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.
In 1985, ten experimental Block-I satellites were up. A complete "constellation" of 24 satellites was in orbit by January 17, 1994.
More accurate GPS receivers are used these days in surveying to accurately locate boundaries, structures and so on.
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.
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.
Hand-held GPS units are often used by hikers and mountain climbers to plot their courses and to accurately reflect exactly where they are. The author once used GPS to organize a helicopter evacuation of an injured motorcycle rider in a remote region of the La Sal mountains in Utah. A reading was taken, coordinates given, and the pilot was able to find us with ease.
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.
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 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.
GPS is coming into common use in cars these days. Most new cars allow you to purchase GPS technology that will help you plot your trip from one location to the next.
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 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 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.