GPS - Global Positioning System


The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS. Its receiver needs to be connected at least to three satellites for calculating two dimensions (longitude and latitude) and look after the displacement of the user. In case when the receiver has connection with for or more satellites, it can already fix three dimensions (longitude, latitude and height from the sea level).

GPS satellites transmit two navigation signals on frequencies L1 and L2. Signal C/A code, which is transmitted on frequency L1, provides positioning precise to 3 metres. High precise P code is transmitted on frequencies L1 and L2 (1227.60 MHz) and provides positioning precise to 1 cm.

GPS satellites go round the earth twice a day on a precisely determined orbit and pose information on its surface via signals.

  GPS receivers get this information and using the triangle method ascertains the exact location of the user. GPS receiver compares the time of receiving signal to the time of its posing. The difference in time gives GPS information about how far the satellite is.

After this, realizing the same operations on the signals received from some satellites, the receiver can ascertain the exact location of the user, which is then fixed on an electric map displayed on the accessories.



Single and double frequency GPS


There are two types of GPS receivers: single and double frequency GPS.

A single frequency GPS is for public use that a common citizen could establish the location of  GPS receiver on the earth. We are able to meet a single frequency GPS in modern phones, automobiles, notebooks. There also exists special hand GPS-es, with modern compass functions and is considered one of the main things for tourists.

The coordinates established by the GPS-es of that kind couldn’t be done with high precise (its error hesitates from 3 to 50 metres), although the advantage in comparison with a user GPS-es is that, they are more compact than geodesic measuring GPS-es.

As for double frequency GPS, it is used for precise measuring works and the coordinates established by it comes to 1 cm.

A double frequency GPS consists of  base and movable stations. A base station is set on high precise definite coordinates so called bench marks and with the movable station one can establish the precise coordinates of an interesting object. Base station defined in advance gives us an ability to establish a coordinate of an interesting point with the precise of  1-5 cm targeted to the base station coordinate. So the more precise is the advanced defined bench mark coordinate the more precise is an established coordinate.