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PMB-248 GPS gives only GGA sentences — Parallax Forums

PMB-248 GPS gives only GGA sentences

carlosgcarlosg Posts: 6
edited 2010-04-08 17:49 in Accessories
I want to get true heading information from this GPS as in $GPHDT sentences
but it only gives $GPGGA sentences.· If I·issue a request as follows:
$CCGPQ,HDT\r\n
I would expect it to give HDT sentences but instead I no longer get anything back.
Anyone know if this should work?

Thanks

Comments

  • max72max72 Posts: 1,155
    edited 2010-04-06 12:18
    Not all the units have all the NMEA senteces available. I guess true heading is pretty rare to be found.

    As far as I know this sentence is not available on this unit.

    Massimo
  • carlosgcarlosg Posts: 6
    edited 2010-04-06 14:47
    Thanks Massimo.· How do you figure HDT sentence is not supported in this GPS?

    You know of any sentence that will give heading on this unit?

    I also tried RMC·but nothing.

    Wish I had a list of sentences it supports.·

    The spec only states NMEA and the website states NMEA0183 v2.2

    I've just started with this and so was not moving at the time --

    I assumed it would just give a bogus value.

    Could it be·it actually needs to be moving in some direction?

    Thanks,

    Carlos
  • John AbshierJohn Abshier Posts: 1,116
    edited 2010-04-06 15:47
    From the manufacturer's website: NMEA0283 V2.2 GGA, GSV, GSA, RMC (optional VTG, GLL)
    RMC will give true course over ground when moving.

    John Abshier
  • max72max72 Posts: 1,155
    edited 2010-04-06 15:58
    Check www.polstargps.com/Polstar_GPSproducts_PMB248.html

    Massimo
  • hover1hover1 Posts: 1,929
    edited 2010-04-06 16:04
    Where did you get the command "$CCGPQ,HDT\r\n" ? I don't know of any commands for this product. As far as I know it only sends NMEA data out and does not accept any input.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-04-06 16:24
    hover1 said...
    As far as I know it only sends NMEA data out and does not accept any input.
    The device does have a serial input pin. It must be there for something. How else would you access the "optional" VTG and GLL sentences? I've looked for docs for this supposed "feature", but Polstar's online datasheets are sketchy, at best.

    -Phil
  • carlosgcarlosg Posts: 6
    edited 2010-04-06 16:32
    It's a query sentence.
    Got it from here http://www.cs.put.poznan.pl/wswitala/download/pdf/NMEAdescription.pdf
    which describes the NMEA 0183 protocol.
    I figured the chip has an RX pin for something!
    Sounds like I'll only get RMC when I get moving. Thanks all for the pointers.

    Carlos
  • hover1hover1 Posts: 1,929
    edited 2010-04-06 16:38
    Phil,
    I have spent many hours searching for code to program the optional baud rates and·features, without even a hint that the GPS will accept data. I just figured The "optional" is programed at the factory with propriatary code.
    Yes, Polstar·docs could use a little improvment. smile.gif
    Jim
    Phil Pilgrim (PhiPi) said...
    hover1 said...
    As far as I know it only sends NMEA data out and does not accept any input.
    The device does have a serial input pin. It must be there for something. How else would you access the "optional" VTG and GLL sentences? I've looked for docs for this supposed "feature", but Polstar's online datasheets are sketchy, at best.

    -Phil
  • carlosgcarlosg Posts: 6
    edited 2010-04-07 17:58
    ·· [noparse][[/noparse]From the manufacturer's website: NMEA0283 V2.2 GGA, GSV, GSA, RMC (optional VTG, GLL)
    ·· RMC will give true course over ground when moving.

    ·· John Abshier]

    John,· at walking pace, I still do not get RMC.· Do you know how fast it needs to be moving?

    I'm thinking of calculating heading my self over a span of a few seconds of lat/long. data.

    Carlos
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-04-07 18:43
    The GPRMC sentence should show up whether you're moving or stationary. How are you observing the unit's output?

    -Phil
  • hover1hover1 Posts: 1,929
    edited 2010-04-07 20:58
    ·· [noparse][[/noparse]From the manufacturer's website: NMEA0283 V2.2 GGA, GSV, GSA, RMC (optional VTG, GLL)
    ·· RMC will give true course over ground when moving.

    ·· John Abshier]

    John,· at walking pace, I still do not get RMC.· Do you know how fast it needs to be moving?

    I'm thinking of calculating heading my self over a span of a few seconds of lat/long. data.

    Carlos
  • carlosgcarlosg Posts: 6
    edited 2010-04-07 23:09
    Do you suspect a problem with my code? It's in JAL for a PIC.
    I've tested the parser with a GPS simulator without problem.
    I did encounter giberish characters with the GPS chip but no problem,
    I ignore everything until I read a $

    Carlos
  • sylvie369sylvie369 Posts: 1,622
    edited 2010-04-08 00:02
    Can you connect it through an adapter so that it's sending the data straight to a terminal window? That'd answer a lot of questions for you.
  • carlosgcarlosg Posts: 6
    edited 2010-04-08 17:49
    Sylvie369, I did just that yesterday, and I see the 4 sentences every second, including RMC.
    So my code, timing, or something I'm introducing is to blame so I'll investigate further.
    Thanks all for the help and suggestions!
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