Using NMEA from 'Fasttrax' GPS devices

DC KelleyDC Kelley Global Mapper UserPosts: 77Trusted User
edited March 2012 in GPS Features
Here is a short NMEA example from a stock GPS set sold by Fastrax (http://www.fastraxgps.com/) and this file is what it produces 'out of the box' as a saved NMEA file. This particular device is a "GNSS" and it is tracking both GPS and GLONASS in this run with some WAAS corrections. Beside the stock $GPGGA and $GPRMC sentences there are some odd (to me) vendor sentences here such $PSTMTS which I presume GM ignores.

nmea.txt

The reasons for the post is that GM plots this data as a vertical line with about a 7km offset, when in fact it is a stationary position. It seems to ping between two such points with a sub meter offset. Apart from the normal random walk effects, this not what I expected from the data I see in this file. [And unlike another recent post, this device seems to create perfectly valid NMEA content itself, it just has a ton of other details not of interest to GM]

I would like to learn why this plots oddly, and what sort of pre-processing filter we might need to build and use to have GM consume these. If I manually reduce it to just a few $GPGGA lines it seems fine.

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2012
    I took a look and it appears there was a bug in the reading of latitudes from $GPRMC sentences formatted as DDMM.MMMMM. I have fixed that now and the data lines up properly. I have placed a new build at http://www.globalmapper.com/global_mapper13.zip with the latest changes for you to try. Simply download that file and extract the contents into your existing v13.xx installation folder to give it a try. If you are using the 64-bit v13 version there is a new build at http://www.globalmapper.com/global_mapper13_64bit.zip .

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • DC KelleyDC Kelley Global Mapper User Posts: 77Trusted User
    edited March 2012
    Mike. Great!, will try but the download seem to be very small (~2megs) and seems invalid, is the link good from your end.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2012
    You probably caught it as I was uploading a new build for someone else. All uploads are complete now so if you try again you should get a complete file.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • DC KelleyDC Kelley Global Mapper User Posts: 77Trusted User
    edited March 2012
    Works fine now with the new upload. I have another one from uBLOX which causes an error fauilt, but let me look it over for anything obvious and will then post it if still having issues. Glad the board is back up again, seemed to be some DNS issues for us yesterday, but none now.
  • DC KelleyDC Kelley Global Mapper User Posts: 77Trusted User
    edited March 2012
    Delay that a bit... The data that I now have at hand seems to be corrupted by semi-drops out so the NMEA-183 sentences stop and restart at odd places (due I presume the the way it was recorded and not from GPS maker).

    GM does not care for this and asserts an error. It seems a bit beyond the call for GM to cope with such data. Mike, I will review what some of the open source NMEA source code does, but suspect I will need to put in some logic to detect "$KEYWORD" then "*xx<cr><lf>" and then toss anything in between them if the char '$' is found in order to overcome this.

    Once I cleaned this stuff out for a short test, it does plot fine in GM.
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