Global Postioning System (GPS)


What Is GPS?

The Global Positioning System is a constellation of satellites which orbit the earth twice a day, transmitting precise time and position information. With a GPS receiver, users can determine their location anywhere on earth. Position and navigation information is vital to a broad range of professional and personal activities, including boating, surveying, aviation, national defense, vehicle tracking and navigation, and more.

The complete system consists of 24 satellites orbiting about 12,000 miles above the earth, and five ground stations to monitor and manage the satellite constellation. These satellites provide 24-hour-a-day coverage for both two- and three-dimensional positioning anywhere on earth.

Development of the $10 billion GPS satellite navigation system was begun in the 1970s by the U.S. Department of Defense, which continues to manage the system, to provide continuous, worldwide positioning and navigation data to U.S. military forces around the globe. However, GPS has an even broader civilian, commercial application. To meet these needs, GPS offers two levels of service, one for civilian access and the second encrypted for exclusive military use. The GPS signals are available to an unlimited number of users simultaneously.

How Does GPS Work?

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 receives these signals, listening to 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.

The position information in a Magellan GPS receiver may be displayed as longitudeAatitude, Universal Transverse Mercator, Military Grid or other system coordinates. 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.

Each satellite continuously broadcasts two signals, L1 and L2. The L1 frequency contains the C/Acode which provides Standard Positioning Service (SPS) for worldwide civilian use. The encrypted P-code is broadcast on both the L1 and L2 frequency, resulting in the Precise Positioning Service (PPS) for military use. The SPS signal will provide a civilian user an accuracy of better than 25 meters. Because they are so accurate, civilian GPS receivers using the SPS signal are sometimes subjected to Selective Availability (SA) interference by the United States Government, to maintain optimum military effectiveness of the system. When engaged, SA inserts random errors in the data transmitted by the satellites. As a result, SPS signal accuracy can be reduced to 100 meters.

However, using a technique calledDifferential GPS (DGPS), the user can overcome the effect of SA interference and increase the overall accuracy of the GPS receiver. With DGPS, one GPS receiver unit is placed at a known location and the position information from that receiver is used to calculate corrections in the position data transmitted by the satellites. This corrected information is then transmitted to other GPS receivers in the area. The resulting real-time accuracy is in the S-to 10-meter range. Sub-meter (over) accuracy can be obtained by using DGPS and post-processing calculations in static positioning.

By 1991, GPS had achieved 24-hour-a-day worldwide 2D coverage and nearly $300 million in worldwide sales of GPS equipment. Global sales of GPS products exceeded one billion dollars in 1996. The U.S. Department of Commerce projects that GPS will become an eight-billion-dollar-a-year industry by the year 2,000 and employ as many as 100,000 people.

Who Uses GPS?Magellan Systems' GPS receivers provide positioning, velocity and navigation information for a variety of markets and purposes. Anyone who needs to know the precise time or the exact location of people or objects will benefit from GPS. This information also can be used in charting and mapping, plotting a course, navigating from point to point, tracking vehicle movement, locating previously identified sites or any number of similar functions.

> Magellan's GPS 2000 XLTM, GPS 3000 XLTM, Meridian XLTM, NAV DLX-10TM, NAV 1200XLTM, NAV 6000TM and NAV 6500TM marine GPS receivers are proving to be invaluable aids in both recreational and commercial boating and sailing. The company's Differential Beacon Receiver (DBR) and DBR-2 enable users to access differential GPS correction signals to achieve 5- to 10-meter DGPS accuracy with their Magellan GPS receivers.

> Land-based professions from survey to exploration employ GPS to provide vital positioning and location information. Surveyors, natural resource managers, wildlife managers, geologists, geographers, mappers, forestry managers, search and rescue teams, public safety professionals, archeologists, utility managers, and oil, gas and mineral explorers are just some of the people taking advantage of GPS with Magellan's ProMARK XTM series, FieldPRO VTM, and the MBS-2TM base station and DGPS reference receiver.

> The company's 10-channel GPS receiver module, the AIV- 10TM, is used by other equipment manufacturers in products requiring positioning, navigation or tracking features. Magellan's versatile OEM GPS receiver modules can be adapted to a variety of environments. Applications include avionics, fleet management, public safety vehicle dispatch, vehicle mapping displays and navigation systems, marine navigation plotters and autopilots, atmospheric balloon testing and marine buoy positioning.

> The Magellan GPS SkyNav 5000TM and portable SkyStar GPSTM and EC- 10XTM avionics receivers hav e been developed for the general aviation market to provide both fixed- and rotary-wing aircraft with position and navigation information, and assist in flight planning and operations.

> Magellan's GPS 2000 XL, GPS 4000 XL, Trailblazer XL, NAV 6000 and NAV 6500 receivers guide hunters, backpackers, fishermen, campers and other outdoor enthusiasts through backwoods deserts and wilderness areas around the world.

> In 1997, with the acquisition of Rockwell International's PathMaster vehicle navigation technology, Magellan launched its Driver Information Systems Group specializing in vehicle navigation, information and route guidance systems for private passenger automobiles and auto fleets.

If you are looking for coordinates in the Lower Laguna Madre area, here's a page full of positions that have been one of our most asked for documents ...

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