Global Positioning System

A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS satellites high above the Earth. Each satellite continually transmits messages containing the time the message was sent, a precise orbit for the satellite sending the messageand the general system health and rough orbits of all GPS satellites.These signals travel at the speed of light through outer space, and slightly slower thrrough atmosphere. The receiver uses the arrival time of each message to measure the distance to each satellite, from which it determines the position of the receiverThe resulting coordinates are converted to more user-friendly forms such as latitude and longitude, or location on a map, then displayed to the user. It might seem that three satellites would be enough to solve for a position, since space has three dimensions. However, a three satellite solution requires the time be known to a nanosecond or so, far better than any non-laboratory clock can provide. Using four or more satellites allows the receiver to solve for time as well as geographical position, eliminating the need for a very accurate clock. In other words, the receiver uses four measurements to solve for four variables: x, y, z, and t. While most GPS applications use the computed location and not the (very accurate) computed time, the time is used in some GPS applications such as time transfer and traffic signal timing.





















How GPS works....

Applications

The Global Positioning System, while originally a military project, is considered a dual-usetechnology, meaning it has significant applications for both the military and the civilian industry.

Military

• Navigation: GPS allows soldiers to find objectives in the dark or in unfamiliar territory, and to coordinate the movement of troops and supplies. The GPS-receivers commanders and soldiers use are respectively called the Commanders Digital Assistant and the Soldier Digital Assistant.
• Target tracking: Various military weapons systems use GPS to track potential ground and air targets before they are flagged as hostile.These weapon systems pass GPS co-ordinates of targets to precicion guided munitions to allow them to engage the targets accurately. Military aircraft, particularly those used in air to ground roles use GPS to find targets
• Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, Cruise Missiles and Guided Missiles.
• Search and Rescue: Downed pilots can be located faster if they have a GPS receiver.
• Reconnaissance and Map Creation: The military use GPS extensively to aid mapping and reconnaissance.
• The GPS satellites also carry a set of nuclear detonation detectors consisting of an optical sensor (Y-sensor), an X-ray sensor, a dosimeter, and an Electro-Magnetic Pulse (EMP) sensor (W-sensor) which form a major portion of the United States Nuclear Detonation System.

Civilian

• The ability to determine the receiver's absolute location allows GPS receivers to perform as a surveying tool or as an aid to navigation.
• The capacity to determine relative movement enables a receiver to calculate local velocity and orientation, useful in vehicles or observations of the Earth.
• GPS enables researchers to explore the Earth environment including the atmosphere, ionosphere and gravity field.
• GPS survey equipment has revolutionized tectonics by directly measuring the motion of faults in earthquakes.
• PS functionality has now started to move into mobile phone. Eg NOKIA N95 has this feature availiable in it.