GPS (Global Position System). It is a system that establish our position with Latitude, Longitude and Height coordinates. It is based on a 21 satellites constellation orbiting the earth at 20100 km of height, needing 11h58m to describe a full orbit.
In order to know our positon we must have a GPS reciever, the system measure the distance from each satellite to the reciever antenna. To get the distance, satellites sends radio waves to 300.000 km per seconds, it measure the time between the moment that the signal left and the moment that reached the reciever. As distance is equal to speed times time (S=VxT), then we can determine without any problems the distance between antenna and satellite. This calculation is done by the GPS reciever.
With a satellite I establish a distance, this mean that the position to be defined can be at any point from a hollow sphere to a distance x from the satellite. The GPS recievers are designed according to types of application such: Geodesia, Topography, Sea Navigation, Air Navigation, Land Navigation, Cartography, AVL (Automatic Vehicle Location). Each reciever has specific functions and accuracies according its own application.
Standar GPS accuracy varies from 1 to 5 meteres depending from manufacturer. Exists professional GPS that cost thousands of euros and has an accuracy of less than 1 cm. Are designed for topography. ¿What is the reason of the cost differences between differnent GPS models ? Basically are due a marketing need. One cheap GPS has a reciever with the same quality, accuracy and benefits of an expensive one. A reciever with more than 20 channels wont give you better benefits as the most part of the time you will only capture the information from 7, 8 or even 9 satellites at the same time. The remaining ones will be on a different part from the planet, not accessibles to your reciever. At the same time a reciever that has 1 cm of accuracy wont provide any difference on the matter of satellite tracking from those that provides 1-5 meters of accuracy.