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 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 1985, ten experimental Block-I satellites were up. A complete "constellation" of 24 satellites was in orbit by January 17, 1994.
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.
When it was first deployed, GPS included a feature called Selective Availability (or SA) that introduced intentional errors of up to a hundred meters into the publicly available navigation signals, making it difficult to use for guiding long range missiles to precise targets. Additional accuracy was available in the signal, but in an encrypted form that was only available to the United States military, its allies and a few others, mostly government users.
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 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 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.
The GPS system uses a satellite constellation of 24 satellites in intermediate circular orbits. The orbits are designed so at least four satellites are always within line of sight from almost any place on earth. The constellation also includes three spare satellites in orbit.
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.
Most ships and airplanes are equipped with GPS systems so that they can accurately plot on a map where they are. In planes, this is especially useful during bad weather, and in ships in knowing where shallow water and other sunken obstacles may lie.
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.
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.
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