http://elorantechnologies.com/

http://www.esa.int/esaNA/index.html

The reference for Global Navigation Satellite Systems

http://www.navipedia.net/index.php/Main_Page

http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

www.edu-observatory.org/gps/tracking.html

http://stakeholders.ofcom.org.uk/enforcement/spectrum-enforcement/gpsrepeaters/

http://www.navcen.uscg.gov/