Global Positioning System

I

Introduction

Global Positioning System (GPS), space-based radio navigation system, consisting of 24 satellites and ground support, that provides accurate, three-dimensional position, velocity, and time, 24 hours a day, everywhere in the world, and in all weather conditions. Because the user does not communicate to the satellite, GPS serves an unlimited number of users.

II

History and Development

Operated and maintained by the United States Department of Defense, the Navstar Global Positioning System was initiated in 1973 to reduce the proliferation of navigational aids. By creating a system that overcame the limitations of many existing navigation systems, GPS became attractive to a broad spectrum of users. Since the earliest satellites, it has successfully proven itself in classical navigation applications; because its capabilities are obtainable in small, inexpensive equipment, GPS has fostered many new applications.

III

Capabilities

GPS is available in two basic forms: Standard Positioning Service (SPS) and Precise Positioning Service (PPS). SPS provides a horizontal position that is accurate to 100 m (109 yds). PPS horizontal accuracy is 20 m (22 yds). For authorized users, usually the US military and its allies, PPS also provides greater resistance to jamming and immunity to deceptive signals (antispoofing).

Enhanced techniques such as differential GPS (DGPS) and use of the carrier frequency enable DGPS users to achieve 3 m (10 ft) horizontal accuracy. Surveyors pioneered the use of carrier methods to compute positions to within 1 cm. SPS, DGPS, and carrier techniques are available to all users.

IV

How GPS Works

GPS satellites carry atomic clocks that measure time to a high degree of accuracy. The time information is placed in the codes broadcast by the satellite so that a receiver can continuously determine the time the signal was broadcast. The signal contains data that a receiver uses to compute the locations of the satellites and to make other adjustments needed for accurate positioning. The receiver uses the time difference between the time of signal reception and the broadcast time to compute the range to the satellite. The receiver must account for propagation delays caused by the ionosphere and the troposphere. With three ranges to three satellites and knowing the location of the satellite when the signal was sent, the receiver can compute its three-dimensional position.

To compute ranges directly, however, the user must have an atomic clock synchronized to the global positioning system. By taking a measurement from an additional satellite, the receiver avoids the need for an atomic clock. The result is that the receiver uses four satellites to compute latitude, longitude, altitude, and time.