GPS navigation systems have become commonplace on boats. Garmin is one of the leading manufacturers and several strong companies, such as Humminbird, are around that specialize only in marine electronic systems. Many gps chartplotter units have an internal antenna and can optionally be hooked up with an external antenna, while other units require have no internal antenna whatsoever and thus require a separate external antenna. An internal antenna is attractive since this means one less piece of boat rigging to handle. This is provided, of course, that the internal antenna gives you a strong enough signal. Most boaters agree that if you do not have a hardtop on your boat, an internal one is fine. Opinions are more mixed in the case of hardtops.