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Old 16-09-2010, 08:00   #1
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Which NMEA Sentences Does an Autopilot Use ?


I am connecting my nmea devices and my board pc.
I am doing ok at the moment but there are areas that are still blurry.

For instance: what data does the autopilot get from the GPS when steering a route or from the pc with the chartplotter software?

I can't find that with google or a forum search.

Either the autopilot receives a waypoint to steer to and the electronics do the calculations in some internal plotter function and sends the "low tech" commands to the reversible hydraulic pump (in my case there is a hydr pump)
Or the APA resp. the APB sentences come with some specification, which i don't know. Can anybody shine his or hers light on this?

Thanks, Len.

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Old 16-09-2010, 15:10   #2
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the book from your manufactures has what will be needed accepted

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Old 16-09-2010, 15:28   #3
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I think what you are trying to say is: RTFM.

IMHO the information about NMEA is pretty scarce. In my Simrad AP300X manual there is very little that helps me understand what is really happening there.

But your manual must be a wealth of information, looking at your response.
Could you tell me what manual that is, so I can download it and take a look?

My warmest regards,
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Old 16-09-2010, 16:24   #4
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Len what I'm saying is each component has a book with the sentences stated, at least mine do on Nexus, Sitex and Raymarine.

If you don't have them contact the manufacturer, they absolutly have it.

You absolutely need it.
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Old 16-09-2010, 16:41   #5
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If you set the GPS to go to a way point or route it sends the sentences that tell the autopilot what the course to steer and is. You need to check both the GPS and autopilot to see what ones they send and receive. The GPS only sends. There is nothing in the autopilot except it can tell the current heading over ground over time via the fluxgate and the memory in the auto pilot. It also knows the rudder angle from the sensor. Wheel pilots are no so good at rudder. There is an extra level of assumption with them.

Where autopilots get different is how they use that data they have and can get from the GPS. If you compute course over time plus add rudder angle the course computer can determine how effective the rudder is over time and determine how far to turn the rudder for how long to make a course correction. They all do this differently and don't need the GPS just the fluxgate and rudder indicator. Based on the various sentences they accept and what the course computer does can vary. Better auto pilots can learn the sea state and others need to be told by you. You can be pointed due east yet heading NE. Crossing the Gulf Stream it can happen on smaller boats.

If you have a wheel pilot - don't bother. Set your GPS and monitor the results it shows you and adjust the wheel pilot accordingly. Based on the GPS compass compared to the magnetic compass you see the difference. What no auto pilot can do is compute set and drift. When the boat speed is slow compared to the current the autopilot can fail to get you there totally or greatly increase the time. In a strong current where you are crossing 90 degrees and the boat speed through the water is less than the cross current the auto pilot will eventually have you going head into the current going back wards and not be able to do any better. The extreme example shows the problem.

Using set and drift you compute the straight line across the current and add the offset from the current based on the time to get across. Steering below the destination will let you end up where you want to be over the time it takes to cross the extra distance. In a current, the boat will always follow the current direction at the speed the water ins moving over ground because the is in the water but it also follows the powered rudder direction too. You add them up to see where you end up over the same period of time. This is what the autopilot can't do.

It is because of this that you need a magnetic and a GPS compass and why they don't always agree. The autopilot doers not know why the two don't agree and assumes it is because of the "effective" rudder vs sea state. For power boats they add a gyroscope to measure the acceleration but in a slower boat like anything under sail it really does little.

To get all the way there you would need a ring laser gyro. Very cool gadgets that run about $80,000.00. With that you could have the ultimate course computer. They can subtract the earths rotation out of the equation too and compute the sum of the changes in velocity to determine exact X,Y, and Z position over time given an exact location. They call it a cruise missile.

Personally I never do it that way. I plot out my course before I leave and load the GPS with a basic route. I manually set my autopilot and watch it and make some adjustments knowing if there is current to compute or not. The idea is you study all this before you set off so you don't do it on the fly. On the fly navigation can be quite dangerous because you can make one mistake and send yourself off the edge of the world. It's easy enough to do it many time correctly but that one time it may be an OMG moment - we are not supposed to be here!

Not all auto pilots can do all of this. I know how straight a course mine can steer and it is enough for when I need it. Once you know your own limits you can use it in a safe navigation approach.
Paul Blais
s/v Bright Eyes Gozzard 36
37 15.7 N 76 28.9 W
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Old 16-09-2010, 16:54   #6
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Or, to answer the question, I have found that most autopilots respond correctly to the NMEA $xxRMB message.

** RMB - Recommended Minimum Navigation Information
** 14
** 1 2 3 4 5 6 7 8 9 10 11 12 13|
** | | | | | | | | | | | | | |
** $--RMB,A,x.x,a,c--c,c--c,llll.ll,a,yyyyy.yy,a,x.x,x.x,x.x,A*hh<CR><LF>
** Field Number:
** 1) Status, V = Navigation receiver warning
** 2) Cross Track error - nautical miles
** 3) Direction to Steer, Left or Right
** 4) TO Waypoint ID
** 5) FROM Waypoint ID
** 6) Destination Waypoint Latitude
** 7) N or S
** 8) Destination Waypoint Longitude
** 9) E or W
** 10) Range to destination in nautical miles
** 11) Bearing to destination in degrees True
** 12) Destination closing velocity in knots
** 13) Arrival Status, A = Arrival Circle Entered
** 14) Checksum

This is what we use in OpenCPN, with good results. YMMV.

Good Luck
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Old 16-09-2010, 17:21   #7
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here's what it should look like

The SPX-5 Wheel Course Computer supports the following NMEA0183 sentences.

NMEA0183 to Course Computer
NMEA0183 from Course Computer
* The computer will only transmit these items if it has received the appropriate data.

Header Data in
APB Cross track error, bearing to waypoint, waypoint number
BWR Bearing to waypoint, distance to waypoint, waypoint number, time
GLL Latitude/longitude, time
HDT Heading
MWV Apparent wind angle, apparent wind speed
RMB Cross track error, bearing to waypoint, distance to waypoint, waypoint
RMC Course over ground (COG), speed over ground (SOG), latitude/longitude,
time, variation
VHW Speed through water, heading
VTG Course over ground (COG), speed over ground (SOG)
XTE Cross track error
ZDA Time, date

from course computer

HDG Heading
RSA Rudder angle. Available only if Rudder Reference option is fitted.

note For instance HDM and HDT are both compass heading sentences 1 magnetic and the other true....when you consider that virtually all systems around these days can automatically convert magnetic to true you will only need both sentences if the kit can't do the conversion, in which case it probably pre-dates the ark

Hope that helps

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Old 17-09-2010, 02:21   #8
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Thanks folks,
Dave's post was exactly what I was looking for. Paul is right of course regarding limitations and risks. But you know how it goes.... When something is technically possible, you want to understand it and you want to see it work. Using the gadget in the real practical situation is a whole other subject. I think nobody in his/her right mind will ever let the GPS/PC and the autopilot steer without being constantly aware of what is happening and where.

Cheers, Len.

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