Autopilot NMEA question for gear heads
 

I just bought a Raymarine ST1000 Plus tiller autopilot for my Endeavour 32.
The astute among your will immediately wonder how this will work on a 13,000
pound 32 foot wheel driven boat. The unit will just drive the vane servo
shaft on the Cape Horn steering gear now on order. We could have bought even
a smaller tiller pilot if one was available.

Since I was trying to buy the smallest and cheapest unit I could get, I was
surprised to find out how sophisticated it is. Given the right NMEA input,
it evidently will follow a track to a waypoint instead of just blindly
maintaining a given course. It will even rollover to a new waypoint and run
another leg.

The Raymarine manual describes the inputs needed to follow a track but in
plain English. The Garmin manual for the GPSmap 76Cx I will be using says:

The following are the sentences for NMEA 0183, Version 3.01 output:

Approved sentences-

GPGCA

GPGLL

GPGSA

GPGSV

GPRMB

GPRMC

GPRTE

GPVTG

GPWPL

GPBOD

GPAPB

If anyone could annotate a quoted copy of this list I would appreciate it. I've
tried to find a listing via Google but the only one I can turn up is quickly
replaced by another page before I can read it. Presumably, the Capt'n wants
you to buy the information.

A link to a similar but readable listing would also help.

Any tips or experience interfacing and using one of these autopilots with a
GPS also appreciated.

--
Roger Long

Autopilot NMEA question for gear heads
 

Never mind. I found a list at:

http://aprs.gids.nl/nmea/#allgp

I'd still be interested in hearing about anyone's experience with such a
hook up.

--
Roger Long

Autopilot NMEA question for gear heads
 

Roger Long wrote:
I just bought a Raymarine ST1000 Plus tiller autopilot for my Endeavour
32. The astute among your will immediately wonder how this will work on
a 13,000 pound 32 foot wheel driven boat. The unit will just drive the
vane servo shaft on the Cape Horn steering gear now on order. We could
have bought even a smaller tiller pilot if one was available.

Since I was trying to buy the smallest and cheapest unit I could get, I
was surprised to find out how sophisticated it is. Given the right NMEA
input, it evidently will follow a track to a waypoint instead of just
blindly maintaining a given course. It will even rollover to a new
waypoint and run another leg.

The Raymarine manual describes the inputs needed to follow a track but
in plain English. The Garmin manual for the GPSmap 76Cx I will be using
says:

The following are the sentences for NMEA 0183, Version 3.01 output:

Approved sentences-

GPGCA

GPGLL

GPGSA

GPGSV

GPRMB

GPRMC

GPRTE

GPVTG

GPWPL

GPBOD

GPAPB

If anyone could annotate a quoted copy of this list I would appreciate
it. I've tried to find a listing via Google but the only one I can turn
up is quickly replaced by another page before I can read it. Presumably,
the Capt'n wants you to buy the information.

A link to a similar but readable listing would also help.

Any tips or experience interfacing and using one of these autopilots
with a GPS also appreciated.

Roger,
A good place for NMEA and GPS info is http://vancouver-webpages.com/peter/
krj

Autopilot NMEA question for gear heads
 

In article ,
"Roger Long" wrote:

Never mind. I found a list at:

http://aprs.gids.nl/nmea/#allgp

I'd still be interested in hearing about anyone's experience with such a
hook up.


You'll love it. We have a few paths plotted that take us just to the
edge of shallows we have discovered we can not cross.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's NEW Pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

Autopilot NMEA question for gear heads
 

On Thu, 1 Feb 2007 18:48:55 -0500, "Roger Long"
wrote:

Never mind. I found a list at:

http://aprs.gids.nl/nmea/#allgp

I'd still be interested in hearing about anyone's experience with such a
hook up.

Most auto pilots want to know cross track error ($GPXTE), course over
ground ($GPVTG) and bearing to mark ($GPBWC).

I read throug the Garmin manual online and can't find any mention of
cross track error. I'd call Garmin tech support.

Autopilot NMEA question for gear heads
 

Wayne.B wrote:
On Thu, 1 Feb 2007 18:48:55 -0500, "Roger Long"
wrote:

Never mind. I found a list at:

http://aprs.gids.nl/nmea/#allgp

I'd still be interested in hearing about anyone's experience with such a
hook up.

Most auto pilots want to know cross track error ($GPXTE), course over
ground ($GPVTG) and bearing to mark ($GPBWC).

I read throug the Garmin manual online and can't find any mention of
cross track error. I'd call Garmin tech support.

GPRMB is what most autopilots use. It gives destination waypoint,
originating waypoint, VMG to waypoint and XTE
$GPRMB,A,0.66,L,003,004,4917.24,N,12309.57,W,001.3 ,052.5,000.5,V*0B

A Data status A = OK, V = warning
0.66,L Cross-track error, steer Left to correct
003 Origin waypoint ID
004 Destination waypoint ID
4917.24,N Destination waypoint latitude 49 deg. 17.24 min. N
12309.57,W Destination waypoint longitude 123 deg. 09.57 min. W
001.3 Range to destination, nautical miles
052.5 Bearing to destination
000.5 Velocity towards destination, knots
V Arrival alarm A = arrived, V = not arrived

Autopilot NMEA question for gear heads
 

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

Autopilot NMEA question for gear heads
 

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.

Getting your hands off the wheel doesn't mean no interaction with it.

Autopilot NMEA question for gear heads
 

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.

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


If for some reason you get off your track, the autopilot will make a
converging track to the destination waypoint. I don't know about the
ST1000 but my ST4000+ has an option that can be selected to turn to the
next waypoint or wait for input before turning. I selected wait for me
to decide. There may be some reason I don't want to turn.
krj

Autopilot NMEA question for gear heads
 

On Feb 1, 6:29 pm, "Roger Long" wrote:
I just bought a Raymarine ST1000 Plus tiller autopilot for my Endeavour 32.
The astute among your will immediately wonder how this will work on a 13,000
pound 32 foot wheel driven boat. The unit will just drive the vane servo
shaft on the Cape Horn steering gear now on order. We could have bought even
a smaller tiller pilot if one was available.

Since I was trying to buy the smallest and cheapest unit I could get, I was
surprised to find out how sophisticated it is. Given the right NMEA input,
it evidently will follow a track to a waypoint instead of just blindly
maintaining a given course. It will even rollover to a new waypoint and run
another leg.

The Raymarine manual describes the inputs needed to follow a track but in
plain English. The Garmin manual for the GPSmap 76Cx I will be using says:

The following are the sentences for NMEA 0183, Version 3.01 output:

Approved sentences-

GPGCA

GPGLL

GPGSA

GPGSV

GPRMB

GPRMC

GPRTE

GPVTG

GPWPL

GPBOD

GPAPB

If anyone could annotate a quoted copy of this list I would appreciate it. I've
tried to find a listing via Google but the only one I can turn up is quickly
replaced by another page before I can read it. Presumably, the Capt'n wants
you to buy the information.

A link to a similar but readable listing would also help.

Any tips or experience interfacing and using one of these autopilots with a
GPS also appreciated.

--
Roger Long

Roger,

The ST1000 manual (http://www.raymarine.com/raymarine/Default.asp?
site=1&SECTion=3&Page=337&Parent=166) on page 47 lists the NMEA inputs
accepted. And what sentences contain each item.

Most likely if you just wire up the Garmin and the ST1000, it will
work. The garmin most likely is set to transmit the most usefull NMEA
sentences allready. Sentence APB (Autopilot B) seems most
appropriate.

Check crossing of NMEA + and - if it doesn't work.

Todd Smith

Autopilot NMEA question for gear heads
 

"Roger Long" skrev i en meddelelse
...
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?
SNIP
--
Roger Long
Unfortunately, I'm not familiar with all the NMEA codes etc.

My comment is based on practical experience using a Raymarine autopilot
4000 controlling the tiller on a 34' sailboat. My experience is, that in
case of XTE, the pilot "tries pretty hard to get you back to the
original track" ... (not in 90 degrees toward the track though) ... but
still with a course towards the next waypoint. I'm not aware of the
specifik formula.

On my system it is possible to reset the XTE to zero, in which case, it
will make a new track to follow - i.e. from the actual position in a
straight line to the next waypoint.

If the system more or les automatic - i.e. without intervention - could
follow another track than the original you have planned, you might get
into trouble if the new track passes over a rock or other unpleasant
obstacles ...

I think that is the same argument/reason why the system - not without
your physical intervention/approval (push a button) - will turn to the
next waypont, when you are following a route.

--
Flemming Torp
Gimle - DEN61

Autopilot NMEA question for gear heads
 

"Roger Long" wrote in
:

Any tips or experience interfacing and using one of these autopilots
with a GPS also appreciated.

--
Roger Long


The autopilot doesn't do the navigating. The chart plotter does the
navigating and sends out directional messages to the autopilot via the
network from its calculations taken from GPS position data. i.e. "We
want to go to this waypoint. We are headed in this direction, so we need
to turn the boat 48.5 degrees to starboard to head that way. Our course
over ground to the waypoint shows there is 1.8 knots of current trying to
drag us off course, so we need to subtract that from the boat heading.
Yo! Autopilot! Turn 44.5 degrees to starboard or I'll tell the captain
to replace you!" Autopilot, terrified of being turned into marine scrap,
comes about smartly 44 degrees to starboard, as measured on its compass
I'm convinced, causing an awful gybe because you got cheap and didn't buy
the network wind instrument so Chart Plotter could tell the slaves it
needed to gybe to the new course. The chart plotter makes constant
course corrections and tells the autopilot what to do to make it happen.

Autopilots aren't all that smart, if you haven't guessed that already.
All this is to force you to buy more boxes...GPS boxes...Chart Plotter
boxes...Speed boxes...Gyrocompass boxes...Compass boxes...Wind
boxes...etc. The more boxes, the more data. The more data, the smoother
that turn is because Chart Plotter and Autopilot can see their rate-of-
turn lots more often than waiting on the calculated ROT from the slow GPS
updates. The more boxes, of course, the more profits. Boats are full of
boxes...

Hope the gybe didn't pull apart the traveler that needs replacing....(c;

Larry
--
Democracy is when two wolves and a sheep vote on who's for dinner.
Liberty is when the sheep has his own gun.

Autopilot NMEA question for gear heads
 

"Larry" wrote in message
...
"Roger Long" wrote in
:

Any tips or experience interfacing and using one of these autopilots
with a GPS also appreciated.

--
Roger Long


The autopilot doesn't do the navigating. The chart plotter does the
navigating and sends out directional messages to the autopilot via the
network from its calculations taken from GPS position data. i.e. "We
want to go to this waypoint. We are headed in this direction, so we need
to turn the boat 48.5 degrees to starboard to head that way. Our course
over ground to the waypoint shows there is 1.8 knots of current trying to
drag us off course, so we need to subtract that from the boat heading.
Yo! Autopilot! Turn 44.5 degrees to starboard or I'll tell the captain
to replace you!" Autopilot, terrified of being turned into marine scrap,
comes about smartly 44 degrees to starboard, as measured on its compass
I'm convinced, causing an awful gybe because you got cheap and didn't buy
the network wind instrument so Chart Plotter could tell the slaves it
needed to gybe to the new course. The chart plotter makes constant
course corrections and tells the autopilot what to do to make it happen.

Autopilots aren't all that smart, if you haven't guessed that already.
All this is to force you to buy more boxes...GPS boxes...Chart Plotter
boxes...Speed boxes...Gyrocompass boxes...Compass boxes...Wind
boxes...etc. The more boxes, the more data. The more data, the smoother
that turn is because Chart Plotter and Autopilot can see their rate-of-
turn lots more often than waiting on the calculated ROT from the slow GPS
updates. The more boxes, of course, the more profits. Boats are full of
boxes...

Hope the gybe didn't pull apart the traveler that needs replacing....(c;

Larry
--
Democracy is when two wolves and a sheep vote on who's for dinner.
Liberty is when the sheep has his own gun.

I'm pretty much of the opinion that the only automated course correction on
a sailboat, under sail, should be "Steer To Wind." That doesn't insure that
you will go where you want to go, but it will help prevent the boat suddenly
deciding to tack or gybe because it reached a waypoint that you forgot
about. Of course, missing that waypoint turn might put you aground, but
under no circumstances should the boat be allowed to sail itself for any
length of time without supervision -- fall overboard and the boat continues
merrily along its way. I use my tillerpilot on Essie, but only with someone
keeping an eye on "Otto." If I'm singlehanding, I have a remote for "Otto"
that lets me make corrections from the foredeck.

Autopilot NMEA question for gear heads
 

KLC Lewis wrote:

I'm pretty much of the opinion that the only automated course
correction on a sailboat, under sail, should be "Steer To Wind."

All my steering under sail will be done by the Cape Horn windvane servo
pendulum. The small electronic unit will just tweak the servo shaft while
under power with the primary effort to turn the rudder provided by the
boat's motion, just as under sail.

--
Roger Long

Autopilot NMEA question for gear heads
 

Larry wrote:

The autopilot doesn't do the navigating.

That's not what the manual for the ST1000 says. The GPS can tell it the
bearing, distance, and track error. The ST1000 has a logic circuit to
resolve this into a new heading.

No chartplotter on my simple boat. Not enough room and power for all that
fancy stuff.

I'll only be using this under power as well. See other reply.

Enjoyed the rest of your post though.

--
Roger Long

Autopilot NMEA question for gear heads
 

"KLC Lewis" wrote in
et:

I'm pretty much of the opinion that the only automated course
correction on a sailboat, under sail, should be "Steer To Wind." That
doesn't insure that


Lionheart's Network Pilot (B&G) has 3 options...Steer to Network Wind
instrument's data, steer to the autopilot's compass or take instructions
from one of the chartplotters or The Cap'n.

I agree steering to wind is correct, ESPECIALLY at night sailing short
handed. We never let her go off by herself without someone on watch.
Most of her sailing controls come back into the cockpit under the
hardtop. Sticking to a waypoint course isn't practical in most sailing
situations we encounter. Sailing in the general direction of some
waypoint is nice, though...(c;

If anyone is in a hurry or has to "be somewhere" at a certain time,
please take a jet, not spoil it for the rest of us who are happy to get
there "some time this month". Thanks....

Beer - Check...Food - Check...Water - Check. Who cares who's first?

Larry
--
Democracy is when two wolves and a sheep vote on who's for dinner.
Liberty is when the sheep has his own gun.

Autopilot NMEA question for gear heads
 

"Roger Long" wrote in news:45c39114$0$1339
:

That's not what the manual for the ST1000 says. The GPS can tell it
the
bearing, distance, and track error. The ST1000 has a logic circuit to
resolve this into a new heading.



Like I said, the chartplotter tells it how to steer the boat. A GPS
receiver puts out lat/long/time and calculates course and speed if it has
a little display with a crumb line. You can tell the simple GPSes with
simple displays what waypoint lat/long you'd like to go to and it will
make a waypoint IT can steer the autopilot to. There's no way to input a
waypoint lat/long into the autopilot that has a few course change buttons
and simple controls. $GPAP data coming from the simplest GPS tell it
what to do:
$GPAPA - Autopilot Sentence "A"
$GPAPB - Autopilot Sentence "B"
$GPASD - Autopilot System Data
So, the little GPS you put the waypoint into is the chartplotter, whether
it has a $200 map plug into it or not.

The autopilot has no computing capacity to store waypoints, do great
circle calculations, etc. On your simple installation, I'd like to
recommend the Garmin 76Cx handheld GPS/Chartplotter in its little snapin
mount at your helm. Power is not an issue. It uses so little it will
still give you full navigation on its own batteries if yours fail. It
comes with America Recreational chart, but the CD has all the bigshot
charts you can load into its standard microSD 128MB memory card. The CD
program loads the 76Cx from your laptop, and you activate the charts you
want to BUY online for it.

Now you have a full-featured chartplotter to run the nav solutions and
tracking on that's really neat. I installed one on my buddy's sailboat,
and he also has a Jolly Boat (fiberglass rib boat with 40hp outboard).
We bought a second snap-in GPS mount for the Jolly runabout and you just
pop the 76Cx out of your pocket into whichever boat you're on and have a
full navigator, even on a boat with a 40hp Yamaha outboard....overkill?..
(c;

The little autopilots I know of only have a fluxgate compass to steer
with.

Larry
--
Democracy is when two wolves and a sheep vote on who's for dinner.
Liberty is when the sheep has his own gun.

Autopilot NMEA question for gear heads
 

Roger Long wrote:
Larry wrote:

The autopilot doesn't do the navigating.

That's not what the manual for the ST1000 says. The GPS can tell it the
bearing, distance, and track error. The ST1000 has a logic circuit to
resolve this into a new heading.

No chartplotter on my simple boat. Not enough room and power for all
that fancy stuff.

I'll only be using this under power as well. See other reply.

Enjoyed the rest of your post though.

You have a GPSMap 76. Doesn't that count as a chartplotter?

Autopilot NMEA question for gear heads
 

"Roger Long" wrote in message
...
KLC Lewis wrote:

I'm pretty much of the opinion that the only automated course
correction on a sailboat, under sail, should be "Steer To Wind."

All my steering under sail will be done by the Cape Horn windvane servo
pendulum. The small electronic unit will just tweak the servo shaft while
under power with the primary effort to turn the rudder provided by the
boat's motion, just as under sail.

--
Roger Long

Ya, Cape Horns are good units. I want to figure out how to add one to my
boat, but with my davits it's a bit difficult.

Autopilot NMEA question for gear heads
 

"Larry" wrote in message
...

I agree steering to wind is correct, ESPECIALLY at night sailing short
handed. We never let her go off by herself without someone on watch.
Most of her sailing controls come back into the cockpit under the
hardtop. Sticking to a waypoint course isn't practical in most sailing
situations we encounter. Sailing in the general direction of some
waypoint is nice, though...(c;

If anyone is in a hurry or has to "be somewhere" at a certain time,
please take a jet, not spoil it for the rest of us who are happy to get
there "some time this month". Thanks....

Beer - Check...Food - Check...Water - Check. Who cares who's first?

Larry
--
Democracy is when two wolves and a sheep vote on who's for dinner.
Liberty is when the sheep has his own gun.

I've charted-out my proposed route for my wee-long July cruise around the
northern end of Lake Michigan. In theory, it's seven days leaving Marinette
(home port), hitting Washington Island, Snail Shell Harbor at Fayette MI,
Manistique MI, St.James Harbor at Beaver Island, Leland MI, Stugeon Bay via
the Canal, and back to Marinette. In theory. I'll be gob-smacked if that's
where I actually get to go.

Karin

Autopilot NMEA question for gear heads
 

"KLC Lewis" wrote in message
...

"Larry" wrote in message
...

I agree steering to wind is correct, ESPECIALLY at night sailing short
handed. We never let her go off by herself without someone on watch.
Most of her sailing controls come back into the cockpit under the
hardtop. Sticking to a waypoint course isn't practical in most sailing
situations we encounter. Sailing in the general direction of some
waypoint is nice, though...(c;

If anyone is in a hurry or has to "be somewhere" at a certain time,
please take a jet, not spoil it for the rest of us who are happy to get
there "some time this month". Thanks....

Beer - Check...Food - Check...Water - Check. Who cares who's first?

Larry
--
Democracy is when two wolves and a sheep vote on who's for dinner.
Liberty is when the sheep has his own gun.

I've charted-out my proposed route for my wee-long July cruise around the
northern end of Lake Michigan. In theory, it's seven days leaving
Marinette (home port), hitting Washington Island, Snail Shell Harbor at
Fayette MI, Manistique MI, St.James Harbor at Beaver Island, Leland MI,
Stugeon Bay via the Canal, and back to Marinette. In theory. I'll be
gob-smacked if that's where I actually get to go.

Karin


That should have been "weeK-long," but the way weather can blow up here,
"wee-long" might actually turn out to be prophetic. :-D

Autopilot NMEA question for gear heads
 

Larry wrote:

On your simple installation, I'd like to
recommend the Garmin 76Cx handheld GPS/Chartplotter in its little
snapin mount at your helm. Power is not an issue.

Exactly the one I bought my boatpartner for Christmas. Great minds think
alike.

I hadn't thought of it as a chart plotter but now that you mention it, it
really is.

I bought a micro SD card with my new cell phone last year and got a 1 gig
card because that was all that was on the rack. I've gotten tired of making
crappy looking videos and pictures with the cell phone so I switched. The
GPS is now loaded with every street map from here to Florida and out to
nearly the Misissippi. I went to Washington DC last month and I would still
be driving around in circles if I hadn't taken the GPS along. I thought the
Thunderingly Stupid Agency might get weird about my taking a GPS on the
plane but they were more concerned with confiscating my 1 OZ, factory
sealed, travel size mouthwash bottle because it wasn't in the plastic baggie
that is supposed to slow down a terrorist enough for the cabin crew to jump
on him.

Even when I missed turns, the Garmin just recalculated and gave me a new
line to follow back to my route.

A ten buck card reader writer from Radio Shack makes loading and unloading
the maps faster and easier. The street maps even have the dirt road into my
family's summer cottages so 2 gig card (the largest it will take) should
handle every chart I'll need to get from here to Newfoundland and back.

I bought this unit because it is the same size as the black and white 76 I
also have but my aging eyes need the color. I'd alread built this mount

http://home.maine.rr.com/rlma/Strider0601.htm#gps

for the old one so I didn't shop but it was a good choice.

--
Roger Long

Autopilot NMEA question for gear heads
 

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
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

Autopilot NMEA question for gear heads
 

"Roger Long" wrote in news:45c3d0bc$0$28089
:

I hadn't thought of it as a chart plotter but now that you mention it, it
really is.



Yep, Garmin has the unlock codes for sale....just hand over your money.

Larry
--
Democracy is when two wolves and a sheep vote on who's for dinner.
Liberty is when the sheep has his own gun.