what type of data is the GPS module feeding the mavic? I have an idea

Discussion in 'General Discussions' started by droneuser, Mar 3, 2018.

  1. droneuser

    droneuser Well-Known Member

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    Hello

    I have been reading post about people loosing GPS signal :( has anyone hooked up a data analyzer to the wires on the GPS module inside the mavic? maybe the signal could be fed in via trackimo gps ?

    have it feed in along with GPS module? meaning if the dji gps module cant get signal the trackimo would kick in and supply it

    just a thought :)
     
  2. Afremont

    Afremont Well-Known Member

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    I'm of the opinion that the GPS problems reside outside the actual hardware module. I think it's more of an Android problem. I really wish they hadn't gone that route on the OS. Java and reliability are mutually exclusive in my experience.
     
  3. droneuser

    droneuser Well-Known Member

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    I am trying to think of a way to improve the GPS
    yes that very well could be... when the weather gets better around here I am going to try to do some testing with ios and android
     
  4. sar104

    sar104 Well-Known Member

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    Except in the cases of flying in narrow canyons, among tall buildings or under heavy canopy etc., the "loss of GPS" means that the FC is ignoring the GPS data, not that it is not receiving it. It is almost never due to a lack of GPS lock or signal. What is usually happening is that the IMU compass heading data disagrees with the IMU inertial system computed heading, and if that is not resolved rapidly by the FC then it is unable to hold position (even though it still knows its position from GPS) and switches to ATTI mode. Changing the source of the GPS data is not going to help.
     
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  5. droneuser

    droneuser Well-Known Member

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    so what you are saying is when the IMU inertial system disagrees it stops getting data from the gps?
     
  6. sar104

    sar104 Well-Known Member

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    It ignores the GPS data. It still receives it, as you can see if you look at a flight log where the FC switched to ATTI for one of those IMU exceptions.
     
  7. droneuser

    droneuser Well-Known Member

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    ohhh ok I see what you mean hummm so the only fix for that would be some type of firmware updated? or no cause the CPU of IMU inertial system is only so fast?
     
  8. sar104

    sar104 Well-Known Member

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    There is no way to fix the issue, even though it seems like it should be simple. The problem is that GPS only gives position and velocity (although only at 5 Hz), and knows nothing about the direction in which the aircraft is pointed other than in a very broad sense from the track, if it is moving. As a result, if the aircraft drifts (due to wind currents) the FC does not know what to do to correct it in terms of propulsion. That essentially makes positional holding (P-GPS) impossible, and so it drops into ATTI where it only holds attitude and altitude until it can resolve the discrepancy in the heading data.
     
  9. droneuser

    droneuser Well-Known Member

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    so its normal to see a quick switch to ATTI mode from gps then back to gps for a sec or two in logs cause of that
     
  10. sar104

    sar104 Well-Known Member

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    Not normal, in the sense that it should not happen. If you see it regularly it suggests that your takeoff location might be magnetically distorted, you are flying in an area of magnetic distortion, or that the aircraft needs a compass (or possibly IMU) calibration.
     
  11. droneuser

    droneuser Well-Known Member

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    ok but if you don't see it regularly then its normal to see that if it does? I mean a fast switch back and forth?
     
  12. sar104

    sar104 Well-Known Member

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    Sometimes it switches back fairly quickly but without seeing the specific log I can't really comment on whether that is normal.
     
  13. droneuser

    droneuser Well-Known Member

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    ohh no I don't have any log I was just wondering so it would depend on how fast it switches it may be fast enough to switch you may not see it in the logs?
     
  14. sar104

    sar104 Well-Known Member

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    No - it will be always be recorded in the log.
     
  15. droneuser

    droneuser Well-Known Member

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    ok that's good to know Thanks!
     
  16. tictag

    tictag Well-Known Member

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    @sar104 not sure I agree with your 'conflicting signals' hypothesis above. I'm thinking the IMU's principle function is maintaining aircraft stability e.g. not allowing pitch/roll/yaw movements beyond limits ... the aircraft knows what pitch/roll/yaw it is at and how fast that pitch/roll/yaw is changing (accelerometer triad, angular momentum triad) and when limits are reached, the prop pitch is 'dialed back' accordingly, which is why you can't roll or invert a Mavic. I guess it could also be used for micro adjustments to counter wind gusts, pressure drops etc. I'm really not sure it has anything to say about position or heading, computed or otherwise . Now the compass, yes definitely, as you say GPS only informs where you are, not your heading (except when moving, as you say). I would argue loosing either compass lock or GPS lock would warrant ATTI mode, and my hypothesis is that the IMU has no part to play in it.
     
  17. tictag

    tictag Well-Known Member

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    Oh, and to the OP's point, I'll best DJI simply purchased a cots GPS module, which would invariably output a UART lat/long at 5 or so hertz. If I could be bothered, I could test this hypothesis as I have both a Mavic and a digital signal analyser!
     
  18. droneuser

    droneuser Well-Known Member

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    that would be great if you can test it! thanks!!!!! there has to be a better way to get a good solid GPS

    or I should say better way for the compass to get a good signal not sure if that would help the error or not
     
    #18 droneuser, Mar 6, 2018
    Last edited: Mar 6, 2018
  19. tictag

    tictag Well-Known Member

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    Well, you caught me on a good day :) I captured 60 seconds of digital signaling from drone switch-on from the four signal lines coming out of the GPS module. It looks like there are two UART signals and one I2C signal (SDA & SCL). I haven't done any further analysis but it looks on the face of it that UART 1 is the GPS location information based on the fact that it's a 5Hz ASCII signal.

    Here's the setup and GPS module connector pinout:

    1 setup.jpg 2 pinout.jpg

    Here is how the data looks:

    3 UART.jpg 4 I2Ca.jpg 5 I2Cb.jpg

    And here are the actual trace data files - just download Saleae Logic from their website to open, view and analyse further: Downloads

    Good luck with your project!
     
  20. droneuser

    droneuser Well-Known Member

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    thanks for doing that ! I don't have a data analyzer to check things out on a trackimo :( but the info you posted will help and is good to know :) heck it may help with some ideas from other people too! :)

    hum makes a person wonder what type of output does the compass have...
     
    #20 droneuser, Mar 6, 2018
    Last edited: Mar 6, 2018
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