1. GPS Basics

GPS positioning is based on triangulation from GPS satellites in precisely defined orbits. Each of the 24 to 27 active GPS satellites transmits a unique code modulated on a carrier frequency. By receiving data from at least four satellites, a single GPS receiver can decipher the codes, determine its distance from the satellites, and pinpoint its position to within a few meters. Four satellites are the minimum required, since the receiver has to solve for four unknowns: x, y, z and time.

1. The How GPS Works on-line tutorial by Trimble Navigation provides an excellent explanation of GPS positioning.
2. The Basics of High-Precision Global Positioning Systems by Javad provides an excellent on-line manual which is also available in PDF format.
3. The Global Positioning System Overview by Peter H. Dana, The Geographer's Craft Project, Department of Geography, the University of Colorado at Boulder, provides comprehensive information on the GPS system and is an excellent reference.

GPS position is determined from at least for satellites to solve for x, y, z, and t.