "visible" is actually passing overhead in an arc that crosses the equator at an angle of about 55 degrees. Each satellite has an orbit around the earth that is perceived by the earth observer to be about 11 hours, 56 minutes.

Each GPS spacecraft continually transmits navigation messages which include:

	satellite identity
	satellite estimated position
	information on time message was sent

The microwave messages travel at the speed of light to GPS receivers which are synchronized to the atomic clocks on the GPS satellites.

The basic GPS receiver consists of five parts:

	an antenna (whose position the receiver reports),
	electronics to receive the satellite signals,
	a microcomputer to process the data, determine the antenna position, and record position values,
	controls to provide user input, and
	a screen to display information to the user.

The GPS receiver can accurately calculate how long it took for the messages to arrive. The time is then converted to a distance to estimate how far the antenna is from the satellite.

A minimum of three satellites are needed to calculate a position on the surface of the earth (latitude and longitude). Altitude requires a fourth satellite. Up to eight satellites may be "visible" to the antenna at any one time. The use of more satellites may improve the accuracy of the position solution.

The accuracy of the solution is affected by several factors:

	satellite clock and position errors,
	receiver clock errors,
	effects of the upper atmosphere (changes the speed of the microwave signals)
	multipath errors (microwave signal "bounced" off another object before reaching the receiver)
	the number of satellites in view, and
	the geometry, or arrangement, of satellites in the sky (those low on the horizon are less accurate than those overhead; satellites that appear close together are less accurate than those spread apart)

The error for any calculated location is generally quoted as +/-40 meters (+/- 130 feet). While any one location solution may be closer (more accurate) than this, if you were to stand in one location and collect a large number of position solutions, you would find that the most extreme are up to 130 feet away from your location.

To complicate the accuracy picture, the Department of Defense introduces intentional random errors in the satellite signal (called Selective Availability) which increase the general error to +/- 100 meters (330 feet). To compensate for these errors, the industry has developed a concept called Differential Global Positioning that is intended to help correct these errors.

This page was last updated on 10/08/01.