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Global Positioning System - GPS
The global positioning system is a location determination network that uses satellites to act as reference points for the calculation of position information. These man-made reference points can be viewed as aerial lighthouses that are visible to user equipment and can also transmit additional information that can provide extremely accurate location information to the GPS function within location determination devices.
Several orbiting satellite systems are used for global positioning, which include:
The USA GPS System
The USA Global Positioning System (GPS) is a location determination system that was developed by the Department of Defense's (DOD) Ivan Getting, and Massachusetts Institute of Technology (MIT).
This system, which consisted of eleven satellites, was called NAVSTAR (Navigation System with Timing and Ranging) and launched between 1978 and 1985. By 1980, 18 satellites were part of the NAVSTAR constellation and used by the military for GPS.
In 1983, President Ronald Regan declassified GPS technology, allowing for public use. This was prompted by the take down of Korean Airline 007 by Russian Military jets when the commercial airliner drifted into Russian airspace. Since then, public use of the satellites for commercial purposes was allowed and by July of 1995, 24 satellites were in place, completing the full system. It was off limits to the public between 1990 and 1993, during the first Gulf War, to allow for exclusive use by the military. Although the constellation currently has 29 satellites in orbit, only 24 are required for normal operation leaving five spares in case of lost operation.
The GLONASS system is a global positioning system that is operated by the Russian Republic, (State Unitary Enterprise of Applied Mechanics). The system has continued to evolve, providing more accuracy and precision, and much like the US system, it also has 24 satellites. Unlike the US satellites, which are launched individually, GLONASS satellites can be launched in groups of three. Additionally, the GLONASS system operates at a higher altitude than the US system. There are some technical trade offs to this approach such as better accuracy in the northern hemisphere at the cost of less accuracy in the southern hemisphere. Since GLONASS serves the Russian Republic, this is in their favor.
European Galileo System
The European Galileo System is a global navigation system similar to the US GPS system, which was started in May 2003 by the European Union and the European Space Agency The system was to be built between 2006 and 2008 at the cost of 3 billion Euros, and is expected to be complete by 2013 with a cost of $3.4 Billion Euro, as of this printing. This system will use 30 satellites for a complete constellation and will be for civilian use only. China, Korea, Israel and other non-EU countries are invested in the project and will use the system as well. The EU system aims to be the most accurate of the world’s systems.
Beidou Satellite Navigation System
The Beidou System is a Chinese regional satellite positioning system. It is a two-way satellite system that allows mobile devices to request position information from the satellite network.
Terrestrial and Other Positioning Systems
Terrestrial positioning systems are position location systems that use land-based transmitters or reference points to act as reference points for the calculation of position information. Some of the terrestrial based positioning systems include long range aid to navigation (LORAN), dead reckoning (DR), and inertial navigation systems (INS).
How GPS Works
Recall that there are 24 satellites being used in the US GPS system at all times. These satellites orbit the earth in such a way that at any given time and location, at least four satellites are visible to a GPS driven device. Each satellite is equipped with an extremely accurate atomic clock so the satellite is always aware of the current time on Earth at the Prime Meridian. The satellites are also aware of their own positions with the assistance of ground stations that give continuous updates. The 24 satellites orbit the earth transmitting their time and position. These pieces of data are received by the antennas attached to radio receivers inside a GPS device.
As a GPS device starts up, it must scan its radio tuner for very faint GPS satellite signals. Once it has collected data (the position of a satellite and the time the satellite sent the position) from at least three satellites, a location fix can be made.
Differential time of arrival and triangulation are the methods used to determine location in a GPS system. Dead reckoning may also be used when satellites are not visible.
Differential Time of Arrival
Differential time of arrival is the method used to determine how far each satellite is from a GPS device. Although each satellite transmits its position and the time it was at that position, it takes time for that signal to reach the Earth. The receiver contains a very accurate clock, which can determine the difference in time between the current time and when the satellite sent the signal. With this differential time and the speed of radio waves, the distance from each of the three satellites can be determined using the simple formula:
Rate x Time = Distance
Trilateration is a method that is used to determine position on Earth in three dimensions. GPS deals with three-dimensions rather than two. Since the distance from the Earth to a satellite results in a sphere rather than a flat circle, the calculation is a bit complex.
Using trilateration, rather than draw circles to determine position we need to draw spheres. For example, if the first acquired satellite is 25,000 miles from position one cannot simply draw a circle around that satellite and determine a position 25,000 miles from it. A sphere must be plotted, extending toward Earth and away from Earth. A second satellite is calculated to be 25,001 miles from position, resulting in another sphere. The two spheres intersect, creating a perfect circle. A circular plane now exists, extending down through the earth and out into space. A large number of potential positions have now been eliminated, but there is not yet an exact location. Many potential positions still exist and a third satellite is needed to define a sphere that intersects with the two current spheres resulting in two points that define possible position. One point is in space and one is on earth. Since the world is roughly a sphere, the point in space can be eliminated and the approximate position of the GPS receiver is located on Earth. A fourth satellite is necessary to account for altitude and provide an exact fix of the location. The plotting of a fourth sphere provides the exact location and altitude of the receiver at the time the four measurements were taken.
This figure shows a global positioning satellite (GPS) system. This diagram shows how a GPS system receives and compares the signals from orbiting GPS satellites to determine its geographic position. Using the precise timing signal based on a very accurate clock, the GPS receiver compares the signals from 3 or 4 satellites. Each satellite transmits its exact location along with a timed reference signal. The GPS receiver can use these signals to determine its distance from each of the satellites. Once the position and distance of each satellite is known, the GPS receiver can calculate the position where these distances cross. This is the location. This information can be displayed in latitude and longitude form or a computer device can use this information to display the position on a map on a computer display.
Global Positioning System Operation - GPS Diagram
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