The global positioning system (GPS) is a space-based satellite navigation system providing location data coupled with time information worldwide. This coverage is predicated on an unobstructed line of sight between the user and the dedicated satellites. The service is funded by U.S. taxpayers and maintained by a department within the U.S. government. Anyone with a GPS receiver can access the system without charge.
As a navigational resource, the orbiting satellites provide critical capabilities to military, civil, and commercial users around the world. It is intended to become the backbone for modernizing the global air traffic system.
The project was conceived in 1973 to overcome the limitations of previous ground-based navigation systems. GPS was created and undertaken by the U.S. Department of Defense (DoD) in 1994 and was fully commissioned with 24 satellites. In recent years, three additional orbiting units have been added to enhance worldwide coverage. As of December 2010, there were 30 operational satellites orbiting the earth actively broadcasting positioning, navigation, and timing messages to users, 24/7, around the globe. In addition, five older satellites are maintained in orbit in a standby mode that can be brought back to operational status if required.
Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS system and implement the next generation of GPS III satellites and Next Generation Operational Control System (OCX).In 2000, U.S. Congress authorized a modernization effort. Renovation of the constellation, which should enhance the performance and capabilities of the system, began with the launch of eight GPS Block IIR-M satellites during 2005-2009 and the first of 12 GPS Block IIF satellites in May 2010. The next generation of satellites, GPS III, is currently in development and on schedule for a first launch in 2014.
In addition to the U.S. owned network, the Russian GLObal NAvigation Satellite System (GLONASS) was in use by only the Russian military, until it was made fully available to civilians in 2007. A Galileo project is in works by the European Union along with the Chinese Compass Navigation System, and Indian Regional Navigational Satellite System.
Signals from satellites
The design of GPS is based partly on similar ground-based radio-navigation systems, such as LORAN. A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the earth. Each satellite continually transmits messages that include the time the message was transmitted, precise orbital information (the ephemeris), and general system health including the orbits of all GPS satellites (the almanac).
The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite. These distances along with the satellites’ locations are used to compute the position of the receiver. This position is then displayed, perhaps with a moving map presentation or latitude and longitude. Elevation information may also be included. Many GPS units show derived information such as direction and speed, calculated from position changes.
Three satellites might seem enough to receive information to resolve a position since space has three dimensions and a position near the earth’s surface can be assumed. However, even a very small clock error multiplied by the very large speed of lightcoupled withthe speed at which satellite signals propagate would result in a large positional error. Therefore, receivers use four or more satellites to solve the receiver’s location and time. The accurately computed time is not displayed in all applications. A few specialized devices do use the time; these include time transfer, traffic signal timing, and synchronization of cell phone base stations.
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