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.
In 1998, Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety.
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 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.
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.
In 1985, ten experimental Block-I satellites were up. A complete "constellation" of 24 satellites was in orbit by January 17, 1994.
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).
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.
The most recent launch was in September 2005. The oldest GPS satellite still in operation was launched in February 1989.
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.
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.
GPS has become a vital global utility, indispensable for modern navigation on land, sea, and air around the world, as well as an important tool for map-making, and land surveying. GPS also provides an extremely precise time reference, required for telecommunications and some scientific research, including the study of earthquakes.
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