Mystery (to me) nmea sentence

[liberte.alone]

Hello all! while writing a tutorial on NMEA parsing I captured a NMEA sentence that I am unable to find any information on.

I would be very grateful if someone, anyone could shed some light on this!

 

Here is the sentence as received:

$GPBT,INFO1,INFO2,INFO3,INFO4,1,0,12,1,100,32,65536,77120,724,724*06

 

I can find zero information on it at all.

 

Much Thanks,

 

Liberte.alone

[RamblinBear]

Not much information on that one at all. The only information I have found is: $GPDBT - Depth Below Transducer

on this page NMEA sentence information

If you scroll up that page you might find some information to help you interpret the syntax of that particular sentence.

 

From what I can see the NMEA strings all begin with $GP*** (6 characters in total) whereas the string you have is only $GP** (5 characters) which leads me to suspect that the source you got it from missed off a letter!

Edited June 1, 2015 by RamblinBear

[+ecanderson]

Would appear to be a proprietary string.

[robertlipe]

Proprietary sentences are SUPPOSED TO start $Pxxx where xxx is a 3 letter code for the maker, like GRM for MGN. I agree it's depth below transducer and you can find some evidence of that at, say, https://joinup.ec.europa.eu/svn/gvsig-mobile/pilots/branches/libLocation/src/org/gvsig/location/nmea/decoder/DbtDecoder.java, but their description doesn't match your example. I don't support it in GPSBabel and have no records of requests for it.

 

That said, there's a lot of "slang" NMEA because NMEA is "old school" standards body that charges $250 for the actual spec, despite large amounts of it being self-evident upon inspection and well documented on a number of sites.

 

http://www.nmea.org/content/nmea_standards/nmea_0183_v_410.asp if you're willing to read the authoritative words.

[liberte.alone]

Thank you very much, all of you!

I found much the same. After many days of logging and watching the string I was able to at least figure out its origin.

It seams that the MTK chipset in this device (Columbus v-800+) has the ability to log all the tracks, data etc into its 64 meg storage. It would appear that the designers of the firmware used a NMEA sentence structure to place all of the "stats" into for retrieval by I assume their app that lets you pull the logs etc. Of course they don't have (or wont respond) any documentation on this at all.

The last field in the sentence increments by 1 every time you power the device on. the other "64k" field is the amount of storage left available. Other than that I have no idea what it all does.

 

Robertlipe, Thank you for the link to the authoritative word! While "researching all of "this" I have become very interested in all the data flowing down from the sky. I think I will be looking for a receiver that receives a lot more of the different sentence types. (From the old to the new formats).

 

RamblinBear, Yes, I thought the same, but, I received that sentence in the serial stream from my gps device and parsed it out as it came from the receiver...

 

Thank you all for assisting on the wild goose chase!

 

Many thanks,

 

Liberte.alone

[robertlipe]

Wait, so despite there being a 32 year old standard that includes an explicit escape hatch for strings NOT defined by that spec, Columbus ignored that guideline and hijacked an EXISTING NMEA sentence and used it for something utterly unrelated? That's sub-awesome for compatibility.

 

This is why I (the GPSBabel guy, not the forum moderator) really really dislike most of these cheapo data loggers. There's no dignity in their engineering. They take a reference schematic and create new plasticware and battery configuration and try to figure out how to sell it a nickel cheaper than the guys down the street while still doing SOMETHING to make it unique at the lowest possible cost. Ugh.

 

FWIW, I've found Semson's to be the most knowledgeable place about the logger class of receivers. If you need to understand bits and bytes (as the GPSBabel dude, I often do) they're the only place I've found that seems to have any actual knowledge of the products. They can actually answer questions like "what NMEA sentences are emitted?" while that's the kind of thing you'll never get from an REI or an Amazon.

 

Good luck.

 

BTW, since you seem insufficiently scared of programmer-y things relating to NMEA, I'd appreciate a hand with http://www.gpsbabel.org/htmldoc-development/fmt_nmea.html and https://code.google.com/p/gpsbabel/source/browse/trunk/gpsbabel/nmea.cc

[+ecanderson]

Ref the https://code.google.com/p/gpsbabel/source/browse/trunk/gpsbabel/nmea.cc link, looks like a nice crack at an NMEA parser. Without digging through all of the sentences it's capable of handling nor checking for content being parsed from the sentences, I'm not clear about intent. Are you looking for help to extend the types of sentences parsed, or cleaning up or correcting the existing code, or ???