Do I Need a GPS?
GPS (Global Positioning System) data are not required for radio use, but use is strongly encouraged. If you do not connect a GPS receiver to your DSC radio, you must enter position data manually at least every 12 hours to retain use of the digital distress hailing feature.
GPS signals are required if you implement AIS (Automatic Identification System) on your vessel.
USPS offers The Boatowner's Guide to GMDSS and Marine Radio, a guide to the application and use of GMDSS systems. This text is written for the recreational boater and is available from International Marine / Mc-Graw Hill publishers.