GPS related websites

The Global Positioning System  -   http://erg.usgs.gov/isb/pubs/factsheets/fs06299.html
United States Geological Survey fact sheet

The Global Positioning System  -   http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
 Peter H. Dana, The Geographer's Craft Project,
Department of Geography, The University of Colorado at Boulder.

GPS Guide for Beginners  -   http://www.garmin.com/manuals/GPSGuideforBeginners_Manual.pdf
From Garmin.

What is GPS?  -   http://www.garmin.com/aboutGPS/
A brief explanatory page by Garmin.

Trimble - All About GPS  -  http://www.trimble.com/gps/