INTRODUCTION

The Global Positioning System, usually called GPS (the US military refers to it as NAVSTAR), is an intermediate circular orbit (ICO) 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.

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

GPS HISTORY

The initial concept of GPS began to take form soon after the launch of Sputnik in 1957. ".... Some scientists and engineers realized that radio transmissions from a satellite in a well-defined orbit could indicate the position of a receiver on the ground" This knowledge resulted in the U.S. Navy's development and use of the "transit" system in the 1960's. This system, however, proved to be cumbersome to use and limited in terms of positioning accuracy.


Starting in the mid-1970s the U.S. Department of Defense (DOD) began the construction of today's GPS and has funded, operated, and maintained control of the system it developed. Eventually $12 billion dollars would take GPS from concept to completion. Full Operational Capacity (FOC) of GPS was reached on July 17, 1995 (U.S.C.G., 1996, www). At one point GPS was renamed NAVSTAR. This name, however, seems to be regularly ignored by system users and others. Although the primary use of GPS was thought to be for classified military operations, provisions were made for civilian use of the system. National security reasons, however, would require that civilian access to accurate positioning be intentionally degraded.



GPS ELEMENTS

GPS was designed as a system of radio navigation that utilizes "ranging" -- the measurement of distances to several satellites -- for determining location on ground, sea, or in the air. The system basically works by using radio frequencies for the broadcast of satellite positions and time. With an antenna and receiver a user can access these radio signals and process the information contained within to determine the "range", or distance, to the satellites. Such distances represent the radius of an imaginary sphere surrounding each satellite. With four or more known satellite positions the users' processor can determine a single intersection of these spheres and thus the positions of the receiver . The system is generally comprised of three segments:
1. The space segment
2. The control segment
3. The user segment

SPACE SEGMENT

The space segment consists of 24 satellites, each in its own orbit 11,000 nautical miles above the Earth. The user segment consists of receivers, which you can hold in users' hands or mount in users' vehicle. The control segment consists of ground stations located around the world that make sure the satellites are working properly.

The GPS space segment uses a total of 24 satellites in a constellation of six orbiting planes. This configuration provides for at least four equally- spaced satellites within each of the six orbital planes. The orbital path is continuous in relation to the earth, meaning that a satellite's orbit will follow the same path on the earth with each orbit. At 10,900nm (20,200km) GPS satellites are able to complete one orbit around the earth every 12 hours. GPS satellites orbit at a 55-degree inclination to the equatorial plane. This space segment configuration provides for a minimum of 5 satellites to be in view from any place on earth, fulfilling the necessary four needed for three-dimensional positioning.