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.
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.
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.
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.
More accurate GPS receivers are used these days in surveying to accurately locate boundaries, structures and so on.
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.
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.
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.
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 come in a variety of consumer formats, and today can be found in cars, watches, phones boats and planes. Major manufacturers include Trimble, Garmin and Leica, low-end, hand-held consumer units are often available at less than $100 per unit.
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.
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.
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.
The most recent launch was in September 2005. The oldest GPS satellite still in operation was launched in February 1989.