Global positioning Systems (GPS) is a system consisting of 24 satellites orbiting around the earth with 4 satellites on each axis.

There are some small rackets called NAVSTAR that keep the satellites on the appropriate axis. They are recognizable from a position of 10 to 100 meters. The satellites apply simple mathematical calculations to broadcast information which is translated as longitude, altitude, and latitude by earth receivers. GPS is a system that is able to work 24/7 all around the world without charging any extra fees for this long-hour working. GPS satellites revolve around the earth on an axis twice a day while sending information signals to the earth.

The first GPS satellite was successfully sent to space in 1978. The first and foremost aim of designing and developing GPS was military purposes however this changed after 1980 and started being used as non-military services. In 1994, all the four satellites were stabilized on Earth's orbit.

The earth controlling include earth control stations that have clear coordinates and their position is obtained by classic positioning like Very Long Baseline Interferometry (VLBI) and Satellite Ranging Laser (SRL). These stations are responsible to calculate mathematical Polynomials with the least squares, orbital parameters, and the satellite positions to Geodetic and Geocentric latitude (the start of the coordinates is at the center of the Earth). There are 5 stations like these that the main one called Colorado Spring is based in the United States and four other ones are located in other parts of the world. The last part of the GPS is the user part which includes two parts:

-The receiver antenna of data sent from satellites

-Receiver (the processor of the data and determining the local positioning of the antenna)

The software and microprocessors inside the receiver determine the distance between the Earth antenna and the related satellites. Then, by using of four satellites of X,Y,and Z, the location of the antenna or the receiver is determined.

Basis of GPS function

The basic function of GPS is positioning that is done by receiving GPS satellite waves which are situated in axis revolving around the Earth. GPS receivers compare the timing received with that of their own; the difference shows the distance between GPS receiver and the related satellite. This is what exactly a GPS does. By making use of 3 satellites or more, GPS can find out its latitude and longitude (called two-dimensional determination), and then in corresponding with 4 -or more- satellites, a GPS can set its own three-dimensional position which include altitude, longitude, and latitude. By continuous conduct of these calculations, GPS can clearly determine its speed and direction. Nowadays, in some parts of Iran, we can receive data from 10 satellites, and have access to 4 or 5 satellites any time and in any place. As the number of observable satellites increases, the basic calculations of positioning increase as well then the required time to position a place decreases, and preciseness of positioning increases. The important point which needs to be considered here is that the altitude provided to us by the GPS is different from the altitude written on maps or atlases. The GPS height is compared to a criterion level called elliptic level while the altitude in maps and atlases is orthometric height calculated from the sea level. The highest amount of difference between these two criteria is 100 meters. GPS presents the position data in form of numbers and in a coordinate system. Today, the most practical coordinate systems are Lat-lon and UTM. GPS machine is a small computer which is programmed to perform specific actions so this small computer can function in other ways providing that it has your coordinate data. For example, it can give the information about the sunrise and sunset time, and also the rising and setting of the moon. It is interesting that GPS, based on your speed, can inform you about the remaining time to arrive to your destination. GPS can also show your speed average, speed maximum, the level of downhill and uphill of the road, vertical speed, the area positioning in terms of hunting or fishing spots, and hunting in any part of the world. Also, the machine can determine an unknown area and may return you from a traveled road.