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GPS stands for global positioning system. Actually, the abbreviation comes from the US system, also known as Navstar, which was the first positioning system ever. There is another system from Russia called GLONASS. In Europe, the system is called Galileo.
The system consists of several (24 for GPS systems) satellites in an earth orbit transmitting the time information and satellite location. From this information the user can determine the position on earth. This is calculated based on the triangulation method, so we need at least three satellites to determine the position.
This is very well known and the most commonly used application, but there is more to it. Better receivers can use the Doppler effect method to precisely calculate the speed. The accuracy of speed measurement can be better than 0.1 km/h. Therefore we can also use receivers like VGPS as speed sensors. A low cost market GPS doesn't have this option. The output update rate for speed can vary from 10-100 Hz.
The third possible application comes from the way how the GPS system is functions. Since the satellites are transmitting exact absolute time and better receivers usually output this pulse with a high precision (below one microsecond), we can use this technology to synchronize remote systems. Actually there is no distance limit - we can accurately synchronize two systems with one placed in California and the second one in India. With that, we can clearly see if there is a correlation from butterflies flapping their wings in San Francisco to the earthquakes in India (just kidding, but this actually very useful to recognize the source of faults on power lines, for example). This technology is available with GPS-CLOCK and VGPS-200C hardware.
The quality of the signals coming from the GPS depends a lot on the number of satellites recognized by the receiver. The signal can't really go around a corner or though walls, so a clear line of sight is very important. High buildings and trees will obstruct or block the signal. High buildings can also produce reflections, which are a major cause of measurement errors, even though the number of available satellites is high.
In general, we can get a good signal if we have six or more satellites; otherwise we get jitters in speed and position.
We can use different antennas for the GPS. Usually there very tiny ones all the way to quite huge ones. The difference between them is that the small ones need a ground plate to deflect from the earth. The ground plate should be at least 30x30 cm big. The car roof, if it is metal, is a perfect ground plate for the antenna. For other applications, we need to be sure to make a ground plate or we should use an antenna with an integrated ground plate.
