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Thread: Wrong altitude readings - is it a receiver problem?

  1. #11
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    This is good analysis. I am actually watching one such issue right now with a Cessna.. While F- feeder is providing data the altitude is 0, but as soon as it switched to my feeder it showed correct altitude (5,000 ft).. and then lost it again when F- took over again.. There does not appear to be an issue with "raw" data sent by aircraft's transponder.

  2. #12
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    cessnafr24.jpg

    White/green color of the track is as local F- feeder and my feed are switching over who's providing the data -- green while I am feeding (with correct altitude) and white for 0ft altitude when F- was feeding the data for same airplane.

  3. #13
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    Similar findings here. See the N5549A Cessna T210N. From left to right:
    F-receiver incorrect 0 ft (White), T-receiver correct 6700 ft (Green) and F-receiver incorrect 0 ft (White) again.

    N5549A-F-RX-1.jpg N5549A-T-RX.jpg N5549A-F-RX-2.jpg

    Must be something wrong when FR24 is using F-receiver and transponder is encoding in 100 ft increments.
    Last edited by mohair; 2014-04-15 at 13:28.

  4. #14
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    http://forum.flightradar24.com/threa...ll=1#post50338

    It would appear the second portion of my original post after requesting more information has turned out be the possible reason. Which is why support etc often ask for receiving station information to assist diagnosing anomalies (opposed to asking 'where is the problem' and listing particular transponder types )

  5. #15
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    http://www.modesbeast.com/resources/...jekt-ADSB3.pdf says
    Code:
    3. Mode-S
      a. Interrogated by ground station
          1. DF0 (56 Bit)
              • ACAS (general)
              • TCAS (as one sort of ACAS)
          2. DF4 (56 Bit) rollcall reply: altitude (100ft resolution)
          3. DF5 (56 Bit) rollcall reply: squawk
          4. DF11 (56 Bit) transponder capabilities
          5. DF16 (112 Bit) never observed
          6. DF20 (112 Bit) rollcall reply: altitude (25ft resolution)
              + BDS registers (Mode-S enhanced surveillance EHS)
          7. DF21 (112 Bit) rollcall reply: identity
              + BDS registers (Mode-S enhanced surveillance EHS)
      b. Squitter mode frames (independently transmitted)
          1. DF11 (56 Bit) with InterrogatorID set to zero
              Used to get transponder capabilities without Allcall and Rollcall
          2. DF17 (112 Bit) 1090 Extended Squitter contains ADS-B data (position, heading e.g.)
          3. DF18 (112 Bit): same as DF17 but from ground traffic
    So does this mean some equipment - when interrogated - responds with a DF4, other equipment responds with a DF20, and one of those responses is not always being decoded correctly?

    If so what radars do decode it correctly and what don't?

  6. #16
    Administrator piopawlu's Avatar
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    Thanks for your reports. We do support Mode-C encoded 100ft increments altitude reports, but there was a problem with returning 0ft rather than "INVALID" when wrong code was received. I've fixed that now and will update receivers soon but I'd appreciate an example of a valid frame that FR24 software decoded as 0ft so I can test it here.

  7. #17
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    Hi piopawlu,

    Not sure does these AVR logs help, but I have FR24 box received data which includes that N446MM (A56206) plane transmitted DF17 (and also all other messages) frames from two different flights.

    https://dl.dropboxusercontent.com/u/...446MM_short.7z
    https://dl.dropboxusercontent.com/u/..._raw_N446MM.7z

    ------
    Laxlou

  8. #18
    Administrator piopawlu's Avatar
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    Thanks for your data, I've fixed the problem and will roll out the new version tomorrow morning. It may take a few days before all receives get the update as we don't want to update them all at once.

  9. #19
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    piopawlu,
    Thank you very much for your support. It seems there are currently only very few aircraft indicating randomly 0 ft altitude. What is the status of the F-receiver new software version roll out?
    -mohair

  10. #20
    Administrator piopawlu's Avatar
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    It's currently installed on about 75% of our receivers.

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