GPS is a satellite navigation system designed to provide instantaneous position, speed and time information anywhere in the world at any time and in any weather.Present usage of GPS for positioning includes personal navigation (hiking, boating, hunting, driving directions, etc.), aircraft navigation, offshore survey and vessel navigation, fleet tracking, dredging, machine control, civil engineering, land surveying, GIS and mapping, deformation analysis, etc. The list is almost endless.
The day-to-day running of the GPS program and operation of the system rests with the US Department of Defense (DoD). Originally designed by the US Department of Defense (DoD), GPS comprises three main components: the control segment, the space segment and the user segment. Nowadays, GPS is used for topographic surveys and hydrographic surveys. The survey grade GPS receivers can achieve 2 mm accuracy for horizontal measurements and upto 5 mm accuracy for vertical measurements.
The control segments are stations situated on the ground on various parts of the world. These are the eyes and ears of the GPS and they keep tracking the satellites, which are rotating around the earth in various orbits in space at a height of 20,183 km above the earth. There are at present 24 such satellites, and they constitute the space segment.
The user segment comprises the receivers that have been designed to decode the signals transmitted from the satellites for the purposes of determining position, velocity or time. To compute the positional value from GPS satellite signals, a GPS receiver must perform the following tasks.
Calculation of the positional value
Selecting one or more satellites in view
Acquiring GPS signals
Measuring and tracking
Recovering navigational data
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