Weather Instrument NMEA sentence

[whiteoaks7]

I'm just completing a software repeater for the wind direction/velocity instrument on my boat which will sit on top of GPSU or the RYA plotter on my laptop. My WI is a NASA clipper and the NMEA0183 sentence it spits out is $WIMWV, for example, $WIMWV,007,R,4.1,N,A*3F which decodes to 007 degs, right of centre (this is relative to boats head) at 4.1 knots. So far no issues, this is easy to parse and display.

The NMEA standard (which I can't afford so I use the (probably old) stuff published on the web) tells me that (a) the checksum (ie the *3F in the example) is optional, (b) the sentence format can contain other groups relating to windspeed in m/s and km/h; and, while the NASA uses a 360 deg output the Standard allows 180 deg and uses L as well as R to distinguish left/right of the WI centre line.

My question is this - does anyone have actual experience of a wind instrument that puts out anything other than the sentence the NASA one uses? As a software writer I would like to cover all eventualities, and while I'm not currently thinking of releasing the program into the wild world, I might one day if there's some demand. I therefore need to cover any "non-NASA" interpretations of the sentence.

If you want to help by capturing the sentence from your own system, you can use Hyperlink to capture the data as it's sent from your WI - I can tell you the setups if you're not sure.

On that point - if anyone is interested in beta testing the program also let me know, I will probably be interested in letting a limited number of beta releases go for about a tenner provided I get feedback.

Cheers, David Berry

 

[tome]

[ QUOTE ]
My question is this - does anyone have actual experience of a wind instrument that puts out anything other than the sentence the NASA one uses?

[/ QUOTE ]

yes, the Raymarine system puts out a relative MWV sentence as you've shown plus a True wind sentence (has a 'T' in place of the 'R'). It also puts out a VHW and VWT plus VPW (speed to windward)

Tacktick puts out just relative MWV as you've shown

Can send you sample data with sentence descriptions. Can even send you time-stamped data so you can see the data rates.

The checksum algorithm is a simple XOR of everything between the $ and the * converted to 2 hex characters. Parsing and displaying the data is pretty straightforward, could even send you the code

 

[whiteoaks7]

Thanks - please send the data. As you say the checksum is easy but my point was that NMEA allow its use or not - it's optional, I just wondered if anyone actually implemented the sentence without the checksum. I understood the "R" in the MWV sentence meant "right", as opposed to "L"eft. Do you know what the final "A" before the "*" means? Apart from that I have "N" for knots, "K" for kilometers per hour and "M" for metres per second, all from the old web page.

 

[PembrokeshirePromise]

Far be it for me to disagree with Tome - as he always seems to know about these things - but I think TackTick can display (via the NMEA interface at least)


MWV - Apparent Wind Angle and Speed
MWV - True Wind Angle and Speed
VWR - Apparent Wind Angle and Speed

Hmm - seems to be something amiss in the doc at this point ?

From:
http://www.tacktick.com/products/userguides/NMEA%20Interface_pc.pdf


Now maybe Tome was meaning from the wind instrument directly - in which case he may well be right - I've not looked at that - only the stream coming out of the NMEA box.

I'll PM you an example.

I've been playing with capturing these, logging graphing etc - though not had much time to put into it.

 

[Aeolus_IV]

If you are interested in the output from a B&G H1000 based system I have a few meg of test data you are welcome to /forums/images/graemlins/smile.gif

Regards, Jeff

 

[tome]

You're probably correct - if you connect compass and gps inputs it will generate the additional sentences.

As you surmised, I was using wind alone via the nmea interface

 

[tome]

Ok PM me an email address and I'll dig out some stuff for you

Here's a description of some nmea sentences at release 2.01. It's an extract of a trials write-up. Doesn't cover them all - only the ones we wanted to look at, but should answer most in common use

If it's a decent graphical interface then worth doing your software. If it's just numbers then save yourself the effort

Good luck

 

[wooslehunter]

Hi Tome,

I'd be interested in seeing what's on the first link but it seems to be broken.

 

[tome]

Having some problems uploading files to the site. Will post a pdf later

 

[tome]

Try this link for the document

 

[whiteoaks7]

Thanks for the inputs. It's clear that I need to read more carefully - I'll now go and untangle my MWV from my VWR !!!