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Waypoint Mission loses altitude on 2nd pass.
1408 24 2015-9-21
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Sir Edward K
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I have been trying to set up this waypoint mission and have had trouble because of its length.  Now it is right at 1000' long and I am standing 200' from the beginning of it.   I set the mission up so that the heading is "Free" and to "Hover at End" and the Speed is set to 0.  So it is a Cable Cam.  All my points were set at 35' AGL.

During the first pass everything seems to be fine.   But I filmed it twice so I did not quit the mission.  I just went back to the beginng along the cable and my son took off on his bike again and the 2nd time through I noticed that I was a LOT LOWER and near the end I was almost ground level!

So is this because I am not using a device with an barometer such as an Ipad Air2?  My device is an iPad Mini 2.   Is this more error than would be expected?  From point 1 to 4 is about 1000'  I went from Point 1 to 4 then back to 1 and then back to 4 then I quit F-Mode and went to P-Mode because I was near ground level at the end of the shot.

Edit: I just noticed that when I landed at the start (home point) point, the altitude read 29.6' watching the flight record see picture after video.

Any Thoughts?





Flight Record (Altitude not close)

Flight Record (Altitude not close)
2015-9-21
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DreamingWatchma
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Your iPad does not know anything about the height of the Phantom.
How could it?
Only the Phantom knows how high she is.
2015-9-21
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Sir Edward K
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DreamingWatchma Posted at 2015-9-21 16:35
Your iPad does not know anything about the height of the Phantom.
How could it?
Only the Phantom kno ...

And the phantom is reporting the wrong height!  As it is displaying the altitude on my iPad Mini via the Go App so yes my iPad is seeing the height reported by the Phantom.   But that value is gets off over the two passes to be off by at least 25 or more feet.   I thought maybe the iPad Air 2 since it has a barometer (I think it does anyway) would be able to help the Phantom stay at the correct altitude throughout.  But I don't know since I don't have one.
2015-9-21
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hundleton1
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Altitude reporting errors have been since v1.26 mine can be as much as 8M out at the same take off point. take off fly arround came back 3 minutes later and it can miss read by 5m,
all baro sensors have a tolorance, its based on speed and altitude,
2015-9-21
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CaveDrone
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Whoa,  that's a problem!  Are you using DJI Go?
2015-9-21
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Sir Edward K
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hundleton1 Posted at 2015-9-21 17:56
Altitude reporting errors have been since v1.26 mine can be as much as 8M out at the same take off p ...

I was wondering if an iPad with the barometer (like an iPad Air2) would minimize the error propagation?
2015-9-21
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Willie Wonka
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Sir Edward K Posted at 2015-9-21 19:01
I was wondering if an iPad with the barometer (like an iPad Air2) would minimize the error propaga ...

That is what autopilot is doing, it is calculating the ground level Barometer reading and giving the phantom orders to maintain altitude based on the barometer of the phantom and comparing both, i am starting to lean towards autopilot being a better implementation of a zip line, i need to test it but thats what the manual says it relies on.
And i don't know if DJI GO does the same or not.


Edit : Also weather conditions change barometer readings, when the phantom starts it records the reading as ground level, and if ground level reading changes while it is flying then it changes the altitude accordingly and it would be either above ground or below ground as in your case.
2015-9-21
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Sir Edward K
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Willie Wonka Posted at 2015-9-21 18:09
That is what autopilot is doing, it is calculating the ground level Barometer reading and giving th ...

Thanks for you input.  I would think since the iPad Air 2 is suppose to have barometer that they would be using it as one of the items continually being sent back and forth in order to maintain a correct altitude.   Altitude is not calculated very accurately with GPS as horizontal coordinates.  

I flew this flight within 14 minutes start to finish and did the same yesterday and I noticed the same thing and both days were great days to fly no apparent barometric changes (aka different weather moving in).  I am trying to find out if I need to get an iPad Air 2 to mitigate the affects of this.  Would rather not because I don't like the size.  But....

Edit:  Also wondering if the error would have been as bad if I would have positioned myself near the middle of the run instead of at one of the ends.
2015-9-21
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Sir Edward K
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CaveDrone Posted at 2015-9-21 18:00
Whoa,  that's a problem!  Are you using DJI Go?

Yes,  DJI Go.   iPad Mini 2.   I am flying low to start out at 35'  and every point is 35' but it is off almost 30' by the time I am at the end.  I was only 5' off the plowed field at the end of the second pass (when I should have been near 35'   
2015-9-21
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Willie Wonka
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Sir Edward K Posted at 2015-9-21 19:42
Thanks for you input.  I would think since the iPad Air 2 is suppose to have barometer that they wo ...

I am itching to test it myself but i can't maybe tomorrow if it does not rain, as i have an iPad air 2.

You on the other hand can try and power up in the middle and the end of the run and see if there is sagging in altitude.

I had similar altitude leak when i was on 1.2.6 but when upgraded it was gone or i was too involved in filming lol and i did not notice as i use the grid lines on the screen to correct the altitude anyways.

Since we have the waypoints now, we need to be sure we don't run into any leaks or someone will be kissing the trees
2015-9-21
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Sir Edward K
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Willie Wonka Posted at 2015-9-21 19:10
I am itching to test it myself but i can't maybe tomorrow if it does not rain, as i have an iPad a ...

Yep, that will be my next test.  Set up mid way between the ends of that waypoint run and see what the results are after running through it back and forth twice.  My points are nearly in a straight line I set them to follow the faint curve of the road.  I really liked the look of the second pass but that was just too close for comfort.   I think for a baseline test it should also be done free flying in P-Mode.  For instance fly at 35' for 1000' and note the apparent altitude compared to that displayed.  Then fly back to the beginning and see if it still looks to be about 35' in altitude and then fly back to that point 1000' feet away and see.

I will try to get that in tomorrow but time will be tight.  I should be able to get it in if I can do it without too many people coming up and asking questions.  So far everyone along the trail has been really nice and interested in it and nothing negative about it yet.
2015-9-21
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Sir Edward K
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hundleton1 Posted at 2015-9-21 17:56
Altitude reporting errors have been since v1.26 mine can be as much as 8M out at the same take off p ...

Thank you for your input.  Do you fly with an iPad Air 2 or something with a barometer built in?  8 M not too bad I guess if you are flying decent height but if you are trying to be around 30 or 40 ft and being off that far is worrisome.  But manageable as long as I know it is happening.   I will just have to restart waypoint runs after landing and taking off again I guess.
2015-9-21
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DreamingWatchma
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Sir Edward K Posted at 2015-9-22 00:05
And the phantom is reporting the wrong height!  As it is displaying the altitude on my iPad Mini vi ...

Yes, the Phantom is reporting the wrong height.
The iPad just displays the height but it does not really know the height.

The Phantom knows the height at launch time and the actual height midflight.
That’s it.

I very much doubt that an iPad with barometer would change anything!
I'm quite sure they would not exchange height information.

A third height, the height of your tablet which could change all the time while you walk around would not really help and would make things more complicate and less reliable.
2015-9-22
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DJI-Tim
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looks like barometer failure
2015-9-22
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Sir Edward K
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DJI-Tim Posted at 2015-9-22 04:46
looks like barometer failure

Tim, what can I do to find out if it is a barometer failure or not?  I purchased mine from Ritz Camera on Amazon in mid August would I have to send it back to them for warranty? Or would I send it to DJI?  And I don't know how long I would have until the warranty would expire.  Because if I were going to send it in I would wait until it is cold here in Illinois.

Also, I would like to know, does a device with a barometer (like and iPad Air 2) communicate barometric readings with the Phantom?
2015-9-22
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Sir Edward K
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DreamingWatchma Posted at 2015-9-22 04:40
Yes, the Phantom is reporting the wrong height.
The iPad just displays the height but it does not  ...

They could exchange barometric information which can be used to compute the "relative difference in height".

Why would I be walking around while I am flying my Phantom?  I will not be using the follow me app (accident waiting for my bird)  but since you mentioned it that is what AutoPilot does and why they recommend the iPad Air 2 (because it has a barometer).  The following is taken from their FAQ:

7.4 Altimeter Reference

Autopilot can utilize several different instruments to measure both aircraft and device altitude, including absolute and relative altitude. Given your desired flight plan, it is crucial to select and properly calibrate the appropriate instruments to ensure a safe flight and produce well-framed video. The reason Autopilot requires this, where other apps may not, is because the normal usage pattern within Autopilot is a moving operator as opposed to a stationary operator (see Understanding Autopilot).

With a stationary operator, there is much less danger in using the power-on altitude as the reference, since the assumption is that the operator will be flying line-of-sight and the terrain cannot change that dramatically within sight range. Furthermore, even if the terrain does change, the operator is manually controlling the aircraft and has the ability to compensate.

With a moving operator, there is a very real danger that as you move, your altitude will change. If altitude does change, Autopilot must know about it or else you risk letting it fly the aircraft into obstacles, or worse, the ground.


So you see, that App does communicate barometric readings with the phantom in order for it to compensate, to keep its "relative height".
2015-9-22
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DreamingWatchma
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Sir Edward K Posted at 2015-9-22 13:08
They could exchange barometric information which can be used to compute the "relative difference i ...

I did not say (or I did not mean to say) that it is impossible for any app to exchange height information.
Of course it’s not.
I said (or meant to say) that the DJI Go app does not send such information to the Copter.

I don’t know about the AutoPilot app, I’m only talking about the Go app.

However, I don’t see how any app or any mobile device could fix a problem with the barometer in your P3. In order to keep its relative height the height information in your P3 still has to be correct.
2015-9-22
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Sir Edward K
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DreamingWatchma Posted at 2015-9-22 08:42
I did not say (or I did not mean to say) that it is impossible for any app to exchange height info ...

I did not know I had a barometer problem until Tim mentioned that as a possibility so Il don't know that I do yet (I will test it with a few flights).  Others have said in this thread that it is not uncommon to have a 8 meter difference between beginning and end of an 8 minute flight.  So if that is the case why wouldn't the communication of the barometers of the two devices help stabilize the relative height?   That is what I am trying to find out.  8 Meters is not small amount in a low altitude flight.   If I was paying more attention to filming my son riding than looking at how close I was getting to the ground I probably would have crashed.   Luckily I pay attention to both.   For now I can plan around it by not running a mission twice without landing and restarting in between.

You are right.  If my barometer is broken or not working properly the communication  of this information would not help.  But if this is something that is quite common where you can have 8 Meters difference between the start of your flight and the end when landing at the same place ( this could help).   8 meters or 26' is a big difference especially in a low altitude flight.

I have asked the question a number of times, if a device like the Air 2 communicates the barometric readings with the phantom and I cannot recall hearing a yes or no in that regard until your last post.   Thank you as this is one of the things  I wanted to know.

My reason is for the question is two fold.  

   1) If The iPad Air would communicate barometer and help stabilize the relative altitude then I would purchase it.  If not I would not purchase one because I don't like the bigger size.
  2) If the iPad Air does not communicate barometer at this time then is DJI contemplating  this on future adaptions of the "GO" app?

These two questions are meant more for the DJI people.  Just hoping to improve on the altitude difference.
2015-9-22
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kleake1
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Sir Edward K Posted at 2015-9-22 06:08
They could exchange barometric information which can be used to compute the "relative difference i ...

This is interesting as the follow me could definitely read altitude from the mobile device to keep the phantom at the same height above the operator.  Good info to test, although I thought that height info would be read from the GPS from both devices as well, but maybe not.  

To the OP, there has been barometer issues in the past.  In fact, my P2 used to drop almost 15ft in about 5 minutes, but after the first battery, the others were pretty stable.  I assumed it was the warming up of the internal components causing the issue.  The P3 seems to be a LOT more stable, but I have noticed a few times that if hovering about 10ft off the ground, it may drift lower, but the then VPS will see the ground and it will climb back up.   I need to watch my altitude reading a little more without the VPS on.   Keep in mind though, this thing is setting altitude with a device that is measuring air pressure.  How much does air weigh?  How much difference is there between 10 and 20ft?  All the while the propellers are disturbing the air pressure and flow while flying.  There is definitely a lot of precision going on there so a little variation is to be expected.  Sounds like yours may be pretty excessive though.
2015-9-22
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Sir Edward K
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kleake1@gmail.c Posted at 2015-9-22 09:54
This is interesting as the follow me could definitely read altitude from the mobile device to keep ...

Thanks for your response.  GPS cannot calculate altitude accurately and it can be off by as much as 100' because altitude is a lot more complicated to calculate do to the Geoid fluctuations.  So the Quad can only use the barometer for the Altitude.
2015-9-22
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Sir Edward K
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DJI-Tim Posted at 2015-9-22 04:46
looks like barometer failure

Tim, just went out to same area and this time I flew manually (No waypoints) set altitude to 40' and flew 1000' out and back 6 times and could  not tell it was dropping elevation but when I landed I was within 8 ft of takeoff.   That I can live with.

I am thinking there is something that is happening during f-mode waypoints that is making this happen and I will have to try and set these points over again and see if it happens every time.  But this is the second time it has happened.

My setup was four way points about 250' apart for a total of a 1000 feet.  The points were nearly aligned with each other as I was trying to set them to follow the edge of a very slightly curved road.  The points were laid out northerly to southerly (see attached image).  I don't know if this matters but if you are trying to replicate the problem this could help.  When I started the mission I set the heading to "FREE" and "Hover" at the end and set the speed to 0.   Then I used the right stick to go from the north end to the south end while I was filming to the east.  When I got to the south end I just came back to the north end by pullng back on the right stick until I got to the beginning.  I could tell here that it was a little lower than the start.   Then I went back south by pushing forward on the stick until I got to the south end where I was just a couple feet off the ground.  The whole time I was filming to the east.  Once I got to that end I put it in P_GPS and went up about 70 feet and few back to me and landed.


I also found this on the PhantomPilots forum talking about the same thing :
http://www.phantompilots.com/threads/near-disaster-warning-on-running-waypoints-w-o-a-device-barometer.53225/
Seems that this  might be something worth checking into.


As an aside, I make these exhibits for when I fly somewhere new.  So I am prepared with the legal jargon etc and can let them read it and see where I am flying on the map.  There is a no fly zone about a mile and a half north of this location so I have one that shows my location in relation to that as well so I can show anyone that I am not flying in a no fly zone.

Layout of Waypoints

Layout of Waypoints
2015-9-22
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DreamingWatchma
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That's an interesting link you posted.
Meta4 (a staff member) said exactly what I tried to say:
"Whether your tablet has a barometer and whether it is accurate or not is completely irrelevant.
Your tablet could only report the altitude of the tablet. It knows nothing of the altitude of your Phantom and the altitude of your tablet is useless information for waypoint flying."
2015-9-23
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Sir Edward K
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DreamingWatchma Posted at 2015-9-23 04:12
That's an interesting link you posted.
Meta4 (a staff member) said exactly what I tried to say:
"Whe ...

Yes, that is what he said but that is not what it could do.  Two different things.
A device on the ground communicating with a device in the air can correct for changing barometric conditions.  Because the device on the ground will always be at a known value "Ground Level".  So if your aircraft is at 200 ft and say you were walking around and you climbed a hill in follow me.  Then the aircraft would rise the same amount (within its error limits). Keeping the AGL relatively the same.  It is not "knowing" its altitude but communicating its barometric pressure.  That is what I was saying not that the device would know an altitude of the aircraft" just communicate the barometric pressure so the aircraft can compensate.   


The theory is just like differential GPS that Land Surveyors use.  A good quality GPS unit on its own called a Rover might be able to calculate its location within a meter.  But if you have another one called a Base at a known location anywhere within about 25 miles of the other one.  Then that one on the ground will take its known location and the one that it is calculating via GPS and subtract them from each other and it will send this difference (the correction) to the Rover, then the Rover can know its real position on the earth within a centimeter.   "Differential GPS" been used for years.  (not to be confused with DGPS used by the Coast Guard which is not as accurate).

Person on ground "known Elevation Ground Level" is the base and the aircraft is the rover.  That is what I feel that the Go app should be using to help stabilize the elevation fluctuations.  And I see that  one other app is using it for that purpose now.    So the GO app will probably use that same process in near future if it keeps the follow me function that is.


2015-9-23
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markfrank
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DreamingWatchma Posted at 2015-9-22 11:40
Yes, the Phantom is reporting the wrong height.
The iPad just displays the height but it does not  ...

With autopilot from autoflight logic in "follow me" mode they do exchange altitude info
With DJI go i tested only in a flat field, I have to test on a steep slope....
it really make sense to exchage altitude infos!
2015-9-25
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DreamingWatchma
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markfrank Posted at 2015-9-25 13:16
With autopilot from autoflight logic in "follow me" mode they do exchange altitude info
With DJI g ...

It only makes sense for "Follow me".
2015-9-25
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