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How accurate are routes on the P4?

Discussion in 'General Discussion' started by Drestin Black, Oct 10, 2016.

  1. Drestin Black

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    Separately: Using the DJI Go app or using Litchi - how accurate will the P4 navigate a line? And I'll start simple. I need to have the drone fly a straight line; from here to there -- the complications? a) it's only 20 feet between tall towers that will fark up the drone and b) there is Always wind, not super strong or often gusting - but it can be. I've tried flying it manually but a) if OA is on it will stop randomly and go really slow as it's picking up the towers on both sides or b) with OA off it goes fast but the wind pushes me side to side and makes me quite nervous doing this manually, quickly. I can do it slow and herky jerky but I need a nice smooth fluid shoot. I need to swoop down from about 50 feet high, to in between the row of 5 on a side 30 foot tall towers, going lower as I reach the end and at the end are two towers attached to an awning where the drone should stop and hover to let the shot complete.

    So, using DJI Go? How would you do this? And, will it be accurate enough? Just need point A-to-B without pause or left/right drifting. I'll attach a photo of the path I need to fly down ... and end up at the far end. Looks brain dead simple, but the winds had me side to side way too close...
     

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    #1 Drestin Black, Oct 10, 2016
    Last edited: Oct 10, 2016
  2. MacCool

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    I flew several Litchi missions yesterday down a long parking lot with light poles about 30 feet apart and it was dead accurate. My map line was in exactly the same place as the line of flight. Seems to me that question is "how accurate are the maps?" rather than "how accurate is the P4?". Before I actually flew below the light poles I ran it up and down the lane at an altitude higher than the tops, just to check.

    I'm not sure how you would do that in DJI Go. I think you'd have to hand fly the route and mark the waypoints. It would be very straightforward in Litchi.
     
  3. Greg Nystrom

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    Since the telemetry is based on GPS the question may be hard to answer. Its a good question that needs real world testing and data with many variables.

    "The United States government currently claims 4 meter RMS (7.8 meter 95% Confidence Interval) horizontal accuracy for civilian (SPS) GPS. Vertical accuracy is worse. Mind you, that's the minimum. Some devices/locations reliably (95% of the time or better) can get 3 meter accuracy. For a technical document on that specification you can go here."

    What is the maximum Theoretical accuracy of GPS?

    -G
     
  4. GMack

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    I'm somewhat familiar with Autopilot. In their write-up, they mention about not flying in winds over 10 MPH and definitely not over 15 MPH with their program. Within Autopilot, they added some "adaptive learning" module where the subsequent flights on the same flight mission will get better as the software learns the flight irregularities (wind?), i.e. second flight will be better than first, etc.
     
  5. Meta4

    Meta4 Moderator
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    That's why DJI don't use GPS at all for altitude.
    Altitude data comes from the barometer only.
     
  6. Sagebrush

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    There are a few things that monkey with GPS accuracy and one of them is signal bounce from high terrain or structures. Flying in the bottom of a man-made canyon? Make sure you report back. I'm curious. The fact that the P3/4s use the Russian GLONASS sats will help.

    SB
     
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  7. daveisim

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    Three meters accuracy means that is the radius around a point. So the circle is actually 6 meters wide. 19.68 feet. So when you first teach the point you don't know where you are in the circle of accuracy. If you are on the left edge then and another day on the right edge that's cutting it close.

    But wait, what if the accuracy is at the 4 meter (or more) level? Not good.

    Remember, when you first teach the points and then replay them minutes later, the gps positions are not going to be too different as conditions have not changed much. But after hours or days later don't count on it. Data the gps satellites broadcast to accurately determine position will change as often as 2 hours. See the Data updates section of GPS signals - Wikipedia, the free encyclopedia for details

    What I'm getting at is teach then shoot. Teach again if shooting later. DJI has a nice waypoint feature that if you pull back the right stick the bird will fly the course in reverse. That way it's easy to fly it forward, pull the stick back, fly it backwards, let go of the stick, fly it forward again, etc, until you have enough footage to morph together or get an ultra smooth single flight. Don't know how to do this in Litchi.

    Some software (co-pilot?) lets you lock on to a visual feature and home in on that. Can you do it that way?
     
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  8. Drestin Black

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    This is what I need - I need to play with some advanced flight modes obviously. But if I could simply and I do mean just this simple. Draw a line: From Waypoint A to Waypoint B and just move smoothly along this straight line (dropping in altitude as I go) that's exactly what I need. Just gotta see how she handles wind in between these things...
     
  9. Drestin Black

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    Are you sure? Seems like throwing away a useful bit of data is ... wasteful... If you've got 10-20 Satellites worth of positioning data, why not average that in with the barometer as well?
     
  10. Meta4

    Meta4 Moderator
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    Yes, I'm sure.
    GPS is great for horizontal accuracy but woeful for altitude accuracy, while the barometer can detect changes of a foot or less.
    Including GPS data would not help at all.
    GPS altitude is commonly out by significant amounts and it also swings considerably.
    It's all about geometry.
    Here's Garmin explaining it:
    How accurate is the GPS elevation reading?

    GPS heights are based on an ellipsoid (a mathematical representation of the earth's shape), while USGS map elevations are based on a vertical datum tied to the geoid (or what is commonly called mean sea level). Basically, these are two different systems, although they have a relationship that has been modeled.

    The main source of error has to do with the arrangement of the satellite configurations during fix determinations. The earth blocks out satellites needed to get a good quality vertical measurement. Once the vertical datum is taken into account, the accuracy permitted by geometry considerations remains less than that of horizontal positions. It is not uncommon for satellite heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating.
     
  11. Drestin Black

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    Caution? lol - I had no idea they were that far off.

    The last time I'd looked at GPS values for altitude I can't say I noticed such huge inaccuracies but then again, I was Geocaching and not trying to fly! Thank you for the information.
     
  12. Sagebrush

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    According to Wiki.... with WAAS–and our P3s and P4s have a WAAS chip–vertical accuracy is 1.3 meters 95% of the time. (NTBS measured)

    WAAS is an FAA system that fixes atmospheric induced accuracy degradation with the U.S. GPS. It's what allows a 737 to shoot a GPS approach in the fog and only get a bounce on touchdown.

    Wide Area Augmentation System - Wikipedia, the free encyclopedia

    SB
     
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  13. Meta4

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    WAAS is only available in North America.
    Do you know that the Phantom has WAAS? I've not seen any reference to suggest it has.
     
  14. Sagebrush

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    Good question. For some reason, I was under the impression that the P3A/P was WAAS enabled when they upgraded from the Standard and added GLONASS capabilities. (I may have included that way-back-when for my 333 exemption application. I just checked the manual and there's no mention of it.

    So until someone can document this, lets' elevate my claim to an unsubstantiated rumor.

    SB
     
  15. GPSMapNut

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    From my experience and observing how well my P3A holds and reports the position, I'm sure that it is much more than a rumor. I know a thing or two about GPS and I know that without real time corrections, the level of accuracy and repeatability, as observed with P3A, could not be achieved. It's not RTK so, it must be WAAS at play.
     
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  16. Meta4

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    But Phantoms hold position just as well away from North America.
    It's because they are using a complex collection of sensors in the IMU - accelerometers, gyros, and a barometer.
    If they relied on GPS alone, they would wander a lot more than they do, particularly in altitude.
     
  17. GPSMapNut

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    Let it hover, out of the range of VPS for 5+ minutes. It will drift with the GPS signal drift. Also; don't forget about accuracy and repeatability that I mentioned.
    I don't know how well they do outside of the WAAS coverage but I know that over here they match WAAS enabled receivers that I used and outdo GPS receivers without WAAS / with WAAS turned off and I did play with more than one receiver.
     
  18. Meta4

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    Yes ...and all that comes from the sensors packed in the IMU.
    That's what I'm saying.
    The Phantom holds position just as well anywhere in the world, not just in WAAS land.
     
  19. GPSMapNut

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    Wow, and which of the sensors in the IMU is supposedly responsible for my P3A coming back, after a long flight, to about a meter from the home position? Are these the same sensors that without working compass can't even keep track of the Phantom orientation?

    Just curious because, as I stated earlier, I had no chance to compare my P3A's repeatability between WAAS and WAAS-less coverage areas. Since you say it's the same, I take it that you did. If you did and it is the same, it's good enough proof that P3A does not employ WAAS.
     
  20. Meta4

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    RTH is a good indication that the GPS in the Phantom isn't using WAAS.
    Much of the time it will be within 1-2 metres of home but it's not uncommon to be out by 3 metres or more, which is the accuracy you expect for a non-WAAS GPS.