Lockheed Martin technicians assemble a GPS Block 2R satellite. Photo: Lockheed Martin

The NAVSTAR Global Positioning System is managed by the NAVSTAR GPS Joint Program Office at the Space and Missile Systems Center, Los Angeles Air Force Base, Calif.

NAVSTAR GPS is a space-based radio-positioning system nominally consisting of a constellation of 24 orbiting satellites that provide navigation and timing information to military and civilian users worldwide. In addition to the satellites, the system consists of a worldwide satellite control network and GPS receiver units that pick up signals from the satellites and translate them into position information.

Delta 2 expendable launch vehicles are used to launch the GPS satellites from Cape Canaveral Air Force Station, Fla., into six circular orbits of nearly 11,000 nautical miles.

GPS provides the following:

• 24-hour, worldwide service
• Extremely accurate, three-dimensional location information (providing latitude, longitude, and altitude readings)
• Extremely accurate velocity information
• Precise timing services
• A worldwide common grid that is easily converted to any local grid
• Continuous real-time information
• Accessibility to an unlimited number of worldwide users
• Civilian user support at a slightly less accurate level

GPS satellites orbit the earth every 12 hours, emitting continuous navigation signals on two different L-band frequencies. The signals are so accurate, time can be figured to within a millionth of a second, velocity can be figured to within a fraction of a mile per hour, and location can be figured to within meters. Positioning accuracy for military users is nominally 16 meters, while accuracy for civilian users is nominally 100m.

There are four generations of the GPS satellite: the Block 1, the Block 2/2A, the Block 2R, and the Block 2F. Block 1 satellites were used to test the principles of the Global Positioning System, and lessons learned from these 11 satellites were incorporated into later blocks. Block 2 and 2A satellites make up the current constellation. A total of 28 were put on contract, with the last four tagged as replacements for earlier satellites reaching the end of their service life.

Block 2R satellites boast dramatic improvements over the previous blocks of satellites, and will have the ability to determine their own position by performing inter-satellite ranging with other 2R vehicles, reprogrammable satellite processors enabling problem fixes and upgrades in flight, and increased satellite autonomy and radiation hardness.

Additionally, the Block 2R has the ability to be launched into any of the required GPS orbits at any time with a 60-day advanced notice and requires many fewer ground contacts to maintain the constellation. All of these improvements result in increased accuracy for GPS users at a cost of 33 percent less per satellite than the previous generation of Block 2A satellites. Block 2R satellites will replace Block 2/2A satellites as they reach the end of their service life and will be launched through the year 2003.

Block 2F satellites are the fourth generation of the navigation satellite and will be used as sustainment vehicles. Improvements in the Block 2F over previous satellites include a design life of 12.7 years and a dramatic increase in the growth space for additional payloads and missions.

U.S. military forces use a GPS signal receiver to determine their position. Photo: USAF

GPS significantly outperforms other position and navigation systems, and it does so with greater accuracy and at a lower cost. Such endeavors as mapping, aerial refueling, rendezvous operations, geodetic surveying, and search and rescue operations have all benefited greatly from GPS's accuracy.

Using lessons learned from Operations Desert Shield and Desert Storm, GPS is being integrated into nearly all facets of the modern battlefield. Forward air controllers, pilots, tank drivers, and ground troops all use GPS to help ensure victory on the battlefield.

The GPS worldwide satellite control system consists of five monitor stations and four ground antennas. The monitor stations use GPS receivers to passively track the navigation signals of all the satellites. Information from the monitor stations is then processed at the master control stations, operated by the 2nd Space Operations Squadron at Schriever Air Force Base, Colo., and used to very accurately update the satellites' navigation messages. Updated navigation information is sent to the GPS satellites from the Master Control Station at Schriever Air Force Base through ground antennas using an S-band signal. The ground antennas are also used to transmit commands to satellites and to receive the satellites' state-of-the-art telemetry data.

As a service to GPS users, the Department of Transportation has established the "Navigation Information Service" (formerly "GPS Information Service") as a point of contact for civil GPS users. Operated and maintained by the United States Coast Guard, the NIS can be reached at (703) 313-5900, seven days a week, 24 hours a day.