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Height loss
1145 22 2015-9-23
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msjh
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I have noticed that my P3 tends to lose height over the course of a 15-20 minute flight.  So today I set up my camera to do a series of time-lapse photos, one every two seconds, launches the P3 and flew it to 20 metres height and let it hover without any control input until it auto-landed after 15 minutes or so.

The result is here:

Does this look normal?  Is this a reasonable altitude drop-off?
2015-9-23
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sploodge
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Flight distance : 809350 ft
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I have done time-lapse with mine for an entire battery and it remains like a tripod in the sky.. Make sure to do an IMU calibration in a cool place.. Atmospheric conditions could also have an impact on its Baro.. Did the altitude on the App show it getting lower?

I would also be concerned about the two UFO's in the first few seconds of the video They are watching you
2015-9-23
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Flight Raptor
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sploodge Posted at 2015-9-23 20:22
I have done time-lapse with mine for an entire battery and it remains like a tripod in the sky.. Mak ...

I, too, was thinking barometric pressure could play a role here.   I see it was relatively windy that day, too,
2015-9-23
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msjh
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sploodge Posted at 2015-9-23 13:22
I have done time-lapse with mine for an entire battery and it remains like a tripod in the sky.. Mak ...

Thanks, sploodge.

I calibrated the compass just before take-off and previously did a cool IMU calibration.  Winds were mild to still air on the ground: perhaps 5-10mph at  the altitude the P3 was at.

The App showed virtually no change in altitude:  here are screenshots from the beginning and the end of the flight




I'm not so worried about the wobbles a few feet either way in the sky but want to do some flying across water.  I estimate that just before landing I had lost 8 metres height here (25 feet) so am nervous doing flying over water which is notorious;y difficult to visually judge height over.
2015-9-23
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msjh
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Flight Raptor Posted at 2015-9-23 13:38
I, too, was thinking barometric pressure could play a role here.   I see it was relatively windy t ...

Actually there was little wind.  The weather forecast said 9mph winds and that was probably about right at height.  I think the time lapse exaggerates the little movement there was.
2015-9-23
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msjh
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sploodge Posted at 2015-9-23 13:22
I have done time-lapse with mine for an entire battery and it remains like a tripod in the sky.. Mak ...

Yeah, the UFOs are a source of concern!  :-)
2015-9-23
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mtnmaddman
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sploodge Posted at 2015-9-23 06:22
I have done time-lapse with mine for an entire battery and it remains like a tripod in the sky.. Mak ...

Same old thing sloodge yours works perfect I have seen this very problem posted many times and I posted directly to you several months ago about this,  this is a problem and you know it, why would you post this same old crap.  You act as though you just heard of this I find your posts to be redundant,  why do you keep misleading people with this same old crap that yours works correctly so this is a random problem you mislead people sploodge why do you persist with this same old BS? Then you cry that all your trying to do is help, and to quit picking on you.  
2015-9-28
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sploodge
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mtnmaddman Posted at 2015-9-29 03:52
Same old thing sloodge yours works perfect I have seen this very problem posted many times and I p ...

No, I haven't had the issue OP has with maintaining altitude on the P3 ( not loosing feet like that anyway, maybe a few inches ), but never denied it existed ( there is a video of it ).. I even gave suggestions as to what the issue could be. IMO, its atmospheric/baro issue. Hot weather/currents of hot air can cause the issue he was experiencing. I had a similar issue with my P2 a couple years back when I visited Thailand, the thing kept bobbing up and gown like it was experiencing turbulence.

I have seen the other threads you posted in regarding baro readings, and as you see in there, some say they see the issue and some say they don't. There are lots of posts about it

Anyway, this will be my last reply to anything you post, feel free to ignore any/all redundant posts I make.
2015-9-28
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msjh
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sploodge Posted at 2015-9-29 08:28
No, I haven't had the issue OP has with maintaining altitude on the P3 ( not loosing feet like tha ...

I'm less convinced it is related to changing atmospheric pressure.  I am a fair weather flyer and hence fly when the barometer is fairly stable (and in any event, over a 15-20 minute flight you're unlikely to see a major change).  In addition, the change in height is always downwards; if it were due to atmospheric conditions, I would expect sometimes it would go up and others down.

I'm more inclined to go with the theory that as the aircraft warms up during the course of the flight it has an effect on the barometer.

But regardless of the cause I just want a system that I can trust to keep its altitude.
2015-9-29
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Sir Edward K
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msjh Posted at 2015-9-29 06:28
I'm less convinced it is related to changing atmospheric pressure.  I am a fair weather flyer and  ...

Yes, I agree the error in height is always negative so it cannot be the random error that other's have talked about (2cm per second) because those kind of errors tend to cancel each other out over time.  But altitude is also effected by temperature like you have mentioned  (see the Barometric formula).  

When you fly the P3 the altimeter  rising as you press up on left stick then when you let go the altitude levels off and stays roughly in this general area.  Once I set it at an altitude I see that altitude displayed within a couple feet unless I change it by pressing the stick.  But the barometer is inside the shell of the craft and as you fly that internal temperature rises and is different than it was during take off.  As the temperature rises  inside the shell the apparent altitude (which is calculated using pressure and temperature)  will also increase giving the P3 an increasing false sense of height until that temperature levels off.   If what I am saying there is true, then theoretically if you fly one battery, land, change batteries quickly and take off again, the second flight should not be affected as much by the temperature differential and should then only be effected by the random error (i.e. +/- 2cm per second of flight).

This is Just a theory.  But if this is what is happening then I would think that they could calculate this systematic error out by changing the formula to just use the temperature at the start of the flight.

Edit: just did some calculations and if it was 86 internally when you took off and 104 internally after you flew a bit, then that should only account for about 2-3 ft in height at 40' altitude and 13' at 400' altitiude  (If I calculated right).  But I still would like to see what it would be like after sending up again with new battery while the craft was still warm.
2015-9-29
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mtnmaddman
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Sir Edward K Posted at 2015-9-29 06:58
Yes, I agree the error in height is always negative so it cannot be the random error that other's h ...

I have observed it is not always negative I have had a p3p It always came back reading at least 3o ft altitude when back at ground level, sometimes as high as +60 ft.  way off.  I got rid of it.  I bought an Inspire,  most of the time it comes back at 3.3 ft negative, eerily so.  and then every so often it comes back at 1 ft. positive,  I can see nothing,(temp,weather change, 2nd flight) nothing.  I can make no correlation to anything.  I then bought a p3a,  it is more accurate than the I1 but it also will come back to the same spot at either a positive or negative baro reading of plus or minus a foot or two, most of the time perfect. again I can not pin the variation to anything.
2015-9-29
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mtnmaddman
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That looks way off my first p3p was doing the same thing if you let it sit on the ground, with everything on except the motors running for 5 min or so you most likely will get an error 43 or 45,  indicating a bad baro sensor at that point your motors will not start, a message to turn the bird on and off  will appear and to contact dji if the problem persists
2015-9-29
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boxerman
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It's because you are using meters.  Imperial units are always much more accurate.
2015-9-29
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boxerman
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It's because you are using meters.  Imperial units are always much more accurate.
2015-9-29
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Sir Edward K
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mtnmaddman Posted at 2015-9-29 20:39
I have observed it is not always negative I have had a p3p It always came back reading at least 3o  ...

In my case it is ALWAYS negative (meaning it is really lower than reported by altitude).   If it were either way within 10 ft I would be happy but 25' is about the norm.
2015-9-30
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suqsid.bobmail
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The reason for the variations is that the barometer isn't giving real altitude information to the quad, it is giving very small adjustments as it detects differences in the pressure. That is what the IMU does.

The quad sums up those changes and reports a height differential from take off point.

All those reading have some inherent error. Those errors add up over time. In a perfect world the errors should balance out high/low. But we don't live in a perfect world where all those barometers are manufactured exactly the same. A small little difference in the actual barometer between two different quads can be a big change in the total sum over time.

I have no solution, but I do understand the frustration. IMU calibration is the only option available to try to correct it, but there is no guarantee whatsoever that it will work.
2015-9-30
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msjh
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Well I've put the blasted thing in the fridge for 30 mins and then I will do a cool calibration.  It had better work or There Will Be Trouble.
2015-9-30
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mtnmaddman
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msjh Posted at 2015-9-30 06:15
Well I've put the blasted thing in the fridge for 30 mins and then I will do a cool calibration.  It ...

You can freeze it in a block of ice and calibrate it upside down and it is not going to make a damn bit of difference your baro sensor is bad.
2015-9-30
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mtnmaddman
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boxerman Posted at 2015-9-29 22:42
It's because you are using meters.  Imperial units are always much more accurate.

that is absurd it will be off at the same rate what ever it is
2015-9-30
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msjh
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mtnmaddman Posted at 2015-9-30 13:34
You can freeze it in a block of ice and calibrate it upside down and it is not going to make a dam ...

You may be right.

I just want to make sure that I have done everything possible.
2015-9-30
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mtnmaddman
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msjh Posted at 2015-9-30 08:41
You may be right.

I just want to make sure that I have done everything possible.

Yeah I know it is pain in the butt to deal with,  as I said I got rid of my P3P because of this problem,  I just flew  my Inspire and it Landed again at 3.3 feet negative,  It flips me that it is always 3.3 feet, that is acceptable but how on earth can it be 3.3 almost everytime,  weirds me out.  my p3a is almost perfect.  it will vary maybe plus or minus a foot.  You probably should call DJI and send it in this winter
2015-9-30
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boxerman
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mtnmaddman Posted at 2015-9-30 07:35
that is absurd it will be off at the same rate what ever it is

Just checking to make sure everybody is paying attention....  
2015-9-30
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mtnmaddman
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boxerman Posted at 2015-9-30 11:34
Just checking to make sure everybody is paying attention....

Good one you got me, are you harrasisng these Brits   I like your avatar by the way it looks like mine when it gets up in the morning.
2015-9-30
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