On Fri, 2 Feb 2007 07:01:12 -0500, "Roger Long"
wrote:
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.
The boat speed may be used to adjust the rudder response - I don't
recall any mention of a speed input on my ST5000.
From observing the operation of my ST5000 and an earlier tiller pilot,
when you first select "Steer by GPS", the pilot will look at the
"bearing to waypoint" reported by the GPS, and will set that bearing
as the desired course, and turn the boat to that direction. Once the
boat is on that heading, the pilot will ignore the "bearing to
waypoint", and instead start watching the cross-track error (xte). If
the GPS reports that the xte is to the left of the desired track, the
pilot will adjust the course to steer slightly to the right. If the
vessel remains off-course to the left, the pilot will make larger
adjustments, until the boat is back on the desired track. If it
overshoots, and gets to the right of the desired track, the pilot will
then adjust the desired heading slightly to the left. The result is
that the boat will remain as close as possible to the track from the
origin waypoint (or from where you told the GPS "GoTo Waypoint) to the
destination waypoint.
There are a couple of situations where having the autopilot follow GPS
instruction can be undesirable - but they are situations where the
navigator should take special steps under any situation.
One such situation is crossing a strong current (Gulf Stream, perhaps)
in a slow boat. If the current is faster than the boat, the
autopilot, in an effort to minimize XTE, will eventually have the boat
headed directly upstream, and making no way across the stream - this
is Not Good! Without the GPS, the prudent navigator would probably
head somewhat upstream before entering the strong current, expecting
the current to carry him downstream towards his destination. Having a
GPS and autopilot does not free the navigator from understanding, and
dealing with, such situations.
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.
My GPS and plotting programs, as well as the autopilot, will alarm on
arriving at a waypoint. I consider this a Good Thing, as I don't want
the boat to automatically turn into the path of another vessel. It
appears that the Raymarine pilots look at the destination waypoint
name in the RMB sentence to determine when a new leg starts - I was
beta-testing a chart program that didn't put the waypoint name in the
RMB sentence, so the autopilot had to watch the xte to change to the
new course - this resulted in excessive overshoot before we got back
to the desired new heading.
--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
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