Setting the correct RTH

Discussion in 'General Discussions' started by Derf, Oct 15, 2016.

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  1. Derf

    Derf Active Member

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    The Mavic will be my first "real" drone and I can't wait for it to arrive. In the meantime I'm trying to educate myself on settings and flying. I've read the unofficial user guide several times. I have flown an AR Drone for 1.5 years but it's definitely more of a toy.

    My question is for experienced flyers: How do you choose or set the appropriate RTH height? What's safe to avoid power lines most trees etc. I live in SoCal so a palm tree may occasionally be around. Do you set one height and leave it? Or do you adjust each time you take flight?

    I appreciate any feedback and suggestions.
     
  2. Anthony Viscomi

    Anthony Viscomi Well-Known Member

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    If you find yourself flying in the same areas most often and you intend on keeping the drone within you eyesight. Then monitor your altitude as you navigate over the tallest obstacles and set that as your RTH height.
     
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  3. msinger

    msinger Well-Known Member
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    Since the Mavic is only able to estimate the altitude (when above VPS range) and you're usually not going to know the exact height of the tallest obstacle in the area, you should add at least 50 feet to whatever you believe the tallest obstacle to be. And, keep in mind that the RTH altitude is the altitude above the takeoff point (not the Mavic's current location). That's important to keep in mind if you're flying in areas with hills/mountains.

    Fly above them or steer clear of them if you know where they are located. The obstacle avoidance features will not be able to see power lines or small tree branches.

    You should always check your RTH altitude before takeoff. When flying in a new location, you'll want to re-estimate the RTH altitude based on the tallest obstacle in the area.
     
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  4. MavicKhan

    MavicKhan Well-Known Member

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    I am yet far from being a Mavic owner, as I intended to let the launch and Xmas craze pass and, most probably, will only be getting one next year, also benefiting from any additional improvements in firmware that might come by.

    Still, I am already avidly following threads on this new forum as, being my first drone, I will want to be as well prepared as I possible can when I finally get my Mavic.

    Regarding this topic: Being that "G" class airspace in the US starts only at 500┬┤AGL outside airport control areas and TMAs (...and these are off-limits for drone flying by nature...) why not just set the RTH function to, say, 350ft?

    Also, why is the return home height always referenced to the point of departure, if the drone always has GPS info?

    Wouldn't it be more logical for the software to be designed for variable altitude, anchored on "height above current point" for such a fail safe feature? That would guarantee complete obstacle avoidance when returning home after a lost signal, right?

    MK
     
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  5. msinger

    msinger Well-Known Member
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    The GPS receiver is only reporting the drone's location (latitude & longitude). It does not know how far the drone is from the ground (or obstacle below it).
     
    #5 msinger, Oct 15, 2016
    Last edited: Oct 15, 2016
  6. MavicKhan

    MavicKhan Well-Known Member

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    No, altitude from MSL (elevation) can always be derived from GPS signals.

    Position can be a tri-dimensional solution with GPS, with accuracy only depending on the number of satellites available. That is why there are VNAV approaches for aircraft, that can be flown independently from air data systems altimetry and that is why current modern autonomous long range ordinance delivery systems don't need ADS in them.

    Of course, to derive the Height Above Terrain, some kind of geodesic information has to be provided but if the drone can derive HAT from the departure point, I don't see why he can't derive it from any route point because, obviously, those can be departure points for any other sorties.

    MK
     
  7. F6Rider

    F6Rider Well-Known Member

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    No DJI Drone has been able to report true altitude ASL, only HAT and that only as referenced from the A/C launch point. (barometric sensor) The issue is either lack of that data in the map stream being used, or a lack in the GPS sensors being used. Due to the relatively short range of the Mavic perhaps DJI decided that HAT was far more important. And now that the Mavic has terrain following it is not such an issue. I almost crashed my P3 into a rising slope of the Eastern Sierra's by not paying attention, I was focused too much on the view forward to look down, and the bird was reporting my altitude as 200' when the ground had risen 175, close call.
     
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  8. MavicKhan

    MavicKhan Well-Known Member

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    Definitely an area to be improved...thanks for the info!

    MK
     
  9. Dirty Bird

    Dirty Bird New Member

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    Obviously this is location dependent but 100 meters (328') is a good setting to start flying over land. What I recommend is climbing straight up with the camera level, then slowly spiral & scan the area within which you intend to fly for any objects extending above the horizon. Objects above the horizon are higher than your bird. Climb high enough so you are above everything on the horizon, note your altitude, then set that +25m as your RTH.

     
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  10. Drowsy

    Drowsy Member

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    That sounds like a completely logical way to figure out the RTH altitude! As long as you don't fly above 400 ft of course. =)
     
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