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Most accurate altimeter + OSD combination??

Discussion in 'Cinestar Misc' started by Jason Smoker, Dec 11, 2012.

  1. Jason Smoker

    Jason Smoker Active Member

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    Friends,

    I need some advice regarding the altimeter and OSD combinations.

    More specifically, I want to be able to very accurately record my height above the ground while having the camera looking out at a straight and level view from the bottom of the helicopter.

    In short, I record "balcony" shots and I need to be able to accurately measure the height of the subject(s) I am viewing.

    I have seen several OSD systems on the market.It is imperative that I have the most accurate altimeter reading possible and be able to overlay the readings on the video thru an OSD system.

    Any suggestions from the experts?

    currently using a eagle tree system
    thanks all
     
  2. Gary Haynes

    Gary Haynes Administrator
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    Jason I use a Smart OSD. But the data is coming from the pressure sensor on the MK so is only as accurate as the MK.

    That being said there are so many variables that affect this that if you are wanting laser measured accuracy you won't get that from these devices. Temperature, altitude, ambient pressure, wind on the sensor, all of these will affect the reading. And these starting altitudes on the MK are set to zero from point of takeoff. So wouldn't necessarily give you the height of a real object.

    So the best solution, if you could rig it up, would be to use a laser measuring device pointed at the ground that could be triggered for taking the measurement. And if you did that the results would vary based on the ground level that was below the copter. So what happens at the moment it is triggered you are over an object like a tent, small building or truck? Non accurate reading.
     
  3. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    [EDIT: I posted this without seeing Gary's message -- sorry for the apparent repetition. He's right!]

    Hi Jason:
    I think I've seen your posting on another forum on this question so I infer you've not found a good answer.

    Here's the problem:
    1. Using the MK Flight Control board pressure sensor is subject to the vagaries of air pressure (which changes from minute to minute) and wind gusts.
    2. Using the MK GPS (or just about any consumer grade GPS) is subject to the vagaries of radio propagation, solar weather, etc.

    In summary, neither system is very accurate.

    Probably the best way to solve your problem would be to mount a visual reflector on your copter and have an assistant use a laser range finder like this to measure the height of the copter from the ground or some other known reference datum (e.g. a table top). That particular Leica model claims to be accurate to 1/16th (1.58mm) of an inch up to 210 feet (64 meters). EDIT: This presumes that your assistant is directly underneath the copter and the laser is therefore pointing vertically. If you could measure any slant angle, simple trigonometry will allow you to correct for the slant -- some laser measuring tools have this built in, I believe.

    I have that Leica unit, but I've not verified the accuracy and range up to their limits.

    You may be able to find one with a PC/USB interface so that you could correlate the altitude with the time (the GPS clock is super accurate even if the altitude reading is not!) I did a Google search for you and this might be the one you're looking for.

    The only other option I can think of is some kind of radar altimeter, but that will cost several thousand dollars.

    Hope this helps
    Andy.
     
    Shaun Stanton likes this.
  4. Jason Smoker

    Jason Smoker Active Member

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    Thanks Andy Yeah I did post this in another forum, but decided I would give this ago just to see if anybody else had some great new invention that could share!! ;)
    that laser meter usb looks interesting
     
  5. Jason Smoker

    Jason Smoker Active Member

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    any idea how to get it to interact off the ground with usb antenna or something?
     
  6. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Glad it looks interesting.

    The real trick, if trick there is, is to turn the problem on it's head and measure the altitude of the copter using a ground-based device.

    That way, you don't have to worry about driving the device remotely and that, I suspect, greatly simplifies the problem.

    If you're running MK Electronics you can use MK Tool's GPS window to get an exact time in UTC and sync that USB's clock to UTC time, set it to continuous measurement and record the altitude to a PC.

    Andy,
     
  7. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Sorry...I posted at the same time as you did.

    Remember that Xbee WiFi is basically a serial port communication. It would be worth trying to use Xbee as part of the serial port link if you really want to fly the laser measuring device.

    Andy.
     
  8. Joe Azzarelli

    Joe Azzarelli Active Member

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    Lasers, GPS, Xbee - yikes! I feel sheepish about offering a low tech solution...

    Do I understand that you are needing shots at certain heights to document a view from where a balcony will be in the future? Why not get some kite string and measure on the ground the distances you will need, make marks on the string, tie it to the bottom of the copter and have the assistant who is letting out the string on the way up tell you when you hit each mark. Practice somewhere safe, of course.

    Sorry if I missed the intent...

    Joe
     
  9. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    There is the risk that if the string gets entangled that you'll pull the bird out of the sky, otherwise it's an elegant, if not highly strung, solution..... (Sorry....I'm really sorry about that pun).

    You could then tie knots in the string every six feet and measure your altitude in fathoms and your bird's airspeed in knots. <evil grin>

    Andy
     
  10. Joe Azzarelli

    Joe Azzarelli Active Member

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    And yell "Mark Twain!!" at 2 fathoms high. :))
     
  11. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Personally, I yell "Sam Clemens." ;)
    I cannot deal well with symbolic links (as computer geeks call them).

    Andy.
     
  12. Jon Fredericks

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    There's gotta be a radar altimeter solution out there, no?
     
  13. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    There are. For $10K+.

    Andy.
     
  14. Matt Wilmot

    Matt Wilmot New Member

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    I have done the kite string option a couple of times now with ribbons tied off every 5 meters. Works great.
     
  15. Jason Smoker

    Jason Smoker Active Member

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    I wish I could do that but I fly on construction sites and they is wires, long poles and sometimes big holes, trucks to dodge and god knows what else in there which makes that option way to dangerous:(
     
  16. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Then I think your best bet might be the "laser from the ground" strategy.
    Do you disable the compass when you fly on those sites -- I would worry that the mass of metal would do really strange thing with the magnetic field and causes errors.

    Andy.
     
  17. Jason Smoker

    Jason Smoker Active Member

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    yeah Andy I do I have my compass disabled and I have to ignore compass errors on start-up its the only way!:)
    I learnt the hard way about the compass having crazy Yaw problems suddenly spinning 180 degrees without warning and a couple of fly aways that I recovered.

    the only problem with the laser is that I cant have somebody underneath to measure it just impossible so i need to have measure the altitude of the copter using a ground-based device but what is best? and how to set-it so it works 100% of the time
     
  18. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Hi Jason:
    Understood. Then I think that GPS is likely to be the only way. The MK GPS is only a consumer-grade device from uBlox. I wonder whether there would be another way of time-syncing a more accurate data recording GPS with the MK GPX file -- the GPX file uses the time from the GPS system so that's super accurate. I'm not sure of the accuracy of the altitude data from the MK GPS system, though.

    Andy.
     
  19. Jason Smoker

    Jason Smoker Active Member

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    from my experience the mk gps altitude data is not accurate enough at higher levels we found that it has about a 10m difference experimenting with different altitude measures.
     
  20. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Right. But I think it's time stamp is likely to be accurate enough so that you could fly another GPS unit (don't know from which vendor, though) that would record time (also accurately) and, more to the point a more accurate altitude. For example, do a Google search for:

    gps with logging

    Andy.
     

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