GPS information is gathered from an internal GPS antenna. No external antenna required. The Faria GPS Speedometer uses a highly accurate 48 channel GPS receiver. You can be sure that the Faria GPS Speedometer is giving you the most accurate GPS information available on the market today.Course Over Ground (COG) and actual heading (compass heading over ground) are displayed on the optional digital LCD.Speed data is shown by an analog pointer. This pointer is driven by a digital stepper motor for increased accuracy and minimized pointer bounce during vessel operation. Premium LED back-lit dials No external GPS antenna required Available LCD displays Compass Rose heading and Actual heading (COG) Ultra fast Satellite acquisition time (TTFF) 1 second from Hot start Speed Accuracy of +/- 1 MPH Heading Accuracy of +/- 1 Degree Digital stepper motor driven pointers Ideal replacement for speed sensing devices (pitot tube and paddle wheel) that typically fail over time Perfect for slow moving vessels where pitot tubes typically don't work 4" - Hole size: 3.375" (85mm)