The acronym GPS (Global Positioning System) refers to a receiver that processes signals, sent from satellites in orbit. The receiver processes and results in a numerical position expressed in latitude and longitude. II system comprises, at present, by 24 satellites that are joined by other stock 03, reaching a total of 27 items distributed on six orbital planes at an altitude of 20,183 kilometers and closely monitored by ground stations. Information about the orbits of the satellites (ephemeris), are constantly updated in GPS every time you turn on the unit.
What happens in practice? Each satellite contains a very accurate clock, this predetermined intervals, sends a series of digital signals; These signals include the orbital position and the time sent. All satellites are synchronised with each other, whereby the GPS receiver does is calculate the time that the signal, satellite party, takes to reach it. By replicating this calculation for all satellites that can receive and applying the Pythagorean theorem, the receiver can determine its position in space. Fortunately for the calculations are automatic.
The system is not foolproof, there are some understandable limitations in its use: you have to have a fairly extensive “visibility” of the sky: what does it mean? It is (understandably) difficult, even for a GPS, receive signals among the skyscrapers, or at the bottom of a canyon, even a thick cover of trees and leaves can sometimes cause problems; to determine the position requires that the receiver can “feel” the signal from at least three satellites to define its position on a 2-dimensional plane (signal triangulation), while serving the signals from at least four satellites to have the notice relating to the altitude. It’s not over, the satellites received must have a “good geometry”, or be placed around the square, which is not too low on the horizon or too close to the vertical.
This limits the detection system, the altitude reported by the instrument is rarely reliable. Generally, the altitude value you get with four satellites, even good geometry, is very far from the real one, to have a legitimate value are required virtually all visible satellites, and even in this situation are still possible errors of the order of several tens of meters. If you need to know your precise altitude, you should also use barometric altimeter.
A bit of history
The Government of the United States is the owner of the satellite system currently in use, kept in the GPS an “induced” called SA (selective availability). This failure reduced the possibility of system accuracy to about 100 metres of approximation and randomly, making very difficult the precise detection of the position. Especially the quest location was difficult because the error generated by the system could be added for the shortage in the number of satellites eventually received; This condition favoured in subsequent years DGPS (see DGPS meaning) system development to connect to GPS to increase its accuracy. To date this system, which was based on terrestrial beacons correction factors for index, has almost no more reason to exist, the accuracy of any unit arrive at ten meters (even less in some cases) more than enough for a roaming user.
In all cases the United States retain ownership of the system and the ability to pause, even selectively, on “friendly”. He is currently developing an alternative system, the Galileo positioning system (Galileo Positioning System) a civil global navigation satellite system developed in Europe as an alternative to the Global Positioning System (NAVSTAR GPS), controlled instead by the United States Department of Defense. Its entry into service is planned for 2014 and will count 30 orbiting satellites on 3 Earth’s equatorial plane and inclined planes at an altitude of about 24,000 km. We must also say, for sincerity, which has suffered several postponements for the implementation.