Global Positioning System (GPS)

Global Positioning System was invented by Ivan Getting and U.S. Department of Defense. GPS is related to satellite navigational system, which is mainly designed for navigation application. It is now used as a timing tool. Six in each of three orbital planes spaced 120 degrees apart out of eighteen satellites and their ground stations forms GPS. To calculate geographical positions GPS uses satellites as reference points. GPS is used as a pinpoint in any ship or submarine on the ocean and to measure Mount Everest. GPS receivers have created with few integrated circuits so that it becomes very economical to use. Today GPS is used in planes, boats, construction equipment, cars, movie making gear, farm machinery and even laptop computers.

GPS satellites are placed high enough to avoid the problems related with land based systems and then also it provides accurate position anywhere in the world. The positions, which are incorrect determined by GPS satellite signals produce accuracy in the range of 50 to 100 meters. By using differential correction technique users can get positions accurate to within 5 meters or less.

There are varieties of applications for GPS because GPS units are smaller and less expensive. GPS assists pilots and drivers in pinpointing their locations and avoiding collisions in transportation applications. Farmers use GPS to control accurate distribution of fertilizers and other chemicals in farm. Recreationally, GPS provides accurate locations and act as a navigation tool for hunters, hikers and boaters.

GPS has its greatest application in the field of Geographic Information Systems (GIS). GPS provides any point on earth with a unique address with some consideration of error. A GIS is descriptive database for the specific part of the earth. GPS tells accurate position by using co-ordinates X, Y, Z while GIS tells X, Y Z is a tree or a spot in an ocean. GPS tells us "where" while GIS tell us "what". GPS/GIS are providing the way to locate, organize, analyze and map of resources.

How GPS Determines a Location
By applying mathematics distances between the receiver and the position of 3 or more satellites are determined. Assuming that the positions of the satellites are known in advance and the location of the receiver is calculated by determining the distance from each of the satellites to the receiver. GPS takes these references and measured distances and "triangulates" an additional position.

GPS satellites are orbiting at an altitude of 11,000 miles from earth. Satellites can be adjusted periodically by large land-based radar systems. Therefore locations of the satellites are known in advance. GPS receivers store this orbit information for all the GPS satellites is known as an almanac. Each GPS satellite broadcasts the almanac continually. Then GPS receiver automatically collects this information and stores this information for future use.

The Department of Defense continually checks the orbit of the satellites looking for deviations from predicted values. Any deviations, which are caused by natural atmospheric phenomenon such as gravity are known as ephemeris errors. When ephemeris errors are determined then the errors are sent back to the satellite, which in turn broadcasts the errors as part of message and supply this information to the GPS receivers. By using ephemeris error data in conjunction with the information from the almanac, the position of a GPS satellite can be determined for a given time.

By measuring the amount of time a radio signal takes to travel from the satellite to the receiver, GPS determines distance between a GPS satellite and a GPS receiver. Radio signals travel with speed of light. If the amount of time the signal takes to travel from the satellite to the receiver is known then the distance between the satellite and the receiver can be determined by using formula distance = speed x time. The signal's travel time can be determined, if the exact time when the signal was transmitted and received are known.

For synchronization purpose, the satellites and the receivers use very accurate clocks so that they generate the exactly same code at exactly the same time. The code generated by the receiver can be compared with the code received from the satellite. The time difference between when the satellite generated the code and when the receiver generated the code can be determined by comparing these codes. This difference between times is the travel time of the code. This travel time is multiplied in seconds to 186,000 miles per second gives the distance from the receiver position to the satellite in miles.

GPS Application

Now days sportsmen, farmers, soldiers, hikers, delivery drivers, sailors, dispatchers, scientists, lumberjacks, fire-fighters, pilots, surveyors and people from many other walks of life are using GPS in such ways that make their work more productive, safer, and easier.

GPS applications are as follows:
1)Location: It is used to determine a basic position of an object.
2)Navigation :It is used to get position from one location to another.
3)Tracking - It monitors the movement of people and things.
4)Mapping - It creates maps of the world.
5)Timing - It brings precise timing to the world.