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magellan sportrak map handheld gpsThe GPS or Global Positioning System is a global navigation system used for military and civilian applications alike.
More accurate GPS receivers are used these days in surveying to accurately locate boundaries, structures and so on. Commercial civilian GPS receivers are purposefully less accurate than they might be. In specific, they are required to have limits on the velocities and altitudes at which they will report coordinates in order to prevent them from being used to create improvised missiles. Each satellite repeatedly re-broadcasts the exact time according to its internal atomic clock along with a digital data packet that includes the satellite's precise position, satellite status messages, and an almanac of the approximate position of every other active GPS satellite. The almanac lets GPS receivers use data from the strongest satellite signal to locate other satellites. The United States Department of Defense developed the system, officially named NAVSTAR GPS (Navigation Signal Timing and Ranging GPS), and the satellites are managed by the 50th Space Wing at Schriever Air Force Base. Although the cost of maintaining the system is significant, GPS is available for free use in civilian and commercial applications. GPS receivers calculate their current position (latitude, longitude, elevation), and the precise time, using the process of trilateration after measuring the distance to at least four satellites by comparing the satellites' coded time signal transmissions. GPS was first made available for commercial applications in 1983, after the Soviet Air Force shot down the civilian airliner KAL 007 in restricted Soviet airspace, killing all 269 people on board. During the Gulf War, the shortage of military GPS units and the wide availability of civilian ones among personnel resulted in disabling the Selective Availability. In the 1990s the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own, less accurate, radio navigation systems. The military resisted for most of the 1990s, but SA was eventually turned off in 2000 following an announcement by then US President Bill Clinton, allowing all users to enjoy nearly the same level of access. In early years, the US military made the civilian application purposefully less powerful than it could. But since August 2000, the GPS civilian application provides the accuracy of GPS signals to within 2 meters (6 ft). Fact is, GPS accuracy can be improved further, to about 1 cm (half an inch) over short distances, using techniques such as Differential GPS (DGPS). The first experimental Block-I GPS satellite was launched in February 1978. The GPS satellites were initially manufactured by Rockwell International and now manufactured by Lockheed Martin. GPS is short for (the) Global Positioning System, a satellite navigation system developed by the US Department of Defense. A fleet of more than two dozen GPS satellites broadcasts precise timing signals to GPS receivers, allowing them to accurately determine location anywhere on Earth. In practice, this means that using a GPS device, you can find out where you are in terms of longitude, latitude and altitude. Selective Availablity typically added signal errors of up to about 10 m horizontally and 30 m vertically. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance the entire eastern US area might read 30 m off, but 30 m off everywhere and in the same direction. In order to improve the usefulness of GPS for civilian navigation, Differential GPS was used by many civilian GPS receivers to greatly improve accuracy. Atomic clocks on the GPS satellites are set to "GPS time", which is the number of seconds since 00:00:00 UTC, January 6, 1980. UTC is short for Coordinated Universal Time. Today, GPS time is 14 seconds ahead of UTC, because it does not follow leap seconds. That's why new GPS units initially show the incorrect time after achieving a GPS lock for the first time. However, this is usually corrected on the display within 15 minutes once the UTC offset message is received for the first time. GPS derives in part from its predecssor, the LORAN navigation system. LORAN was designed for ship and airplane navigation in the 1940s for use during World War II. In 1985, ten experimental Block-I satellites were up. A complete "constellation" of 24 satellites was in orbit by January 17, 1994.
1. GPS Units
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