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IS20 speed calibration for NMEA0183 devices

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Nick_R View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Nick_R Quote  Post ReplyReply Direct Link To This Post Topic: IS20 speed calibration for NMEA0183 devices
    Posted: 15 May 2014 at 07:59
Hi,
As described in this mast repeater thread I noticed a problem. The boat speed value sent from Simnet to NMEA0183 devices (using an AT10 converter) appears to be the uncalibrated value from the sensor instead of the calibrated value displayed on the IS20 instruments. The difference corresponds to the 1.15 correction factor which I have entered in the IS20 Graphics instrument. This affects the values shown on the NMEA0183 speed and wind repeater on the mast, as well as my PC racing tactical software.

My question is, have you noticed this discrepancy and is it dependent on how you calibrate the IS20? What I mean is that the speed calibration can be done in two ways: either by entering a manual correction factor, as I have done, or by using the "Calibrate w/SOG" function, whereby the correction factor resets to 1.00.

The text (from the user manual) appears to suggest that if I manually enter a correction factor, it is only stored in the IS20 instruments. Whereas if I use the "calibrate w/SOG" option, the calibration data would be stored in the sensor itself and then the calibrated speed would also show up on the NMEA0183 output.

This would be what I need. But I'm a little hesitant to try it out because then I lose my current 1.15 reference value, so if any of you can give advice I'd appreciate it. The speed sensor is an Airmar DST800.
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Johan Hackman View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Johan Hackman Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2014 at 09:24
I don't know if the NMEA0183 output from the IS20 sends out any correction data at all.

What I do know is that when you calibrate the boat speed the correction value is stored within the instrument and are broadcast to all the other instruments in a proprietary SimNet message. This message cannot be read by any other brand. It also does not matter if the calibration is done by using the GPS or by typing in the correction value.

I have a Furuno Wind instrument that replaces the non-functional IS20 Wind. I was making sure to replace it with an instrument where the correction factor could be entered locally.

For comparison, the wind sensor off-set is saved in the sensor so if I change that value in one of my IS20 Graphic instruments, the off-set will change in the Furuno as well. In addition, the off-set will be read by the Furuno locally but that is a superfluous feature.

I don't know about your other instruments but I fear that they will only read the uncorrected value and if it cannot be adjusted locally, they will show the uncorrected speed.

The speed correction value could be stored in the DST800 but you need an instrument that is capable of doing that, and IS20 isn't.

Johan
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Mark_J1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mark_J1 Quote  Post ReplyReply Direct Link To This Post Posted: 01 June 2014 at 11:46

Nick_R and Johan,

I came across this snag too. The 'feature' became apparent when replacing my old analogue log transducer with a new analogue version of the DST800 to give me a port and starboard pair of depth transducers.  

Depth information translated from Simnet to NMEA0183 seems to be picked up by some devices as the uncorrected depth.  I was blaming the AT10 converter, but on analysing the 0183 message traces I found that the correct offset was there in the NMEA DPT message (AT10 manual suggests it only forwards DPT messages.  DBT though likely to be present on Simnet, has no offset & is not forwarded).  The downstream devices eg a McMurdo Navtex display repeater was not dealing with it correctly.  I had similar problems with PC based NavMon, PolarNavy and SeaClear software initially.  

The only 'total' solution I've come up with so far is to put the NMEA0183 stream through an intelligent NMEA buffer/filter (eg ShipModul or Actisense) to manipulate the DPT message and insert the manipulated correction into the primary depth field.  To date I've only prototyped this with a PC as for me the problem hasn't warranted the cost of a dedicated buffer.  Though I have looked at a DIY Raspberry PI project to do this a little more cheaply and to incorporate other features eg NMEA0183 over WiFi.

JFYI NavMon and PolarNavy are able to be corrected 'locally' in the software configuration, and can be the intelligent buffer if you don't mind the power drain of running them constantly on a PC while sailing.  SeaClear is lacking development these days and does not have this feature.

For others reading this and just for clarity, the DST800 comes in 'intelligent' and dumb 'analogue' types. I know this confused me for a while as I hunted a replacement transducer.

The intelligent versions have the necessary electronics to create NMEA2000 PGNs or NMEA0183 sentences directly and need connection to Simnet / SeatalkNG etc or to a NMEA0183 listener like the chart plotter.  A software tool is needed for calibration in the sensor or an instrument that has the capability to do this via NMEA2000.  

The analogue DST800 version needs an instrument in the loop to take the raw transducer data and provide this to the boat network in whatever form you want.  In my case, I still have a SIMRAD IS12 Combi instrument taking the raw depth data and presenting it as NMEA2000 on Simnet. 

Got to love all this network nonsense.  My boat is not unusual in running kit from multiple manufactures and as a consequence it has the following networks:

  • 3 x NMEA0183 layers
  • Canbus SIMNET NMEA2000
  • Seatalk
  • Canbus SeatalkNG NMEA2000
  • IP on Ethernet SeatalkHS
  • Simrad Robnet2
  • 'Domestic' IP on Ethernet and WiFi
That's 7 different networks to become familiar with and for data to move between!  I think the only one in the Hanse world I'm missing is Simrad's ethernet NSS as I don't have a Simrad chartplotter.  Frankly I'm amazed that the interoperability works at all :)  

regards

Mark


Hanse 400e "Grey Goose" Hull #31
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