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 II expendable launch vehicles are used to launch the GPS satellites from Cape Canaveral Air Station, Fla., into six circular orbits of nearly 11,000 nautical miles.
GPS provides the following:
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 I, the Block II/IIA, the Block IIR, and the Block IIF. Block I 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 II and IIA 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 IIR 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 IIR vehicles, reprogrammable satellite processors enabling problem fixes and upgrades in flight, and increased satellite autonomy and radiation hardness.
Additionally, the Block IIR 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 IIA satellites. Block IIR satellites will replace Block II/IIA satellites as they reach the end of their service life and will be launched through the year 2003.
Block IIF satellites are the fourth generation of the navigation satellite and will be used as sustainment vehicles. Improvements in the Block IIF over previous satellites include a design life of 12.7 years and a dramatic increase in the growth space for additional payloads and missions.
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.
Weight (in orbit): 2,175 pounds
Orbit altitude: 10,988 nautical miles
Power source: solar panels generating 700 watts
Launch vehicle: Delta II
Dimensions: 5 feet wide, 17.5 feet long (including wing span)
Design life: 7.5 years
Weight (in orbit): 2370 pounds
Orbit altitude: 10,988 nautical miles
Power source: solar panels generating 1136 watts
Launch vehicle: Delta II
Dimensions: 5 feet wide, 6.33 feet in diameter, 6.25 feet high (38.025
feet wide including wing span)
Design life: 10 years
Weight (in orbit): 3758 pounds (accounts for 196 lbs for payload adapter
& 885 lbs for RAP & Flex)
Orbit altitude: 10,988 nautical miles
Power source: solar panels generating up to 2900 watts (BOL) & 2440
watts (EOL)
Launch vehicle: EELV (Delta IV and Atlas V)
Dimensions: 8 ft x 6.47 ft (stowed) 70.42 ft (deployed 4 panel solar
arrays) x 12 ft
Design life: 15 years
BLOCK I (Rockwell)
1974 Contract for eight Block I satellites
1978 Contract for three Block I satellites
BLOCK II/IIA (Rockwell)
1981 Contract for qualification satellite (GPS12)
1983 Contract for 28 Block II/IIA satellites
BLOCK IIR (Lockheed Martin)
1989 Contract for 21 Block IIR satellites
BLOCK IIF (Boeing-North American)
1996 Contract and options for 30 Block IIF satellites
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