Global positioning system

Definition


The global positioning system (GPS) is

"a satellite-based technology that reveals information about the location, speed, and direction of a targeted subject. While it was initially developed for the U.S. military, countless civilian applications of GPS have appeared in the marketplace."

"a constellation of orbiting satellites that provides navigation data to military and civilian users around the world. These satellites orbit the earth every 12 hours, emitting continuous navigation signals. With the proper equipment, users can receive these signals and use them to calculate time, location, and velocity."

Overview
The GPS system consisting of space, ground control, and user equipment segments that support the broadcasts of military and civil GPS signals. Each of these signals includes positioning and timing information, which enables users with GPS receivers to determine their position, velocity, and time 24-hours a day, in all weather, worldwide. GPS equipment is used on aircraft, ships, and land-based vehicles, and mobile hand-held units provide individuals with these capabilities as well.

GPS produces latitude/longitude coordinates by relying on established trigonometric principles, timing, range measurements, and several statistical models. Analogous to traditional land surveying, GPS requires a minimum of four simultaneous satellite observations to precisely position a point on the earth.

Initiated as a test program by the Department of Defense in the early 1970s, the Global Positioning System has provided position, navigation, and timing data for military and civilian applications since 1992. The system uses 24 satellites that orbit the Earth at an altitude of 10,900 nautical miles, Portable or vehicle-mounted GPS devices receive signals from the satellites and calculate the user’s position. GPS operates 24 hours a day, can serve an unlimited number of users, and operates in all weather conditions. The United States provides GPS data free of charge.

GPS began operations with a full constellation of satellites in 1995. Over time, GPS has become vital to military operations and a ubiquitous infrastructure underpinning major sections of the economy, including telecommunications, electrical power distribution, banking and finance, transportation, environmental and natural resources management, agriculture, and emergency services. GPS is used by all branches of the military to guide troop movements, integrate logistics support, enable components underlying battlespace situational awareness, and synchronize communications networks. In addition, U.S. and allied munitions are guided to their targets by GPS signals and GPS is used to locate military personnel in distress.

GPS usage
Civil agencies, commercial firms, and individuals use GPS and GPS augmentations to accurately navigate from one point to another. Commercial firms use GPS and GPS augmentations to route their vehicles, as do maritime industries and mass transit systems. In addition to navigation, civil departments and agencies and commercial firms use GPS and GPS augmentations to provide high-accuracy, three-dimensional positioning information in real time for use in surveying and mapping and other location-based services.

The aviation community worldwide uses GPS and GPS augmentations to increase the safety and efficiency of flight. GPS and GPS augmentations are also used by the agricultural community for precision farming, including farm planning, field mapping, soil sampling, tractor guidance, and crop scouting; the natural resources management community uses GPS for wildfire management and firefighting, pesticide and herbicide control, and watershed and other natural resources asset management. GPS is increasingly important to earth observation, which includes operational roles in weather prediction, the measurement of sea level change, monitoring of ocean circulation, and mitigation of hazards caused by earthquakes and volcanoes.

GPS helps companies and governments place satellites in precise orbits, and at correct altitudes, and helps monitor satellite constellation orbits. The precise time that GPS broadcasts is crucial to economic activities worldwide, including communication systems, electrical power grids, and financial networks.

GPS system description
GPS operations consist of three segments &mdash; the space segment, the ground control segment, and the user equipment segment. All segments are needed to take full advantage of GPS capabilities.

Space segment. The GPS space segment is a constellation of satellites that move in six orbital planes approximately 12,500 miles above the earth. GPS satellites broadcast encrypted military signals and unencrypted civil signals. The baseline constellation consists of satellites occupying 24 orbital slots &mdash; 4 slots in each of the six orbital planes. However, because the U.S. government commits to at least a 95% probability of maintaining this baseline constellation of 24 satellites, the typical size of the constellation is somewhat larger. Moreover in recent years, because numerous satellites have exceeded their design life, the constellation has grown to 31 active satellites of various generations. However, DOD predicts that over the next several years many of the older satellites in the constellation will reach the end of their operational life faster than they will be replenished, thus decreasing the size of the constellation from its current level, reducing satellite availability, and potentially reducing the accuracy of the GPS service.

Ground control segment. The GPS ground control segment comprises the Master Control Station at Schriever Air Force Base, Colorado; the Alternate Master Control Station at Vandenberg Air Force Base, California; 6 dedicated monitor stations; 10 National Geospatial-Intelligence Agency monitoring stations; and 4 ground antennas with uplink capabilities. Information from the monitoring stations is processed at the Master Control Station to determine satellite clock and orbit status. The Master Control Station operates the satellites and regularly updates the navigation messages on the satellite]s. Information from the Master Control Station is [[transmitted to the satellites via the ground antennas. The U.S. Naval Observatory Master Clock monitors the GPS constellation and provides timing data for the individual satellites. The U.S. Naval Observatory Master Clock serves as the official source of time for DOD and a standard of time for the entire United States.

User equipment segment. The GPS user equipment segment includes military and commercial GPS receivers. A receiver determines a user’s position by calculating the distance from four or more satellites using the navigation message on the satellites to triangulate its location. Military GPS receivers are designed to utilize the encrypted military GPS signals that are only available to authorized users, including military and allied forces and some authorized civil agencies. Commercial receivers use the civil GPS signal, which is publicly available worldwide.

Modernization of GPS
The U.S. Air Force, which is responsible for GPS acquisition, is in the process of modernizing GPS. It is uncertain whether the Air Force will be able to acquire new satellites in time to maintain current GPS service without interruption. If not, some military operations and some civilian users could be adversely affected.


 * In recent years, the Air Force has struggled to successfully build GPS satellites within cost and schedule goals; it encountered significant technical problems that still threaten its delivery schedule; and it struggled with a different contractor. As a result, the current IIF satellite program has overrun its original cost estimate by about $870 million and the launch of its first satellite has been delayed to November 2009—almost 3 years late.


 * Further, while the Air Force is structuring the new GPS IIIA program to prevent mistakes made on the IIF program, the Air Force is aiming to deploy the next generation of GPS satellites three years faster than the IIF satellites.


 * If the Air Force does not meet its schedule goals for development of GPS IIIA satellites, there will be an increased likelihood that in 2010, as old satellites begin to fail, the overall GPS constellation will fall below the number of satellites required to provide the level of GPS service that the U.S. government commits to. Such a gap in capability could have wide-ranging impacts on all GPS users, though there are measures the Air Force and others can take to plan for and minimize these impacts. In addition to risks facing the acquisition of new GPS satellites, the Air Force has not been fully successful in synchronizing the acquisition and development of the next generation of GPS satellites with the ground control and user equipment, thereby delaying the ability of military users to fully utilize new GPS satellite capabilities.