Background briefing – latitude, longitude, and GPS

Before we can get geographic information, we'll need to review some essential terminology. One powerful piece of modern technology that helps civilians as well as secret agents is the Global Positioning System (GPS), a satellite-based system to determine location. The GPS allows a terrestrial device to pinpoint its location in both space and time.

The idea underlying GPS is quite elegant. Each satellite produces a stream of data that includes position and super accurate timestamps. A receiver with multiple streams of data can plug the positions and timestamps into a matrix of simultaneous equations to determine the receiver's position with respect to the various satellites. Given enough satellites, a receiver can precisely calculate latitude, longitude, elevation, and even the current time.

For more information see http://en.wikipedia.org/wiki/Global_Positioning_System#Navigation_equations.

A position's latitude is an...