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Thread: Required GPS sentences

  1. #1

    Required GPS sentences

    I am installing an OEM GPS module I had on hand, to supply data to my VFR panel (Garmin 18x-5Hz). Which sentences are required by the EFIS to drive all of its functions? 5Hz updates at 9600 I don't think leaves enough room to have all available sentences turned on.


  2. #2
    We use GPGGA, GPGSA, GPRMS, GPVTG, and PGRMT @ 9600. Use AFS GPS as the Serial Port config selection on the EFIS.
    Trevor Conroy
    Technical Support
    Advanced Flight Systems

  3. #3
    I don't have an option for GPRMS on my unit; did you perhaps mean GPRMC?

    I'm also curious -- are you actually getting 5 updates/sec at those settings? From the Garmin NMEA guide, the max number of chars per second at 9600 is 960, but all those sentences, at least with my cheap math, total 1370 chars; 3 updates/sec would fit in 960. This seems to correlate with the data I'm seeing from the GPS with those settings; every other update is skipped (so, an update every .4 sec instead of .2sec). Am I missing something, and will this impact the performance of the mapping & synthetic vision?

  4. #4
    Yes, GPRMC is the correct one. If all the strings are full of data (which usually they are not) then it would take up more than 200 char/millisecond. However, 99% of the time the sentences do not contain all the possible pieces of information. NMEA does not have placeholders like 0's if the value is null, just ,,,,,, when empty. This is nice because it doesn't take up extra characters allowing us to use all 5 sentences at 5hz 99% of the time. I hope this clarifies it for you.
    Trevor Conroy
    Technical Support
    Advanced Flight Systems

  5. #5
    Hi Trevor, thanks for the info. I still only see updates coming in every .4 seconds (the data is timestamped at seconds 0.0, 0.4, 0.8, 1.2, 1.6, 2.0, and so on), essentially 2.5Hz. I'm viewing the data both with Garmin's sensor config utility, and VisualGPS.

    If it works, though, it works -- I'll hook it into the EFIS when it arrives from the dealer and see what happens. It only raised my eyebrow because I seem to remember reading that the 5Hz gps's were preferred as it made the synthetic vision and mapping displays smoother.
    Thanks, Joshua

  6. #6
    Either way it won't affect the SVT.
    Trevor Conroy
    Technical Support
    Advanced Flight Systems

  7. #7
    My panel is all taken apart for painting otherwise I would test this myself. I am using one GPS to drive 4 devices: among them an EM and an Ack E04 ELT. According to this thread on VAF:
    the ELT requires 2 digit precision on the $GPGGA data, which looks like this:
    $GPGGA,161907,3321.67,N,08434.25,W,1,00,3.2,221.7, M,-30.7,M,,*78
    so the lat/lon is in xx.xx' format

    Will this format create any problems for the AFS input? At the very least, even if the AFS can read and process this data correctly, it would seem to me that with 2 digit precision, this data will not be changing nearly as often as it would with higher precision. I can imagine that this could affect the gyro aiding and also the SVT processing. Trevor, can you check this question with the programming wizards?


    Last edited by NoahF; 05-02-2011 at 01:43 PM.
    Highest Regards,

    Noah F
    Rhode Island

  8. #8
    The AF-GPS outputs about 5 or 6 digits behind the decimal. Not a problem at all. I would not feed the EFIS GPS data with only 2 digits behind the decimal. It will cause the SVN to be jumpy due to the inaccurate data. 2 decimal places is 60ft.
    Trevor Conroy
    Technical Support
    Advanced Flight Systems

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