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(GPS) technology is a great boon to anyone who has the need to navigate either great or small distances. The Global Positioning System (GPS) is a burgeoning technology, which provides unequalled accuracy and flexibility of positioning for navigation, surveying and GIS data capture. This wonderful navigation technology was actually first available for government use back in the late 1970s. The Global Positioning System (GPS) is a radio based navigation system that gives three dimensional coverage of the Earth, 24 hours a day in any weather conditions throughout the world. The technology seems to be beneficiary to the GPS user community in terms of obtaining accurate data up to about 100 meters for navigation, meter-level for mapping, and down to millimeter level for geodetic positioning. The GPS technology has tremendous amount of applications in Geographical Information System (GIS) data collection, surveying, and mapping.
The first GPS satellite was launched by the U.S. Air Force in early 1978. There are now at least 24 satellites orbiting the earth at an altitude of about 11,000 nautical miles. The high altitude insures that the satellite orbits are stable, precise and predictable, and that the satellites' motion through space is not affected by atmospheric drag. These 24 satellites make up a full GPS constellation. The satellites orbit the Earth every 12 hours at approximately 12,000 miles above the Earth. There are four satellites in each of 6 orbital planes. Each plane is inclined 55 degrees relative to the equator, which means that satellites cross the equator tilted at a 55 degree angle. The system is designed to maintain full operational capability even if two of the 24 satellites fail.
The GPS system consists of three segments: 1) The space segment: the GPS satellites themselves, 2) The control system, operated by the U.S. military, and 3) The user segment, which includes both military and civilian users and their GPS equipment.
The GPS system is passive, meaning that the satellites continuously transmit information towards the Earth. If someone has a GPS receiver they can receive the signal at no cost. The information is transmitted on two frequencies: L1 (1575.42 MHz), and L2 (1227.60 MHz). These frequencies are called carrier waves because they are used primarily to carry information to GPS receivers. The more information a receiver measures the more expensive the unit, and the more functions it will perform with greater accuracy. When one receiver is tracking satellites and obtaining position data, the information received has traveled over 12,000 miles and has been distorted by numerous atmospheric factors. This results in accuracy of about 25 meters. Moreover, the department of Defense (the agency running the GPS) degrades receiver accuracy by telling the satellites to transmit slightly inaccurate information. This intentional distortion of the signal is called Selective Availability (SA). With SA turned on and one receiver is used, the greatest accuracy a user can expect is 100 meters.
To improve the accuracy of GPS, differential, or Relative Positioning can be employed. If two or more receivers are used to track the same satellites, and one is in a known position, many of the errors of SA can be reduced, and in some cases eliminated. Differential data can be accomplished using common code or carrier data (L1 or L2). The most accurate systems use differential data from a GPS base station that continually tracks twelve satellites and transmits the differential data to remote units using a radio link. With these systems centimeter accuracy and real-time navigation is possible.
All of these features make it a very desirable and useful technology for a mirid of activities including Search and Rescue, Aviation and Nautical navigation, hiking, hunting, camping, fishing, and many more. All of these various GPS users have unique needs which require different levels of understanding and skill in using this technology.
The Russian government has developed a system, similar to GPS, called GLONASS. The first GLONASS satellite launch was in October 1982. The full constellation consists of 24 satellites in 3 orbit planes, which have a 64.8 degree inclination to the earth's equator. The GLONASS system now consists of 12 healthy satellites. GLONASS uses the same code for each satellite and many frequencies, whereas GPS which uses two frequencies and a different code for each satellite.

Galileo is Europe's contribution to the next generation Global Navigation Satellite System (GNSS). Unlike GPS, which is funded by the public sector and operated by the U.S. Air Force, Galileo will be a civil controlled system that draws on both public and private sectors for funding. The service will be free at the point of use, but a range of chargeable services with additional features will also be offered. These additional features would include improved reception, accuracy and availability. Design of the Galileo system is being finalized and the delivery of initial services is targeted for 2008.

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