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GPS is managed by an Interagency GPS Executive Board. The US Air Force is designated as the executive service for system management. The system is operated and controlled by the 50th Space Wing’s (Air Force Space Command) 2nd Space Operations Squadron, Schriever Air Force Base, Colorado. The Space and Missile Systems Center’s (Air Force Space Command) Navstar GPS Joint Program Office, Los Angeles Air Force Base, California, is the DoD agency for acquiring GPS satellites, ground systems and military user equipment. Delta II expendable launch vehicles are used to launch the GPS satellites from Cape Canaveral Air Station, FL, into six planes of circular 20,200 km / 12-hour orbits . GPS provides the following:
The signals are so accurate that time can be figured to within 100 nanoseconds, velocity can be figured to within a fraction of a km per hour, and location can be figured to within meters. GPS satellites orbit the earth every 12 hours emitting continuous navigation signals on two different L-band frequencies, L1 and L2. Fundamental research on timing and navigation technology for Navstar was conducted by the Timation satellite series in the 1960's. There followed four generations of GPS satellites: the Block I, the Block 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, IIA and IIR satellites made up the constellation by the turn of the century. A total of 28 II/IIAs were put on contract, with the last four tagged as replacements for earlier satellites reaching the end of their service life. Block IIRs replaced older Block II/IIAs as they wear out. Block IIF fourth generation satellites were planned to begin flying in 2005. The Joint Program Office Advanced Plans Branch is actively studying the opportunity to improve accuracy, availability, integrity, and survivability to meet emerging military and civil needs through 2030. The next block of satellites, called GPS III, would continue to deliver the new civil signals and improved military codes that were initiated on the GPS IIR-M and IIF programs. The first launch of research and development GPS III satellites was not expected before 2010. Navstar continues to perform as the world’s premier positioning and navigation systems. Endeavors such 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 User Equipment is being integrated into nearly all facets of modern warfare. Forward air controllers, pilots, tank drivers, and ground troops all use GPS to help ensure victory on the battlefield. GPS User Equipment consists of the receivers that use the signal from GPS satellites to compute position, time, and velocity primarily for military users. GPS receivers are integrated into every type of system used by the DoD: aircraft, spacecraft, munitions, ground vehicles and ships. Stand-alone, hand-held receivers for individual use are also widespread among all branches of the Department. These receivers provide real-time situational awareness for the warfighter much more quickly and accurately than previous, conventional methods. GPS-guided munitions showcase this increased capability and are delivered with unprecedented accuracy, thus improving crew survivability while decreasing the number of weapons required to achieve military objectives. Inherent to the future of military search and rescue is the Combat Survivor Evader Locator (CSEL). The CSEL system is a worldwide architecture of UHF ground stations, Joint Search and Rescue C2 systems and will include 46,000 robust handheld radios. The radios communicate through military, national and civil satellites for use by joint-service aircrews, including special operations. Designed to take the “search” out of “search and rescue,” CSEL acquires a precise GPS derived location and communicates directly with rescue forces and, perhaps just as important, communicates to the survivor/evader that rescue is imminent. 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. GPS Development In the late 1950’s, scientists at John Hopkins University developed a way to use radio signals originating from a satellite in space to provide accurate position updates to navigation equipment located on the US Navy’s ships and submarines. By the mid-1960’s, the Air Force initiated a program consisting of several satellites with very accurate clocks onboard that could give off timing signals of their location in space which would accurately determine the position of a vehicle moving on land or in the air. In 1973, the Navy and the Air Force programs combined to form the Navigation Technology Program, which later evolved into the Navstar Global Positioning System (GPS) in operation today. Navstar GPS became the world’s premier position, navigation and timing information service. Civilian users around the globe rely upon Navstar GPS as well. In fact, the system serves millions of civil users with over a 1.4 million handheld and vehicle-mounted GPS receivers having been produced each year since 1997. The rapidly growing GPS market, including equipment and applications, reached $6.2 billion in 2000 and was expected to surpass $50 billion by 2010. The fundamental concept of GPS is to use simultaneous distance measurements from four satellites to compute the position and time of any receiver on or above the Earth’s surface. The GPS satellites broadcast signals on two different frequencies so that sophisticated user receivers can correct for distortion effects due to the ionosphere, a layer of the atmosphere several hundred miles above the Earth. It takes between 65 and 85 milliseconds for a signal to travel from a GPS satellite to a receiver on the surface of the Earth. The signals are so accurate that time can be figured to much less than a millionth of a second, velocity can be figured to within a fraction of a km per hour, and location can be figured to within a few meters. Typical horizontal positioning accuracy for military users is 7 to 10 meters. Prior to the mid-1990's a purposely 'degraded signal' was provided for civilian users, limiting their accuracy to 70 to 100 meters. This was eliminated by order of President Clinton as it became obvious that civilian and commercial users had many unimagined uses for precise location and timing information as well. Each Block II or IIA satellite has two cesium atomic clocks and two rubidium atomic clocks, while each Block IIR has three rubidium atomic clocks. The stability of these clocks is estimated to be approximately 1 second per 300,000 years. Only one clock is in use on each satellite at a time. The others are backups. The cost to the Air Force in 1973 – 2000 to develop and procure the GPS satellites (not including military user equipment or launch costs) was approximately $5.6 billion in 'Year 2000' dollars. The approximate annual cost to operate and maintain the constellation, including research and development and procurement of new satellites, is $750 million. The GPS worldwide satellite network consists of six monitor stations and four ground antenna stations. The monitor stations (located at Ascension Island, Cape Canaveral, Colorado Springs, Diego Garcia, Kwajalein and Hawaii) use specially-designed GPS receivers to passively track the navigation signals of all of the satellites. Data from the monitor stations is continually sent to the GPS Master Control Station (MCS), located at Schriever Air Force Base, Colorado, for processing. The MCS computes precise, updated information on the satellites’ orbits and clock status every 15 minutes, 24 hours a day, seven days a week. Updated navigation information is sent from the MCS to the ground antenna stations (located at Ascension Island, Cape Canaveral, Diego Garcia and Kwajalein) and then to the satellites. These “uploads” are done once or twice per day for each satellite. This is sufficient to maintain the high accuracy of the GPS constellation. By the year 2000 civil users outnumber military users by 100 to 1 and the ratio was increasing. The Compound Annual Growth Rate of the GPS market was growing by approximately 22%. GPS satellites broadcast on two links to users: L1 and L2. L1 is transmitted at a frequency of 1575.42 MHz, and carries both a Coarse/Acquisition (C/A) ranging code and a Precise (P(Y)) ranging code. L2 is transmitted at a frequency of 1227.6 MHz and carries only the P(Y) code. Only the C/A code on L1 was originally available to all users. The military users have access to both the C/A code on L1 and the P(Y) code on L1 and L2. To access GPS for general use, a civilian user may purchase a hand-held or vehicle-mounted GPS receiver. The civil signal is free to all users worldwide. No subscription, license, fee or registration is required. The future of plans for GPS included increased power and accuracy, as well as increased civil navigation safety with the addition of a new civil signal on the L2 link and a new civil-only signal on a new link, L5, to be broadcast at 1176.45 MHz. A new military-only signal (M-code) on the L1 and L2 links will be fully operational in 2010. It will have increased power and reduced vulnerability to signal jamming. Major Events:
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