Global Positioning System (GPS), Defined

(GPS Defined and Indications of Improvements in Accuracy)

GPS: Global Positioning System
1. A system of satellites, computers, and receivers that is able to determine the latitude and longitude of a receiver on Earth by calculating the time difference for signals from different satellites to reach the receiver.
2. A satellite-based radio navigation system run by the U.S. Department of Defense.

It was designed so that signals from four satellites would be available at any point on the earth. By triangulation of these signals, a receiving unit can pinpoint its current location (longitude and latitude) as well as altitude to within 20 meters. The first GPS satellite was launched in 1978.

MEOs in Six Orbits

In six different orbits approximately 12,500 miles above the earth, the system's 24 MEO satellites circle the earth every 12 hours and constantly transmit location as well as time of day from atomic clocks. They are tracked on the ground by a monitoring network that uplinks data for synchronization. The system uses four frequencies in the L-band from 1.2 to 1.6GHz (see satellite bands).

MEO is an acronym for "Medium-Earth Orbit" which is a communications satellite in orbit from 1,600 to 15,000 miles above the earth. It is higher than a "low-earth orbit" (LEO) satellite and lower than a "geostationary earth orbit" (GEO) satellite. Widely used for navigation systems, MEOs generally take about six hours to orbit the earth and are in view for a couple of hours.

National Geospatial-Intelligence Agency (NGA)

Civilian users should see a 15-20 percent improvement in real-time navigational accuracy since the National Geospatial-Intelligence Agency (NGA, formerly the National Imagery and Mapping Agency) brought additional monitoring stations online in early September, 2005. Tracking data from the six GPS monitoring stations are now being added to data from the U.S. Air Force network to generate the real-time operational orbits for GPS. NGA is feeding its monitor station data in real-time to the GPS Master Control Station (MCS) at Schriever Air Force Base in Colorado, where they are input to the Kalman filter that ultimately produces the broadcast navigation message.

A study funded by the European Space Agency (ESA) seeks to determine if GPS data, augmented by European Geostationary Navigation Overlay Service (EGNOS) corrections, can be used to correctly calculate road usage in Europe, and collect tolls for distance driven.

Tests carried out in Dublin, Ireland, by Mapflow and co-sponsor Dublin Transportation Office during August and September 2005, planned to employ 10 tracking devices, take more than one million GPS samples, and monitor approximately 6,000 journeys.

Part of the Active Road Management Assisted by Satellite (ARMAS) project to develop a virtual road toll system, the trials use in-vehicle black boxes and GPS data to calculate car position in real time and send data on location and journey length to a control center for distance-based charging.

Eliminating tollbooth stops will cut traffic congestion and reduce roadside infrastructure, advocates claim. The in-car unit can also notify motorists of upcoming hazards or traffic delays, and enable traffic managers to vary toll charges based on time of day or levels of congestion — for example, rewarding drivers traveling at off-peak times with reduced charges. Such a driver-tracking system has, however, raised privacy concerns.

Dublin-based Mapflow completed similar trials in London. “The built environment in London creates significant challenges for monitoring road usage by satellite. We believe that the low-rise nature of buildings in Dublin will make it significantly easier to track cars in Dublin, and that this technology will eventually lead to the elimination of toll booths in Ireland,” said Harvey Appelbe, Mapflow chief technology officer.

PNT Research Center at Stanford

Stanford University will open a new research center dedicated to advancing global navigation satellite system capabilities. “Research at the Stanford Center for Position, Navigation, and Time (SCPNT) is aimed at vastly extending and expanding the already revolutionary benefits of GPS in society,” stated Per Enge, the center’s research director and a professor of aeronautics and astronautics.

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