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| The Global Positioning System (GPS) is a satellite-based radio navigation system initially developed and operated by the U.S. Department of Defense (DOD). In 1996, a Presidential Decision Directive, later passed into law, transferred "ownership" from DOD to an Interagency GPS Executive Board (IGEB), co-chaired by senior officials of the Departments of Transportation and Defense to provide management oversight to assure that GPS meets civil and military user requirements. GPS permits land, sea, and airborne users to determine their three-dimensional position, velocity, and time 24 hours a day, in all weather, anywhere in the world with a precision and accuracy far better than other radio navigation systems available today or in the foreseeable future. GPS receivers collect signals from satellites in view. They display the user's position, velocity, and time, as needed for their marine, terrestrial, or aeronautical applications. Some display additional data, such as distance and bearing to selected waypoints or digital charts. The GPS concept of operation is based upon satellite ranging. Users determine their position by measuring their distance from the group of satellites in space. The satellites act as precise reference points. Each GPS satellite transmits an accurate position and time signal. The user's receiver measures the time delay for the signal to reach the receiver, which is the direct measure of the apparent range (called a "pseudorange") to the satellite. Measurements collected simultaneously from four satellites are processed to solve for the three dimensions of position (latitude, longitude, and altitude) and time. |
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| The GPS system, at full Operational Capability (FOC), was designed for a minimum of 24 Satellites (21 active, 3 spare), with 4 in each orbital plane. This produces the design probability that at least 4 satellites will be in view to users worldwide, over any 24-hour period, with a Position Dilution Of Precision (PDOP) of six or less, at least 99.9 percent of the time. The exact number of satellites operating at any one particular time varies based on the number of satellite outages and operational spares on orbit. GPS is now used to support land, sea, and airborne navigation, surveying, geophysical exploration, mapping and geodesy, vehicle location systems, farming, transportation systems, and a wide variety of other additional applications. Telecommunication infrastructure applications include network timing and enhanced 911 for cellular users. Global delivery of precise and common time to fixed and mobile users is one of the most important, but least appreciated functions of GPS. |
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| It costs about $750 million annually to operate and maintain the constellation, including research and development, as well as procurement for and replacement of satellites. These numbers might seem high until you examine the economic impact of the GPS system on civilians and the economy in general. Over 1.4 million civilian GPS receivers have been produced each year since 1997. The economic impact of GPS technology is significant, reaching $6.2 billion in 2000, with expectations of surpassing $50 billion by 2010. .What is Differential GPS? DGPS is a technique used to improve GPS accuracy by incorporating error corrections provided by a GPS monitoring station. The monitoring station calculates the corrections by comparing its known location with that reported by GPS. The difference between the two represents a "differential correction" that can be applied to result in a more accurate position than that provided by GPS alone. .What is WAAS? You've heard the term WAAS, and maybe even know it stands for Wide Area Augmentation System. Okay, so what the heck is it? Basically, it's a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy. How much better? Try an average of up to five times better. A WAAS-capable receiver can give you a position accuracy of better than three meters 95 percent of the time. And you don't have to purchase additional receiving equipment or pay service fees to utilize WAAS. The origins of WAAS The Federal Aviation Administration (FAA) and the Department of Transportation (DOT) are developing the WAAS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA's navigation requirements for accuracy, integrity, and availability. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite. How it Works WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal. Benefiting from WAAS Currently, WAAS satellite coverage is only available in North America. There are no ground reference stations in South America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment, while DGPS does. Other governments are developing similar satellite-based differential systems. In Asia, it's the Japanese Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems. |
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100 meters: Accuracy of the original GPS system, which was subject to accuracy degradation under the government-imposed Selective Availability (SA) program. 15 meters: Typical GPS position accuracy without SA. 3-5 meters: Typical differential GPS (DGPS) position accuracy. < 3 meters: Typical WAAS position accuracy. |
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