NAVSTAR 

GPS consists of a group of 24 NAVSTAR satellites launched on behalf of the United States Department of Defence that provides highly accurate, worldwide positioning and navigation information. 

GPS satellites orbit in very precise orbits about 19,000 kilometres (12,000 miles) above the earth's surface. For comparison, the space shuttle orbits at about 300 kilometres (200 miles) and geo-synchronous telephone and data satellites orbit at about 39,000 kilometres (24,000 miles). There are often more than 24 operational satellites as new ones are launched to replace older satellites. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each twenty four hours (four minutes earlier each day). There are six orbital planes (with nominally four satellites in each), spaced 60 degrees apart, and inclined at about fifty-five degrees with respect to the equatorial plane. 

This constellation provides the user with between five and eight satellites visible from any point on the earth. Terrestrial GPS receivers simultaneously receive information from three to twelve satellites and, from that, determine the precise location of the receiver. GPS uses the triangulation of signals from the satellites to determine locations on earth. GPS satellites know their location in space and receivers can determine their distance from a satellite by using the travel time of a radio message from the satellite to the receiver. After calculating its relative position to at least three satellites with strong signals and geometrically well spaced, a GPS receiver can determine its latitude and longitude using triangulation. A fourth satellite will provide a calculation for height. Using successive positions, a GPS receiver can calculate speed and course. 

GPS satellites have four highly accurate atomic clocks on board. They also have an almanac of the current and expected positions for all of the satellites that are frequently updated from earth. That way when a GPS receiver locates one satellite, it can download all satellite location information, and find the remaining needed satellites much more quickly. When a GPS receiver is first turned on, it takes in the order of twenty minutes to receive this almanac and determine its position. Subsequent turn ons permit it to locate satellites in less than 30 seconds. Accuracy

Even with highly accurate atomic clocks, certain errors do creep into the process of determining the GPS receiver's position. Military user equipment provides a predictable positioning accuracy of at least 22 meters (95 percent) horizontally and 27.7 meters vertically (95 percent) and time transfer accuracy to UTC within 200 nanoseconds (95 percent). 

The most significant of the errors in positional accuracy is due to variations in the earth's ionosphere, which effects the speed of GPS radio signals. Another source of error is from water vapour in the troposphere. Multi-path problems where satellite transmissions bounce off large buildings, effectively lengthening the time a signal takes to reach the receiver also introduces errors, especially in the centre of a large city. The accuracy of GPS can be improved with DGPS (differential GPS) capabilities, which is the ability of your receiver to read signals from a nearby DGPS beacon receiver. with DGPS,  locations can be  determined to within 10 centimetres (6 inches). 

HOME PAGE

Military applications

The GPS system was initially set up for use by the military to provide precise positional information. GPS products are used by the military to coordinate and track the movement of soldiers and equipment in the field, to guide military ships at sea, and to provide position and navigation information to military aircraft. Commercial applications

GPS products have been developed for use for many commercial applications. These include surveying and mapping, aviation and marine navigation, vehicle tracking systems and mobile computer and cellular platforms. Surveying and mapping consist primarily of the collection and processing of position information and usually requires specialised GPS equipment. In the surveying market, applications include construction and engineering surveying, route surveying and geodetic research. Businesses and government agencies use GPS to track their vehicle locations using wireless communications. Some GPS receivers have been integrated into mobile radios, cellular phones and mobile data terminals to meet the needs of vehicle fleet managers.

HOME PAGE