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comparison gpsThe GPS or Global Positioning System is a global navigation system used for military and civilian applications alike.
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. 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. More accurate GPS receivers are used these days in surveying to accurately locate boundaries, structures and so on. Because GPS is as much an accurate time keeper as a locator, one of the most common applications for GPS units is as a reference clock for time code generators or NTP clocks. For instance, when monitoring earthquakes, each seismic sensor can be synched with the GPS system to provide a synchronized, precise time source for measurements. The most recent launch was in September 2005. The oldest GPS satellite still in operation was launched in February 1989. GPS allows the military to accurately target its missile arsenal, largely composed of cruise missiles and precision-guided munitions. This was born out by the use of SOFLAM (Special Operations Forces Laser Acquisition Markers) targeting during the attacks on Tora Bora in Afghanistan. It also improves the accuracy of the US submarine launched ballistic missiles by providing precise locational information to the submarine commanders. Finally, command and control over troops is improved because commanders know precisely where their troops and the enemy are located on the battlefield. 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. 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. 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. 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. 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. One of biggest problems for GPS accuracy is that changing atmospheric conditions change the speed of the GPS signals unpredictably as they pass through the ionosphere. The effect is minimized when the satellite is directly overhead and becomes greater toward the horizon, as the satellite signals must travel through the greater "thickness" of the ionosphere as the angle increases. Once the receiver's rough location is known, an internal mathematical model can be used to estimate and correct for the error. 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. 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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