GPS Speedometers offer significant advantages over marine speedometers that utilize pitot tubes or paddle wheels as speed sensors. State-of-the-art satellite technology measures true boat speed over ground, not through the water, with superior accuracy across their entire range. GPS Speedometers also provide an LCD heading display of your vessels Course Over Ground so that youll know at a glance not only how fast youre moving, but also where youre headed. Ideal for everything from low-speed trolling to high-speed running where conventional instruments may be affected by current, fouling or marine growth. With no moving parts or sensors in the water to corrode or break, they are highly dependable, reduce drag and eliminate potential thru-hull leaks. Both white-faced Premier or black-faced Amega models feature enhanced illumination for excellent readability at night and scratch-resistant, domed glass lenses with 4" diameter (3 13/16" cutout) polished stainless steel bezels. All units use either your boat's existing NMEA 0183 GPS receiver or Model 11886934 GPS Receiver/Antennae.