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Quick help? Calibrate in field but takeoff from parking garage roof?

Discussion in 'Pro/Adv Discussion' started by Tricky, Aug 19, 2015.

  1. Tricky

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    That's the gist of it. I can calibrate tonight in a wide open field, but then I'm driving half a mile and taking off from a garage roof...it'll be empty, as will surrounding areas...I'm going for a specific shot, safely and away from people. Roof gives me the angle I need.

    Will taking off from that location screw w my calibration?

    Thanks

    P3P stock
     
  2. bobmyers

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    If you hand launch, probably ok-- if you set it on the concrete probably an issue.
     
  3. Tricky

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    Hmmm, haven't done that yet...may have to rethink take off location, thx for the tip.
     
  4. Tricky

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    Will my compass values reflect a bad location for takeoff beforehand if I did try from the roof?
     
  5. bobmyers

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    you will probably get a compass error-- in which case a takeoff may be very problematic-- all kinds of crazy things with loss of control can happen.
     
  6. SteveMann

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    Compass error cannot cause a loss of control. When someone can explain how bad compass data can cause a loss of control, I'll shut up. Just saying it happened to you is not an explanation of how it happens.
     
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  7. acherman

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    You will be fine. Even if a compass error pops up, it will go away after you lift off and get a few feet above the surface. Just go up and hover around 20-30' for half a minute to double check that it's stable.
     
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  8. Tricky

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    Holy F that was awesome. Flew perfectly, full GPS compass value 1500, got the pics I wanted, totally found a perfect nighttime perch, I'll be back tmrw night!
    Thx all for feedback, that was so much fun I don't think I'll be able to sleep tonight.
     
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  9. bobmyers

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    Well it did for me-- my friend-- compass error-- would not take off-- just tip over on the right side-- twice in a row and two broken props, Moved it for the area to another area-- not compass error and perfect take off and flight. Now try it from the bed on a pickup and see how that works... cannot may very well become can. Cheers!:)
     
  10. acherman

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    The compass should only provide directionality, it shouldn't have anything to do with attitude control - that's what the IMU does. When it says there is a compass error and to switch out of GPS mode, it's because it uses the compass to maintain it's heading, in ATTI mode it just maintains attitude (staying level), and heading is up to the operator.
     
  11. bobmyers

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    That still doe not negate the fact that a compass error will and does cause errant behavior on the craft. If the Phantom is set on a steel deck and shows a compass error-- is that just to be ignored. If so, then there is no reason for that message. If the Phantom cannot determine north in a 360 degree circle with GPS on and the computer does not need that information, what is the purpose of the compass? If it takes off with a compass error and flying out of sight, how does it determine the direction to fly on a RTH command.

    I an old guy, but I am listening-- educate me:):)
     
  12. acherman

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    First, I suppose I should have prefaced my post with "In theory..." haha So, There absolutely is a need for the compass error message.

    So, I will preface this post with..... this is my basic understanding. If I am wrong, or incorrect, please don't take anything I say as fact or law or any personal attack. :p

    GPS alone can not tell where North is. At a very high level, a GPS receiver takes signals received from multiple satellites, and, based on a database of known satellites positions and timestamps (the difference of when the beacon was sent to when it was received), calculates how far away each satellite is. The receiver then uses trilateration to determine it's location on the planet - there are many variables that are taken into account including how time behaves in different gravitational fields (general relativity affects how accurate an atomic clock on a satellite will be vs one on the ground). So, all it gives you is a location. Unless the received is moving, and updates are taken at regular intervals, the receiver doesn't know what the heading is. A GPS receiver can travel in a 360° circle and know what it's heading is at any given point (averaged - based on "I moved from this point to this point in the last 60 seconds so my heading is X°), but not if it's stationary - it needs a start and end point to know heading. That's where the compass comes into pay - it will tell you the orientation/heading even while stationary.

    Consider an iPad that doesn't have GPS - they all still have a compass (I think). So, when flying your P3, without GPS and dynamic home point, it will still give you the orientation window to show if you are facing the P3 or not. Even in airplane mode I think the compass is still active (I think). Same with the P3. It will always try to use GPS to know it's location, and use it's compass to tell you whih way it's pointing, but it won't necessarily use that info to fly. The only info it will always use to fly is the inertial input from the IMU.

    For your last question, I will make assumptions. If you are flying under a compass error condition, and it enters and RTH situation, I can only assume it will just land. Or be really confused. haha I'm not really sure on that one - that would be a firmware thing. You're right, with knowing it's heading, it would need to fly for a bit to figure out which direction it is moving based on GPS updates) and make corrections so it can get home. That would be some pretty good programming.

    As I read back, this all looks like blah blah blah blah.....:confused:
     
  13. Wibble

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    I think most people who have trouble straight after takeoff maybe because they are too cautious. Never use auto takeoff and just slam it into the air as fast as you can.
     
  14. acherman

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    I always use auto takeoff, with some assisted throttle. Never had a problem.
     
  15. Tricky

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    im going back tonight for better shots. i self take off and self land. i did auto takeoff once...it was okay. :)
    ill tell ya though, hovering about 4-5 feet from the roof of the garage, it did dip and wobble for a second...but that could have been the wind. i let it hover some more, did my pre-flight maneuvers, and it responded appropriately, so off i went.
    again, i had good values for compass and plenty o' satellites. and total privacy ;-)
     
  16. bobmyers

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    A great explanation, sounds like you know your stuff-- but the question remains, why did DJI design a compass for the Phantom if it doesn't need it to fly -- from the explanation-- it obviously has value in correcting the flight direction in instances when the GPS data is not sufficient to put it exactly on the inputted course. The second issue, compass error message, has no purpose if it does not affect the flight of the Phantom and does not need to be conveyed to the pilot..
    Obviously the DJI engineers know something about the compass interaction with the GPS system that we don't know.
    My personal observations regarding compass errors that I gave earlier regarding the behavior of my Phantom following compass still beg for an answer as well as the are recorded flyaways following compass errors which apparently happened because the Phantom computer could not access the true traveled direction. Another that I failed to convey, The first days I had the Phantom, I was stupid enough to take it off from the driveway, next to my pickup, showing a compass error. She took off and flew beautifully until I tested the RTH. When I put it into RTH, it started flying 90 degrees left and I had to switch into ATTI to bring it back to me.

    So based on just on my personal experiences with the Phantom and other cases of flyaways and erratic flight behavior some documented in this forum following compass errors, with all due respect, I cannot say that the compass errors have no adverse effects on Phantom flight. I also don't believe we will hear that from DJI.

    I appreciate the discussion of the discussion on the way the GPS works-- I didn't know those facts, but I do now. I stand by my original answer to the OP-- Flying the Phantom off steel reinforced concrete or flying the Phantom following a compass error is not recommend and could result in erratic flight behavior or even a crash.

    Cheers :):):):)
     
  17. acherman

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    The same could be said about the GPS - it's not NEEDED to fly, but it sure makes it more stable, easier to fly, and perform better. In ATTI mode, the wind can cause drift and yaw. The compass, in conjunction with the GPS, allows it to remain stable.

    It still has a purpose - same purpose as the No GPS message. The GPS and compass work together. Now, the gimbal overload message has no purpose. haha

    My observations are different. I quite often take off from reinforced concrete sidewalks, driveways, roads, etc, without issue. On Sunday I took off from a steel table (roughly 4'x4') at a park, flew to another at the far end of the park, landed by FPV (~500' away), shutdown, and took off again to fly back without issue.

    Since the compass only tells the P3 what direction it's facing, it should never cause a flyaway. Bad GPS can do that, if the Phantom needs to RTH and it's last known home position is far away. A bad home position can also cause a flyaway on RTH. The nice lady's voice tells me that the home position has been updated and to please check it on the map. Since she's so polite I always do - I make sure it's on top of the blue dot on the map. haha

    Now, all of this said, who knows what DJI has done in their firmware. haha I haven't had a single issue with mine since day 1, 70+ flights in now, and all good. Others have had nothing but nightmares every flight and won't even fly now until beter firmware is released. Maybe I'm a lucky one. :D
     
  18. bobmyers

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    All that being said-- doesn't need compass, doesn't need GPS, you are right in the repsect that I have a couple of copters that don't have either those features either, but you have to fly them..
    DJI put the compass and GPS on the Phantom so it will basically fly itself if a person can take off and land it. Really all you do is point it in the direction you want to go or the altitude you want to reach and it is there.

    So we are back to square one.. The compass and GPS are on the Phantom for a purpose, and so are those error messages-- even though in 'theory" they don't mean anything and based on that "theory" there will be no issues if they are ignored. Pretty hard for this old dumb Texan to buy that line of "theory". We will just have to agree to disagree on this one. Cheers
     
  19. acherman

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    Actually, they take off and land themselves too. Haha