The global positioning system is arguably one of the most important navigation innovations of the 20th century. The satellite network allows for anyone with a receiver to pinpoint his location on Earth to within approximately 225 feet. This said, the system does have its limitations and is not always 100 percent accurate.
A GPS receiver needs to get a signal from at least four satellites to determine its location. However, four is the absolute minimum and the reported location may not be accurate. Reception of more satellites by the receiver will result in increased accuracy. Signal strength also plays a part in accuracy too; weak signals lead to errors in pinpointing the receiver’s location.
The location of the GPS satellites affects accuracy and operation of a GPS receiver, as the system uses a process known as triangulation to determine location. Triangulation requires that the satellites be in different directions from the perspective of the receiver. Each of these satellites then report the location of the receiver as it sees it, which the receiver then averages out. If the receiver only sees satellites from one direction, triangulation will work poorly if at all.
Geography and Weather
Environmental factors limit the effectiveness of GPS. Weather conditions such as snow, rain or fog can degrade a GPS signal. Tall buildings like those found in cities can block a path to satellites altogether, and geographical features like valleys, canyons or tunnels causes a condition known as multipath. Multipath occurs when the signal bounces off an obstruction, which causes the receiver to detect the satellite as being further away than it is in reality.
Civilian use of the GPS system comes with a built-in error. The U.S. Department of Defense does this for security reasons, and it adds at least a ten meter error on top of any other issues due to preexisting conditions. Errors can also occur due to the slowing down of the transmissions as they pass through the Earth’s atmosphere. While receivers typically compensate for this issue through built-in error correction, it is not foolproof.
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