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Does rebar affect satellite lock

Discussion in 'Pro/Adv Discussion' started by leekiya, May 24, 2015.

  1. leekiya

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    Took of from the sea wall yesterday, had 16 satellites while it was sat on the ground, took of to 10mtrs high and it went into P-ATTI mode. The place i took of from was on the sea front on a new main stretch of sea wall which will have defiantly had lots of the rebar in. I didn't know if this would have played a part in loosing gps signall or not, It was fine when it was on the ground, it was just when it got 10mtrs in the air it lost gps.
     
  2. Meta4

    Meta4 Moderator
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    Zero effect on satellite reception.
    Your Phantom receives the radio signal from the (US) sats on ~1.5 & 1.2 GHz.
    Unless the rebar is dense and above your Phantom to block the signal, it can't possibly affect radio reception.
    Its magnetic field can disrupt the earth's magnetic field locally and so have an effect on your compass - but this would be detected when you were on top of it rather than when you get 10 metres up.

    Did your Phantom stay in atti mode or was it just a momentary glitch?
     
  3. leekiya

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    It stayed in p atti until I flew forward about 10 mtrs and then 16 satellites came back, it was very strange. I even looked down to check I had not knocked the flight switch with out knowing.
     
  4. RoyVa

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    Only obstructions above affect GPS. They are radio signals from satellites so those have to be blocked to loose signal. From sky to earth obstructions.
     
  5. leekiya

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    Why would it go from 16 to p atti and then back again after flying forward. Really hope this is not going to turn into a regular thing and have gps lock problems like the vision 2 when that first came out. Not heard any others complaining of gps lock problems tho, think there was one member who said it went into p atti when he pressed the button to start recording. Be interesting to see if others are having this problem.
     
  6. IflyinWY

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    Going all the way back to the P1, when the compass is goofed up bad enough, it will affect the GPS mode of the aircraft. I didn't make this up in my mind. 47 seconds into this video, Colin Guinn says the compass can be so out of whack that the GPS mode may not function properly.

    If someone can convince me that the compass is not directly connected to the performance of the GPS mode of the Phantom, I will step off my soap boxes, and bow out of the conversation.



    If it's a P1, you have to fix this yourself, with a magnet. Perhaps with the P3 it can fix itself but there is some "fixing period" which takes you into "P-ATTI" mode temporarily while the computations/adjustments are made. If that's the case, DJI should have said so.
     
  7. leekiya

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    Well this would make sense as my compass would have been very confused i would guess because of all the rebar. Once i flew away from it everything went back to normal. Maybe it was the compass that caused it like you say.
     
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  8. SteveMann

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    Colin's video doesn't say anything different from what I have frequently said. The compass has nothing to do with the GPS. At 5:58, Colin says:
    "Our compass error is so big at this point that if we try to fly this in GPS Mode, it would not fly right because it thinks it's pointed one way and it knows it needs to go somewhere to fix it and it's trying to go there but it's going the wrong direction, it's going to be very confused. It would fly just fine in Attitude Mode, but it would not fly right in GPS Mode."
    He doesn't say it will fly away or crash, he says it won't fly right and it's going to be confused. Which is all I've ever said.

    The compass is only used to determine the heading, or which way is the Phantom pointed. This heading is only used when flying between two waypoints. Your home point is one waypoint and where the Phantom is currently located is another waypoint. So if you enter FS or RTH the Phantom will calculate the heading needed to return to home. It turns to that heading and starts flying home. The Phantom MC will frequently recalculate the heading to go home to compensate for wind. If the distance to home increases it will adjust the heading even more to compensate. It will get home, battery permitting, but it will be a wide arc instead of a straight line.
     
  9. IflyinWY

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    You say so much stuff, it's hard to follow steve.
     
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  10. SteveMann

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    Be specific.
     
  11. Motorotor

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    Here, an over-simplified explanation of what he meant at 47 seconds and compass so out of whack the GPS doesn't function properly.

    I think it's just semantics.

    The GPS works fine with a messed up compass...

    ...its just the "results" of GPS don't function properly.

    i.e. the GPS can not tell the quad to flip a biznatch and go course 180 for 100m to return to course when the phantom thinks 180 is...I dunno....strait down!?

    ..that was an awesome video by the way!!
     
  12. ianwood

    ianwood Taco Wrangler
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    And sadly, it's not all true. Google "AHRS sensor fusion". The compass is more than just for heading. We've been through this already.
     
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  13. ianwood

    ianwood Taco Wrangler
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    To answer the OP's question: rebar does not effect the function of the GPS itself as Meta4 said. However, as you can see in the next thread down, taking off over rebar will have an adverse effect on the compass and the compass is critical to many things including flying in GPS mode and straight and level flight in general. Bad compass data is by far the most common cause of losing control. So, don't take off on or near rebar.
     
  14. Motorotor

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    I think just the fact that you have to calibrate the thing on more than one axis tells you it does more than just heading... but I couldn't pretend to tell you what that was.

    *shrug*
     
  15. ianwood

    ianwood Taco Wrangler
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    There is a vertical and a horizontal magnetic field.
     
  16. Motorotor

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    right, I was saying, that alone should let you know it does more than just heading. Like a magnetometer...

    You can determine heading with just one. You know....like a compass. ;):D

    Just being silly...:confused:o_O:rolleyes::oops:

    EDIT: But hey! while I got your attention Ian....do you expect similar cycles thru these newer batteries? 50 - 300? What are your expectations?
     
  17. IflyinWY

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    That's worth a thread of it's own.

    Hijacking, or whatever you want to call it, causes great info to get buried. Just my opinion. :)
     
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  18. SteveMann

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    Exactly. The magnetic lines, or flux fields, are not smooth. They look more like the waves or water in a shallow lake - choppy, undulating and anything but flat. This is only one of the reasons that you need to recalibrate when changing locations by a few miles. The azimuth of the field is recorded during calibration to make the data linear in the horizontal plane. All the MC uses for flight is the normalized heading data. (Based on my experience building a hex using the Ardupilot APM 2).