References
http://en.wikipedia.org/wiki/Global_Positioning_System
http://adventure.howstuffworks.com/outdoor-activities/hiking/how-to-use-gps.htm
http://www.gps.gov/applications/
http://www.montana.edu/gps/understd.html
http://www.webopedia.com/TERM/G/GPS.html
http://electronics.howstuffworks.com/gadgets/travel/gps.htm