Ah, so these are different formats of data groups and not variables as I
thought. The ST1000 needs the cross track error to follow a track and it
looks like it will get it from the GPRMB which my GPS does put out.

The ST1000 manual isn't very clear but, reading between the lines, it uses
either input from a speed device or a default cruising speed set in
calibration to set a new course according to the cross track error. This
makes me think that it doesn't take the boat back to the track but simply
sets a new course directly to the waypoint.

Can anyone clairify this?

It also appears that, although it will roll over to the next waypoint, it
won't actually turn the boat until you push the keypad. That's good if
something is in the way; bad if you miss the waypoint alarm. Of course, as
the manual says over and over, you should be independently tracking your
position constantly. One of the thinkgs I look forward to in being able to
get my hands off the wheel is to start doing a lot more navigation.

All this makes me wonder. Can more sophisticated autopilots be made to
follow a series of legs without intervention? If so, it's only a matter of
time before someone figures out that a small ship filled with explosives
could be turned into an marine guided missile. The probability is about
zero since the terrorists like to blow up with their bombs and there are
much more effective ways to create mahem. I have to wonder though if the
people obsessed with taking away our 6 oz tubes of toothbpaste and hair gel
might be coming after our navigation electronics at some point.

--
Roger Long