The Basics of GPS

What is GPS?
By now, most of us have come into contact with some form of GPS units .They are in our cars,
motorcycles, outdoor traveling, and other applications.

Global Positioning System or GPS, is a satellite navigation system that provides positioning and clock time to the terrestrial user.The system consists of more than just satellites. While the satellites make up the space segment, the system also includes a control segment that monitors and maintains the satellites, as well as the user segment .

The Global Positioning System was the brainchild of the U.S.Department of Defense, and was named as NAVSTAR GPS which was given by Mr. John Walsh. NAVSTAR is not an acronym, as is widely believed.

This satellite constellation is managed by United States Air Force 50th Space Wing.
The cost is approximately 750 US dollars every year, including the maintenance cost, replacement, research and development. After shoot down of Korean Air Lines Flight 007 in the year 1983, a directive was issued which made the GPS available for civilian use and has been used extensively since then. It has become a very useful tool for making maps, surveying landscapes, commerce and many scientific uses. It also provides time reference which can be used in many applications which include study of earthquake and telecommunication network synchronization.

A GPS receiver simply calculates the distance between itself and three more GPS satellite. Each satellite has an atomic clock in it continually transmits certain data containing its exact time, location of the transmitting satellite and the almanac. The receiver then measures the reception time of the signal. Thus the distance to each satellite is known. Knowing three such distances, a trilateration is formed. By using a fourth satellite, need for a clock at receiver is avoided.

The Global Positioning System is used in a variety of Military and Civilian Applications. It allows soldiers find their objectives in a dark or completely unfamiliar territory and coordinate troop movement and supplies. GPS receivers which military personnel use are called Commanders and Soldier Digital Assistants. A combination of GPS and communication through radio enables real time vehicle tracking.

It is also used in marking targets as hostile and enables the precision guided munitions to allow them engage these targets with high accuracy. Air to Ground roles of military aircrafts use GPS to find targets. GPS also allows targeting for military weapons like ICBMs, Cruise missiles, precision guided missile. Artillery based projectiles are embedded with GPS receivers and can withstand forces up to 12,000G. These are used for 155 mm Howitzers. Any Downed pilot can be easily located if he has GPS receiver. It is widely used by military for reconnaissance and mapping. Some GPS satellites also have nuclear detonation detectors.

GPS units helps civilians a lot in surveying and navigation. Its ability to calculate local speed and orientation is extremely useful. Time transfer is possible because of its capability to synchronize clock. A widely used example of use of GPS is CDMA digital cell phone. Each base uses a GPS timing receiver to synchronize the codes with different base stations and thus making it easy inter-cellular hand off and thus support emergency phone calls and other many applications. GPS equipment has also revolutionized tectonics by measuring the fault motion during earthquakes.





While the GPS receiver is listening to the satellites, it is also downloading information about the satellites' orbit and trajectory.

Each satellite produces a unique code sequence of ones and zeroes.By matching the time difference of the code generated by the satellite's atomic clock and the user's clock , the GPS receiver is able to match the code and calculate a time difference. Based on the calculated time difference and known value of the speed of light, the distance between the SV and the receiver can be determined (speed of light multiplied by time.) The receiver position can then be calculated by intersecting distances from multiple satellites.Three satellites are required to determine a 2-dimensional position and four or more are necessary for 3D.