T&T: NAV mode and autopilots

[capteric36]

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

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

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

 Now back when I was doing the interface testing 
at Trimble Navigation I found that most autopilots 
accepted the XTE sentence, a few accepted the APA 
sentence but none accepted the APB sentence.
 If you are not sure which sentence your autopilot 
accepts and uses for steering your boat, try each 
one in turn and find out!

 It could save your life to make your autopilot 
ONLY use the XTE sentence as that is the one which 
will return you to your intended track the 
fastest. APA will also return you to your intended 
track if the autopilot does not accept the XTE 
sentence.
 In NO CASE should you send the APB sentence to 
your autopilot as it may allow the autopilot to 
ignore all your careful planning and just steer 
you towards the destination waypoint without 
regard for the intended track you so painstakingly 
plotted.

 Just my $0.02 worth.

Eric Thompson
S/V Procrastinator
South San Francisco
capteric36 at sbcglobal.net