GPS Systems FAQ and Explanation

GPS stands for the Global Positioning System. GPS is a navigational system that can accurately locate your position anywhere on the Earth. This technology is available to everyone, everywhere, day and night, and best of all, at no cost for use of the navigational data. GPS uses 24 satellites which are located 11,000 miles above the Earth. The satellites transmit data back to Earth and by locking onto this transmitted data, a GPS receiver can process this data to triangulate its precise location on the globe.

GPS operates 24 hours a day, in all weather conditions, and can be used worldwide for precise naviagtion on land, on water and even in the air. Some of its many current applications include: boating, fishing, hunting, scouting on land or from the air, hiking, camping, biking, rafting, pack trips by horseback, hot air ballooning, general aviation, snowmobiling and skiing, search and rescue, emergency vehicle tracking, 4 wheeling, highway driving and a host of other activities where accurate positioning is required.

How GPS Determines Your Position

Every point on Earth can be identified by a specific address. By using two sets of numbers, referred to as coordinates, which represent the exact spot where a horizontal line (latitude) crosses a vertical line (longitude), you can represent any location precisely. GPS receivers report and record your current position – or the position of any place you’ve been or would like to be – with latitude/longitude coordinates. GPS receivers also produce other critical navigation information, including heading, bearing, distance-to-go, time-to-go, and more – anytime, anywhere, in any weather.

The basis of GPS technology is precise time and position information. Using atomic clocks (accurate to within one second every 70,000 years) and location data, each satellite continuously broadcasts the time and its position. A GPS receiver uses signals from three or more satellites at once to determine the user’s position on earth.

By measuring the time interval between the transmission and the reception of a satellite signal, the GPS receiver calculates the distance between the user and each satellite. Using the distance measurements of at least three satellites in an algorithm computation, the GPS receiver arrives at an accurate position fix. Information must be received from three satellites in order to obtain two-dimensional (latitude and longitude) fixes, and four satellites are required for three-dimensional (latitude, longitude and altitude) positioning. The position information in a GPS receiver may be displayed as longitude/latitude, Universal Transverse Mercator, Military Grid or other system coordinates.

GPS Accuracy

The U.S. Department of Defense began development of the $12 billion GPS satellite navigation system in the 1970s to provide continuous, worldwide positioning and navigation data to U.S. military forces around the globe. However, GPS has even broader civilian applications. Position and navigation information is vital for many professional and personal activities, including boating, surveying, aviation, vehicle tracking and navigation, and more.

To meet these different needs, there were previously two levels of GPS services, one for civilian access and the second encrypted for exclusive military use. The civilian GPS signals were subjected to Selective Availability (SA) interference by the United States Government, which meant there were random errors in the data transmitted by the satellites to reduce the civilian GPS signal accuracy to 100 meters. However, on May 1, 2000, the U.S. government removed SA from GPS signals, which resulted in ten times greater accuracy for public users of GPS – position fixes that are usually within 10 meters.

Differental GPS (DGPS)

Differental GPS, or DGPS, has been developed to improve GPS accuracy to within a few meters. DGPS was originally initiated by the U.S. Coast Guard to counter the accuracy degradation caused by Selective Availability. Even with S/A now eliminated, DGPS continues to be a key tool for highly precise navigation on land and sea. DGPS technology adds a land-based reference receiver - located at an accurately surveyed site - to the other GPS components. This non-moving DGPS reference station knows where the satellites are located in space at any given moment, as well as its own exact location. This allows the station to compute theorectical distance and signal travel times between itself and each satellite. When those theoretical measurements are compared to actual satellite transmissions, any differences represent the error in the satellite's signal. All the DGPS reference station has to do is transmit the error factors to your DGPS receiver, which gives the information to the GPS receiver so it can use the data to correct its own measurements and calculations.

The two most common sources of corrective DGPS signals currently are: (1) Coast Guard, land-based beacon transmitters, broadcasting the data at no charge to the public, covering all coastal areas and much of the inland USA as well; and (2) FM radio sub carrier transmissions available both in coastal and inland areas, but limited to paid subscribers. In order to receive DGPS correction data from Coast Guard beacon transmitters, a mobile GPS unit requires a separate beacon receiver. And to receive FM sub carrier DGPS signals from local subscriber radio stations, the GPS unit requires a separate FM receiver, normally the size of a pager. Naturally, your GPS unit must have the capability to both receive and process DGPS data.