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What is GPS
GPS or Global Positioning System is a constellation of 27 satellites orbiting the earth at about 12000 miles. These satellites are continuously transmitting a signal and anyone with a GPS receiver on earth can receive these transmissions at no charge. By measuring the travel time of signals transmitted from each satellite, a GPS receiver can calculate its distance from the satellite. Satellite positions are used by receivers as precise reference points to determine the location of the GPS receiver. If a receiver can receive signals from at least 4 satellites, it can determine latitude, longitude, altitude and time. If it can receive signals from 3 satellites, it can determine latitude, longitude and time. The satellites are in orbits such that at any time anywhere on the planet one should be able to receive signals from at least 4 satellites. The basic GPS service provides commercial users with an accuracy of 100 meters, 95% of the time anywhere on the earth. Since May of 2000, this has improved to about 10 to 15 meters due to the removal of selective availability. GPS technology offers several advantages: First and foremost, the service is free worldwide and anyone with a receiver can receive the signals and locate a position. Second, the system supports unlimited users simultaneously. Third, one of the great advantages of GPS is the fact that it provides navigation capability. 
Limitations of GPS As with any technology, GPS also has some limitations. It is essential that the users are aware of these limitations. 
One must recognize that a GPS receiver gives a location reading, which is subject to some inherent errors some under our control and some outside our control. Unless specific steps are taken to improve the accuracy, even with the Selective Availability (SA) off stand-alone receivers can be as much as 15 meters off. 
In order to obtain a GPS position reading, one needs to occupy the point. Often one cannot get there (maybe you don’t want to cross a highway with heavy traffic) or you do not want to get there (wildlife etc.).With GPS, if you cannot occupy a point, you cannot obtain the GPS reading. Even if one can reach the point, the area may be covered with a canopy (thick forest) where GPS signals cannot reach and therefore cannot get the reading. GPS needs clear view of the sky. 
The elevation readings from GPS receivers are not very accurate. Even with differential GPS, the elevation readings can be 2 to 3 times worse than horizontal readings. 
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