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GPS Questions

Discussion in 'Phantom 2 Vision + Discussion' started by Mopar Bob, Oct 14, 2014.

  1. Mopar Bob

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    According to things I read on the internet concerning GPS reception, the general opinion is that clouds do not affect the GPS reception. Yet, when I fly my bird on most sunny days I get 10-12 sats. On cloudy days it is reduced significantly. Today was a very cloudy day and the most I got were 8 sats and while flying I actually dropped to 5 for an instant.

    Question #1: Does anyone else have this same issue or do I have a weak GPS component.

    Question #2: If you get GPS lock before the flight, then drop to 5 sats mid-flight, what happens to the lock? Is it gone and therefore a fails safe "go home" is going to travel to an unpredictable location?
     
  2. DrJoe

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    #1: I have not noticed an issue with sunny vs. cloudy days. Just my experience

    #2: When you drop below 6 satellites, you lose your home point. Once the Phantom gets back to 6 satellites, it resets its homepoint (you may 2000 feet away, but there's your home point). It also enters ATTI mode, which means it will drift with the wind when you release the controls. It does not enter a "fail safe go home" or RTH. But if you were to keep flying, notice the sporadic behavior of the drift, and command a RTH, it would not know where to go (if you had <6 satellites), and is one reason for "flyaway" behavior.

    I performed a "foil mod" and tucked in my GPS cable to increase GPS reception to great effect, I regularly receive fast satellite lock and 11-12 satellites found. I believe this forum post can help you:
    viewtopic.php?f=27&t=19099&hilit=foil+mod+for+gps&start=30
     
  3. Mopar Bob

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    I actually did do the foil mod with little or no affect. I used a copper sheet that is used for shielding amps. I covered it with package tape and re-routed the cable under the existing shield.
     
  4. Meta4

    Meta4 Moderator
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    Clouds should have no effect at all.
    The phantom has a well known weakness in shielding of its GPS. Search for foil mod to find the cure. It's easy, very effective and a big improvement to you Phantom.

    If you drop below 6 sats, the Phantom will be in atti mode - still flyable but without GPS position holding.
    It will not attempt to come home -that's what happens when it loses the control signal from you.
    If it did try, it would be unable to because without sat lock it has no way of knowing where it is or where home is.
    Do the foil mod and have no sat worries unless you fly in a canyon or under tree cover.
     
  5. Mopar Bob

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    OH, I forgot to mention this: in my original text I mention cloudy. In fact, I meant to say completely overcast. Not a single break in the sky.
     
  6. Meta4

    Meta4 Moderator
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    ???? Are you sure about that?
    DJI designers aren't that dumb and we would all have lost our birds if the Phantom reset its home point whenever it regains sat lock.
     
  7. Meta4

    Meta4 Moderator
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    GPS satellites are moving and the number seen from your location will vary with time, how good your view of the sky is and interference from the onboard electronics.
    Rain, hail or shine, the GPS signal gets through.
    Any amount of cloud has no effect at all.
     
  8. Mopar Bob

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    Meta4, if that's true, then there must be something wrong with some element of my GPS unit. Would you concur?
     
  9. Meta4

    Meta4 Moderator
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    Get a GPS app for your smart phone or even better a hand held GPS to check what sats are visible and compare that with what your Phantom sees.
    If you can still get 10-12 sats sometimes it doesn't sound like a problem (beyond the known shielding issue).
    There can't be anything wrong with your system that would make the GPS system fail only on cloudy days.
     
  10. Panamon Creel

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    Atmospheric conditions will have an affect on the strength of the satellite reception and it also can effect accuracy.
     
  11. IrishSights

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    I presume you mean Solar flares measured by the k-factor. There are apps to monitor that too. It mainly affects the accuracy of the satellite hold postion reflected by a wider drifting around circle than the normal 1-2m.
     
  12. jwegener

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    Yeah, meta4, that's incorrect. The home point DOES NOT reset itself midflight, that'd be a disaster. What happens (and I think this is what you're referring to but got confused) is that the INITIAL homepoint gets set as soon as it finds 6 satellites wherever that spot Is.

    What this means in practice for me is that my backyard patio where I take off from it only sees 4 satellites. So it's not until my drone gets in the air a bit away from my house and above all the trees that it gets full 6+ satellites --and once it does, the home point gets set to that spot. It certainly doesn't get RESET later if I dip below 6 satellites.

    Hope that clarifies!
     
  13. BlackTracer

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    Note that in atti caused by sats < 6 if it loses control signal it will land right where it is with all the glory of atti mode.
     
  14. Mopar Bob

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    How about the location of cell towers. There is a fake flagpole that is a cell tower within 100 yards of my flying area. Could that be causing interference at 100 yards?
     
  15. Meta4

    Meta4 Moderator
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    Probably not but it would be simple to compare your Phantom sats with what a GPS or GPS app picks up at the same site.
    If there is interference, it would affect them equally.
     
  16. Mopar Bob

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    I put a GPS navigation app on my cell as suggested. According to it, I get 10 sats in my house. I never get any on the phantom in the same location in the house. Not sure what to make of that.
     
  17. N017RW

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    No guarantees.

    Be sure the 'app' is only locating [US] GPS SVs.

    Secondly there can be a huge difference in receiver quality between the DJI, cellphone, and 'dedicated' GPS navigation devices.

    Wise man once said: "man with one watch always knows what time it is, many with two (many) watches never knows what time it is".
     
  18. Panamon Creel

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    No I'm talking about clouds, air pollution, temperature, humidity, Rain, Snow .... all of these things degrade the Satellite GPS radio signals. Ground based augmentation systems can help minimize the variations caused by these uncontrollable factors if they are available in the area but that may not always be the case.
     
  19. Meta4

    Meta4 Moderator
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    It means that there are more sats than your Phantom is picking up.
    One possibility is as N017RW points out, the Phantom only receives the US system sats and your app may be displaying others too. My GPS app also displays Russian sats (squares) as well US sats (circles).
    The other factor is that the Phantom, particularly without the foil mod, will not pick up all the available sats.
     
  20. Mopar Bob

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    For those that have been following this thread, I have some conclusions based on comments from other readers and further research on my part.

    Conclusion #1: Clouds do affect GPS reception. At least Phantom reception. Not all clouds however. Those puffy white things don't seem to have any effect on the reception. However, dark clouds that cover the entire sky dome do affect it. I went to the same field for a few days. Overcast one day, 6-8 sats. Clear skies, 10-12 sats. I repeated this test twice with the same conclusion. Within the course of an afternoon, the heavily overcast skies cleared up, and my reception did also. I think it may come down to each of our specific locations however, as to how much this affects each of us.

    Conclusion #2: Flying near a cell tower does not affect the Phantom. Not sure if you get right on top of it, but I saw no difference when flying in a field miles from a tower and one sitting 100 yards from a tower.

    Conclusion #3: The foil mod, as it is referred to does help. Basically, the Phantom's GPS is a real disappointment to me. Something as complicated as the Phantom, and as reliant on good GPS reception should have a better GPS configuration. One that doesn't require us to tear the unit apart to try to insulate it better. Having the GPS antenna located where it is helps ensure the aerodynamics of the craft, but its just too close to all those leaky radio waves.

    Conclusion #4: The foil mod does require the use of aluminum and not copper. I had done mine with copper sheets that are used to insulate humming in a guitar amp. I saw no change in my reception. I pulled it out and used aluminum. Big difference. Although copper is a better conductor of electricity, apparently, aluminum is a better insulator of EMI (electro magnetic interference).

    Thank everyone for their comments. I hope I did not offend anyone with my conclusions. My next step is to ditch this GPS antenna system.