How GPS Receivers Work

Global Positioning System (GPS) receivers are compact hand-held or vehicle-mounted units designed to receive and process data from the array of special-purpose satellites placed in geosynchronous orbit by the United States military to aid in land, sea, and aerial navigation. The Global Positioning System was designed to provide an accurate means of determining one’s exact location on the earth in terms of latitude and longitude coordinates.

Before the advent of the Global Positioning System, position fixes could only be determined by celestial navigation, which requires measurements using an instrument called a sextant taken by visual sights of celestial bodies such as the sun, certain stars, or other planets. Angle measurements taken by the sextant were then processed by a complicated series of mathematical measurements from which a position fix could be determined. Sights of celestial bodies could not be taken with a sextant during overcast weather, and the complex math required introduced human error that could put a position fix off by several hundred feet, or at worst, many miles.

The Global Positioning System works like this: Satellites placed into geosynchronous orbit remain fixed at a predetermined altitude and position above the earth as it rotates so that radio signals transmitted from them can be used to determine the location of the receiver on earth. This requires the reception of a signal from at least three satellites, all of which are in different locations above the earth’s surface.

These three intersecting signals can then be used in a process called triangulation to determine the exact latitude and longitude of the receiver. The math required to work out this data is performed almost instantly by the built-in computer in the GPS receiver, so that accurate positions can be determined even in a fast-moving vehicle, such as an airplane.

A total of twenty-four such satellites in geosynchronous orbit assures worldwide coverage of the system, and with an average consumer-grade receiver, position fixes are accurate to within about 15 feet. Specialized GPS receivers used by surveyors can pinpoint the head of a nail in the ground to an accuracy measured in hundredths of an inch.

GPS receivers have revolutionized navigation both at sea and on land. Boaters and hikers can go anywhere they wish with confidence that they can find their way back home even at night and in bad weather conditions. Pre-determined locations, called waypoints can be programmed into the GPS receiver, and simply pushing the Go-to button will tell the user how far away each waypoint is and also give a compass bearing showing which way to steer to reach that waypoint.

Waypoints can also be created at any point along the way, forming electronic breadcrumbs that allow the user to retrace his route back exactly the way he or she came in. GPS has countless uses in the military, industrial, and recreational sectors. Earlier bugs have been worked out of the system, and dependability is assured and reliable. This dependability has led to the exclusive use of GPS for navigation on board both military and merchant ships, and celestial navigation with the sextant is practically a relic of the past.

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