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WHY HOOK UP A GPS TO AN AUTOPILOT?
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Many boaters wonder about the benefits of connecting their autopilot to a GPS. Here are some answers to this and other commonly asked questions about GPS and autopilot interfacing:
Why should a boater connect a GPS to his autopilot?
A vessel's course is much more accurate when an autopilot operates from GPS input than from compass input. A compass-specified heading keeps the vessel's bow pointed in a particular direction. However, wind or current can cause lateral drifting that results in the boat not arriving at the destination at which its bow was originally pointed.
With GPS input, the autopilot will stay on the cross track, or the line between the starting point of the trip and the final waypoint of the trip. This boosts average speed and reduces fuel consumption.
How does the GPS communicate with an autopilot?
The GPS and autopilot essentially have a one-way conversation in which the GPS "talks" and the autopilot "listens." The GPS sends an electronic "sentence" down a two-wire interface to the autopilot. The sentence contains information such as how far the boat is from the cross track, the bearing to the waypoint and whether the vessel has entered an arrival circle around the waypoint.
The autopilot uses this information to adjust the heading of the boat to remain on the cross track. Thus, if there is wind or current, the vessel's bow will not point at the waypoint but in the appropriate direction to compensate for the wind or current.
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What should one look for in a GPS that will be linked to an autopilot?
The GPS should output an NMEA 0183 autopilot sentence, preferably of the APB type. If a GPS does not output APB, it will sometimes output other NMEA sentences that provide the information the autopilot needs. An autopilot manual should indicate which sentences it can use.
What should one look for in an autopilot that will be linked to a GPS?
An NMEA 0183 interface should be standard equipment. High end units may also work with the old standards, NMEA 0180 and 0182. Some sophisticated autopilots will have "automatic waypoint sequencing," which allows the autopilot to automatically steer the next leg of the route once it reaches a waypoint. With this feature, a boat can follow a long route through a maze of islands without a boater touching the helm. Combining a GPS with a modern autopilot, like one of the ComNav models, results in a well-integrated navigation and control system that substantially improves a vessel's performance.
From ComNav, 1915 Stainsbury Avenue, Vancouver, BC, Canada V5N 2M6. Phone: 800-428-0212 or 604-872-3000; fax: 800-470-9611 or 604-872-3955. Website: http://www.comnavmarine.com.