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Satelite signal question.

Discussion in 'Phantom 2 Vision + Discussion' started by lthomas, Sep 15, 2014.

  1. lthomas

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    Question about satellite signal. Lets say you launch with 7 satellites in NAZA mode. A solid green flash. I don't know if this is possible, but with all man made mechanisms anything is possible when it comes to error. What happens if a couple or possibly three of those satellites drop their signal while in flight? How many satellites are required for the drone to absolutely know exactly where home is located if the return to home feature is activated? Thank you.
     
  2. itchybeard

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    Your Phantom needs at least 6 satellites in order to know where it is and be able to "return home". If it drops below six satellites then it will go into "atti mode".
     
  3. rrmccabe

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    I agree. But what I find odd is you can get location with less than 6 on other systems. You would think dji would use less to get you home in emergency situation.
     
  4. itchybeard

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    You would indeed, rr, but that's DJI for you :lol:
     
  5. N017RW

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    Must be an arbitrary decision.
    4 SVs is all that is needed for reliable 3D fix and Z is not needed as they use the baro. for altitude.
    DJI is looking for some additional certainty???
     
  6. rrmccabe

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    It would be pretty darn nice if they required you to get 6 for home lock, but if you dropped below that while flying you got a warning saying "Satellite Count Low - Do you want to RTH?".

    Or if you lost control connectivity I would take my chances on it trying to come home with 4 compared to drifting away in ATTI mode !
     
  7. lthomas

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    If this hypothetical situation were to occur, would activating the home lock feature even work? I ask this because I am wondering if any of these fail safe, rescue my phantom features utilize the on board compass to record direction and the last known satellite gps coordinate to calculate a vector to home?
     
  8. flyingtim

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    I was wondering that too. Just tested my at a major pop location in the country. 7 satellites dropped to 6 and then 5 and then 6...

    I'll email DJI and see how many it uses for coming Home.


    ... Done.
     
  9. flyingtim

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    Got a reply, but they misunderstood the question.

    So I sent them another email asking specifically if the Quad's HOME'ing still works with 3,4 or 5 satellites if the number suddenly drops while it is out thar a fly'n.
     
  10. ToThePoint

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    It doesn't. Goes to ATTI and has a mind of its own when flipping to often in and out of homelock.
     
  11. flyingtim

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    ok. thanks.
     
  12. NM_Quad

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    In very general terms ...
    It takes a minimum of 3 satellites to determine your latitude/longitude POSITION (but not elevation)
    It takes a minimum of 4 satellites to determine your ELEVATION, but typically with +/- 50 ft. accuracy, or worse.

    Of these minimum 4 satellites for POSITION and ELEVATION, ideally, one would need to be nearly over your head and the other three near the horizon and 120 degrees apart for best accuracy (i.e., one north, one SE, and one SW). This seldom happens, and if it does, it doesn't stay that way for long. Therefore, having a few more satellites available allows the GPS "firmware" to select the satellites with the best distribution in the sky to determine the most accurate position and elevation. It's not really the number of satellites above your head, or those with blasting signal strength, but their more-or-less equal distribution in the sky that is important.

    DJI has determined 6 satellites is the minimum number to find the best "fit" based on distribution. For example, if several satellites are clustered relatively close to each other, say to the east, the firmware may discard a couple of them for being too close to each other and will attempt to use the best 4. And, of course, if one of those 6 satellites drops below the horizon or gets obstructed by a building, you no longer have 6 satellites for the DJI firmware to use and it can quickly become "lost."

    GPS uses a mathematical model of the Earth's surface, but we all know the Earth is not perfectly round and not all ocean sea levels are zero feet ASL (otherwise, you wouldn't need locks in the Panama Canal). This is why calibration is so important ... you're telling the DJI GPS exactly your starting elevation.

    As a general rule, elevation error is about 1.5 times worse than your position error. If your DJI returns home RTH and lands 15 feet away from the start, generally no biggie. But that same error in elevation (15ft x 1.5 = 22 feet) could make for a hard landing, thinking the ground is still 22 feet away, or (gasp) 22 feet in the air.

    If you take off with only the minimum 6 satellites, you are pushing fate that one won't drop below the horizon or become obstructed during your flight, or 2-3 are closely clustered together in the sky for poor elevation derivation, or there are none above your head. Therefore, strive for as many satellites as you can get, like at least 8. If you take off with only the minimum 6, at least keep it LOS for a manual recovery. For those of you achieving 10+ satellites, you are almost guaranteed a good distribution of birds.

    Use one of the GPS calculators often cited on the forum, or bring along a handheld GPS unit that shows you where the satellites are. The moral of the story: see where the satellites are for at least 6 being fairly well distributed throughout the sky. Distribution is more important than just the signal strength. Avoid flying if most of the satellites are clustered together in one part of the sky or too close to the horizon. Usually, you only have to wait 10-15 minutes for the constellation to change to a more favorable condition. That 10 minute wait might ensure your DJI returns properly.

    (We use GPS at my work for determining ionospheric dispersion based on GPS timing and lock signals).

    Hope this helps explain why DJI requires at least 6 satellites, why even that can often cause problems, and why striving for more satellites with good distribution is really the key over signal strength, especially for RTH.

    Good luck to all.

    Paul
     
  13. dpdpkpcom

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    Very interesting read Paul, thank you !!
     
  14. N017RW

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    Thanks for that.

    With respect to the below highlighted comment:

    As a general rule, elevation error is about 1.5 times worse than your position error. If your DJI returns home RTH and lands 15 feet away from the start, generally no biggie. But that same error in elevation (15ft x 1.5 = 22 feet) could make for a hard landing, thinking the ground is still 22 feet away, or (gasp) 22 feet in the air.

    The Phantoms use a baro pressure sensor for altitude and a rate-of-change to determine ground level when auto-descending.
     
  15. Panamon Creel

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    Yes altitude is determined by a barometric pressure sensor and only DJI can answer why they maintain the min 6 sat lock condition for RTH. IMO a min 6 sat lock condition is appropriate for horizontal position hold but should be reduced to min. 4 Sat locks for RTH conditions.
     
  16. NM_Quad

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    I was getting paranoid about my long post and should have included that due to GPS elevation errors explained, this is why DJI added a barometric altimeter (for more accurate rate of change) to supplement the GPS elevation - for a smoother landing than GPS alone might provide.

    Paul
     
  17. NM_Quad

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    Actually, dropping to 4 sats for an "emergency" RTH is a pretty good idea. I'd rather see mine have a hard landing in front of me instead of who knows where, or an uncontrolled take off into the trees. Did you read this one, DJI! :p

    Paul
     
  18. kirbinster

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    What program on an iPhone or the web does one use to see where the sats are in the sky?
     
  19. kxkraze

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    Update with your location:

    http://in-the-sky.org/satmap.php?gps=1


    .
     
  20. kirbinster

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    Thanks!