GPS how does it work?

G.P.S. stands for Global Positioning System. There are at least 24 satellites in orbit around the earth. These send out signals that units on earth can receive and calculate to determine your position on the earth. Calculating the time it takes to receive the signal from at least 3 satellites, the ground unit can trilaterate its position relative to the satellites. Since the locations of the satellites are in a known position, the ground unit can and do apply these to a map and compute its location relative to the satellites. If the unit is able to receive more than the 3 required signals from the satellites, the unit can improve the precision of the estimated position by factoring in the new signal and then calculating a refined position. Below is an example of 2D trilateration.

If you wanted to tell a friend where you were (point B) and couldn't describe it any other way, a friend could trilaterate your position. If you knew the distance (r1) to a nearby landmark, lets say a mall (P1), this would narrow your position down to a circle around P1..

If you knew the distance to a second landmark (r2), your house (P2), you could tell your friend how far away you were from both locations your friend could calculate where the 2 circles (distances) would intersect. Your friend can assume you are either at point A or point B.

A distance from a third landmark, lets say a school (P3), would determine if you were at point A or B and your friend would know your current position.

GPS receivers run equations like the one above constantly with distances to the satellites being calculated by the lag time from when the signal is sent (with data on the sending time included in the signal) and when the receiver receives it compared to the receiving time. The speed of the signal from the satellite is a known; it moves at the speed of light in outer space and slightly slower in the atmosphere. Doing the calculations this way turns the 2D trilateration into 3D trilateration using spheres instead of circles. The process of determining your position is similar to the 2D version except you need 4 points of information and the earth can be used for that 4th point (other points not on the earth are not used because it assumes your not in space.) Using constant location updates, the GPS receiver can calculate your direction and velocity.

Atmospheric conditions like heavy cloud cover in storms can affect the speed of the signal creating discrepancies. With time and several transmissions , the signal speed can be averaged out and a calculation can be made and/or additional signals from other satellites can be used to determine a correct position..