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Old 08-05-2013, 07:24   #1
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Open CPN - ROT & S63 charts

Hi all,

New to this forum and also to Open CPN so would appreciate some advice from those more experienced in this software. Two queries (for now):
  1. Does Open CPN support UKHO AVCS (S63) charts or only the unencrypted S57 vector charts?
  2. Checking NMEA messages, there doesn't seem to be support for rate of turn $GPROT. Is this correct and, if so, are there any plans to add this useful functionality?
Many thanks!
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Old 08-05-2013, 10:38   #2
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Re: Open CPN - ROT & S63 charts

Welcome to OpenCPN, and this forum.

1: No, only S57 unencrypted charts. Chart Formats | Official OpenCPN Homepage
2: OpenCPN displays rate of turn for AIS targets. AIS | Official OpenCPN Homepage

Thomas
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Old 08-05-2013, 15:23   #3
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Re: Open CPN - ROT & S63 charts

Thanks for the links Thomas.

That's a shame about S63 support - it seems US tax payers get a better deal with their free S57s. I'll persevere.

Yes I agree the AIS messages show ROT of other vessels, but OCPN doesn't seem to make use of own vessel ROT from the NMEA messages. This is a shame since you can only definitively describe a body's motion in 2D from its translational (position/COG/SOG) and rotational (HDG/ROT) components.

I suppose the workaround could be to use your own AIVDM message, not the AIVDO, and extract ROT from that. As a beginner with this programme it's not clear to me yet whether OCPN would make any use of this information for reporting own vessel trends.
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Old 08-05-2013, 15:31   #4
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Re: Open CPN - ROT & S63 charts

Quidnunc...
What would ROT from a GPS be good for on a vessel smaller than very big? HDG from a compass vs. COG from a GPS for sure and we have it, of course, but ROT????

Pavel
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Old 09-05-2013, 01:06   #5
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Re: Open CPN - ROT & S63 charts

Pavel,

Maybe I'm not thinking small? COG from past track is fine if you're moving at a reasonable rate, but detecting the rate of change of heading (few deg per minute) is a struggle for a compass.

I'm interested to see how this system develops and hope it will expand to make it even more useful to new groups of users.
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Old 09-05-2013, 01:53   #6
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Re: Open CPN - ROT & S63 charts

If it is a standard message I believe it should be supported. If not, we need it anyway.

If gps is the only data comming in, it is almost useless, however not completely, if the gps can give accurate velocity as well as position (not by derivative of position, but from doppler shift on signal) then it can also give a more accurate heading rate than difference from last two headings.

Most IMU (which is becoming standard instruments now) give absolute as well as rates for all 3 axes of rotation.

Eventually, I plan to write a raw-autopilot plugin to command arbitrary electric motor configurations to steer the rudder, trimtab, or control pendulum vane, rate of change is essential to implement a basic PID-loop, and gps rate of change derived purely from gps positions at 1hz does a poor job (I have tested on my prototype) So to work on only a gps, a more accurate message is important. I prefer magnetometer heading which is much faster, but a rate gyro would be even better.

Maybe one day we will have a plotting plugin that displays all the data as a sweep plot as well as can perform fourier transforms (make it easy to determine roll and pitch period of vessel) and analyze performance in various sea states, calculate boat characteristics etc.. we need rate of change!
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Old 13-05-2013, 17:34   #7
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Re: Open CPN - ROT & S63 charts

I do not understand all of this discussion, but I think Airmar has a very expensive masthead sensor that does a lot of this, also one suggestion was having a number of small puck GPS at various distant locations in the boat and collect that information for pitch and yaw, etc, but someone said that on small boats like ours it would not work, particularly for bearing.

It seems like two people I really respect, don't agree completely with each other about the need for acceleration data, pitch, yaw, turning, etc. which is perfectly ok. I have a small inkling that these sirf chips, etc have opened up some new opportunities, and Sean may be utilizing the technology in the near future. Hopefully Opencpn will benefit as well.
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Old 13-05-2013, 17:47   #8
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Re: Open CPN - ROT & S63 charts

Rick...
We all agree perfectly on usefulness of turning, pitch, yaw acceleration/deceleration etc. data. I also suppose we know that a (normal) GPS is not a device to get that info... This is simply task for other sensors.
For example ROT would be nice, but is a bit less useful for anything related to own vessel when coming from AIS every X seconds (for Class B typically 30) - You will get better info from HDG / COG you have every second at worst... And much better from a couple of gyroscopes.

Pavel
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Old 13-05-2013, 18:02   #9
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Re: Open CPN - ROT & S63 charts

Agreed, you make good points. Two gyroscopes, hadn't thought of that. In small boats with less real estate to work with, two gyroscopes might be a real help clarifying what is happening because of the differentail...I wonder if they've become miniaturized and less expensive, and they'd be more accurate than measuring from a satellite. I don't know that much about the technical aspects of most of these systems....so I apologize.
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Old 13-05-2013, 19:16   #10
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Re: Open CPN - ROT & S63 charts

Rick...
For boat physics, you would need a toy in it's construction similar to http://forum.arduino.cc/index.php/topic,58048.0.html Playing with this stuff is incredible fun and you can sure build more of them than you can carry for the price of something like Product Detail - SC50

Pavel
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