The GPS or Global Positioning System is a globalnavigation system used for military and civilian applications alike.
As a positioning system, GPS allows users to map waypoints, tracking their position on a map to within a few meters of their exact location. Civilian use GPS systems are made by companies such as Magellan, Garmin and Trimble.
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.
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 US military has developed the ability to locally deny GPS (and other navigation services) to hostile forces in a specific area of crisis without affecting the rest of the world or its own military systems. Such Navigation Warfare uses techniques such as local jamming to replace the blunt, world-wide degradation of civilian GPS service that SA represented.
The most recent launch was in September 2005. The oldest GPS satellite still in operation was launched in February 1989.
The author would have loved to have used GPS years ago in college. We had a class where we used maps and compasses. We were given precise coordinates and had to plot a course through woods and streams and swamps to find and sign in at specific latitude and longitude coordinates. Today, this sport, now called "geocaching", is a popular activity, particularly for outdoor hiking enthusiasts.
In 1996, President Bill Clinton issued a policy directive declaring GPS to be a dual-use system, meaning that it could be made avilable for a wide range of civilian and military purposes.
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.
GPS signals can also be affected by multipath reflections of the radio signals off the ground and/or surrounding structures (buildings, canyon walls, etc). For long delay multipath signals, the receiver itself can filter the signals out. A variety of receiver techniques, most notably Narrow Correlator spacing, have been developed to mitigate multipath errors.
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