Some GPS's (Like Northstar) have a contact closure on the Data Cable. This can
be set up to go to an external Alarm I'E Siren or I have even hooked this up to
a vessels horns to alert the crew when they are in the bar that there anchor is
dragging. What kind of GPS do you have? Check and see if it has a contact
closure wires in the manual. If so it is a very easy setup.

BC