T&T: NAV mode and autopilots

[Peter Bennett]

Tuesday, November 27, 2007, 3:51:59 PM, capteric36 wrote:

c>  Peter responded to a note I sent him with this:
c> <snip>
c> Eric, Eric Eric!!!

c> course steered and course to waypoint?? Both are 
c> vividly displayed on the laptop screen attached to 
c> one of my handheld Magellans. And YES they are 
c> only a visual reference. Using AUTO would mean I 
c> don't have a reference between the two courses
c> <snip>

c>  Well Peter you are right that the information IS 
c> available on the display of just about any GPS 
c> system.
c>  BUT (you knew there would be one, right?) that 
c> information is not sent to the autopilot, except 
c> in one case.
c>  There are three NMEA 0183 sentences used in 
c> autopilots to allow the NAV mode to work. The 
c> first and most commonly used one is the XTE 
c> sentence. This sentence contains NO information on 
c> the course at ALL. It only contains cross track 
c> error information and so has no relation to the 
c> angle to the destination waypoint but only the 
c> distance from the intended track to your boat's 
c> current position.
c>  The second is APA. Also a commonly used sentence 
c> and also contains NO information about the course 
c> to steer, just cross track error and magnetic 
c> bearing FROM ORIGIN waypoint TO destination 
c> waypoint, information useless in the real world.
c>  The third is APB. Now THIS sentence does contain 
c> the information we were talking about. It has 
c> cross track error, the magnetic OR true heading 
c> from origin to destination waypoints, the magnetic 
c> OR true Bering from present position to the 
c> destination waypoint (This is the item of 
c> discussion here), and a "magnetic or true heading 
c> to steer". These last two bytes (separated by a 
c> comma) in the sentence are not carefully defined 
c> and may, in fact, be the direct course to the 
c> destination waypoint.

I believe APA and APB are considered obsolete, and have been replaced
by RMB, which includes both bearing to destination and cross-track
error.

Although I haven't any direct knowledge of autopilot internal
workings, it has always seemed to me that when the "Nav" or "Track"
mode on an autopilot is first engaged, the pilot will look at "Bearing
to Waypoint", and steer the boat to that heading.  It will then turn
its attention to XTE, and adjust the boat's heading as needed to keep
the XTE to a minimum.

Incidently, Raymarine has installed a bug in their recent
Smartpilots - if the XTE exceeds about 60 ft (0.01 mile) the pilot
will make a 30 degree turn to get back on track.  As many GPS
receivers and charting programs only report XTE to 0.01 mile, this can
make the pilot operation in NAV mode very hairy!!  I was unable to use
Nav mode this summer due to this problem, and Raymarine says that I'm
not the only one to complain, but so far don't know when or if they'll
fix it.  Apparently this "feature" was added so that users would
arrive closer to the destination waypoint than with previous versions
of the software. (They did offer to "downgrade" my pilot to the
previous version of the software, but I'll wait til spring and see if
they have a fix...)


-- 
Peter Bennett, VE7CEI    Vancouver, B.C., Canada
Ennos 31 "Honeycomb"
GPS and NMEA info: http://vancouver-webpages.com/peter 
Vancouver Power Squadron: http://vancouver.powersquadron.ca