P3P Losing altitude in GPS mode; telemetry lies

[nhoover]

I've had this problem on some but not all long flights: flying at a constant altitude, the app and log files all show it is doing a great job of maintaining that altitude, but in reality it slowly descends at some point in the flight. Here is a video that shows it, although it's subtle as it happens.

I launch and climb to 325' while flying in a straight line in GPS mode. I fly for just over 4 miles then turn around and fly back. I knew something was wrong because my signal strength dropped but while flying I didn't notice I was actually much lower than what the app said. When I landed at the take-off point, the altitude read 197'. I know from flying here previously to a similar distance that both video and control should be a solid 5 bars at the turnaround point - if flying at 300'. I believe I was really only at about 125'. While I briefly used ATTI mode on the return, it was long after I had lost the altitude and didn't make it worse as far as I can tell.

I am flying with old firmware (1.1.9) and app - Android 1.0.10 on a Nexus 9 with downgraded Google components which has been very stable and lag-free now for almost 100 flights. I might try upgrading once I hear that people are still able to do long distance flights.

Has anyone else noticed this?

Here's the flight on healthydrones.com: HealthyDrones.com - Health Checks for your Phantom

And here's a recording from my tablet:

 

[Mako79]

IMU cold vs IMU hot.
The barometric sensor works on pressure - heat affects pressure and therefore your readings start to become inaccurate/accurate the longer you fly.

 

[nhoover]

Thanks Mako - I guess that makes sense. This was the first flight of the day and I did not let it warm up at all really. I armed the motors and then blasted off right away.

I have noticed that my P3P requires very little time from power up to all green, ready to fly. I've seen others that sit there flashing yellow for a minute or more, every time.

 

[4wd]

Was it a very hot day?
If the internals were getting hotter the barometer air chamber (that's all it is) would think you were higher - however that's a very substantial - and concerning - descent.

Must say it often feels like it's not high enough as you get further away into less familiar territory and you aren't sure how tall that approaching tree or hill is. A bit worrying it might really have descended quite a bit too!

 

[nhoover]

It was indeed a hot day today, but at that location, right by the ocean, it was only about 80F at the time I was flying this morning. Normal for that time of day at this time of year would be more like 65F. I haven't flown much in really hot temperatures.

I may have seen this before more than I really think, but not usually this bad. Landing at 197'!

Before trying this again, I'll first do a warm-up flight.

 

[Meta4]

That's quite a drop in altitude. I'd be doing an IMU calibration.

I am flying with old firmware (1.1.9) and app - Android 1.0.10 on a Nexus 9 with downgraded Google components which has been very stable and lag-free now for almost 100 flights. I might try upgrading once I hear that people are still able to do long distance flights.]

Time to put your false fear away. The current firmware is safe and stable, just like the previous version was.
Updating firmware won't change your distance. You'll still be able to fly as you please.

 

[holoboss]

I have the same problem in all modes. I pull my P3 pro to around 6 ft, and it hoovers just near my head. Satellites locked, and they give it quite impressive position lock, much more precise than my old P2 had. but the fluid change of altitude is really a problem. If i let it hoover or slowly fly, but I don't touch altitude stick - it will go slowly down - WITHOUT reporting it on screen. Imagine if I fly over a lake or sea, and 100 meters from the shore it crashes in water! disaster! (FPV image has a lag of about 1 second and sometimes image freezes, so i can't rely on vison too much either).
my old P2 did not do it, at least not to such extent.

So, when i have to correct the height, it reacts fine and goes up. so i hoovered again near my head. but after about 5 minutes - i noticed it reports... 18ft above ground. Well, I am only 5.6" tall, so i know it is a lie
When it drops, it does not drop smoothly, but by about 1 ft drops, then it stays for a few moments, then it drops down again.

I have a job coming involving actually flying low over sea quite a bit, and i am afraid i will hit the water!
Any ideas? any settings I can tweak through the app to keep it more aggressively up in the air?

 

[alokbhargava]

Loosing height indicates either barometer is gaining height feedback or its electronics has a drift. IMU calibration may or may not help but that's what one can do.

Does any one know if the stick generates analog reference which is then converted to digital or it directly generates digital values? If it's analog, then there are chances of drifts here too.

 

[holoboss]

Loosing height indicates either barometer is gaining height feedback or its electronics has a drift. IMU calibration may or may not help but that's what one can do.

Does any one know if the stick generates analog reference which is then converted to digital or it directly generates digital values? If it's analog, then there are chances of drifts here too.

thank you, so i will do full imu calibration, just in case. system app says calibration not needed. but to be sure i will do it.

 

[alokbhargava]

thank you, so i will do full imu calibration, just in case. system app says calibration not needed. but to be sure i will do it.

That's what you can do as you can't touch the electronics. Do it under cold condition. Keep every thing ready on Go app, start AC and immediately press a button on GO app to start IMU calibration before it warms up due to its internal heat.

 

[SGs]

That's what you can do as you can't touch the electronics. Do it under cold condition. Keep every thing ready on Go app, start AC and immediately press a button on GO app to start IMU calibration before it warms up due to its internal heat.

If the IMU pressure changes as it warms might it be better to do an IMU calibration at as close to flying conditions as possible.

If everyone is calibrating their IMUs at as low as tempature as possible, including some refrigerating their bird, it just seems like maybe it could lead to altitude differences once the bird warms up especially in hot conditions.

I'd rather have to wait for the all green and get more accurate altitude readings than be able to fly quickly (the main reason for cold IMU calibration?). Does that make sense?

 

[Mutterranch]

If the IMU pressure changes as it warms might it be better to do an IMU calibration at as close to flying conditions as possible.

If everyone is calibrating their IMUs at as low as tempature as possible, including some refrigerating their bird, it just seems like maybe it could lead to altitude differences once the bird warms up especially in hot conditions.

I'd rather have to wait for the all green and get more accurate altitude readings than be able to fly quickly (the main reason for cold IMU calibration?). Does that make sense?

 

[Mutterranch]

Sorry I don't know how to quote. But yes that does make sense!

 

[Trackman1]

If the IMU pressure changes as it warms might it be better to do an IMU calibration at as close to flying conditions as possible.

If everyone is calibrating their IMUs at as low as tempature as possible, including some refrigerating their bird, it just seems like maybe it could lead to altitude differences once the bird warms up especially in hot conditions.

I'd rather have to wait for the all green and get more accurate altitude readings than be able to fly quickly (the main reason for cold IMU calibration?). Does that make sense?

These are my thoughts also. But I am no expert. But it only makes sense to me that instruments need certain perimeters for proper calibration.
I would think that temperature would be a major one.
DJI species 20-25c and on a level surface.

I have tried 11c. P3 seemed ok. Did not get warmup message.
Did a cal. at 20c. Get about 30sec. warm up message. P3 flys well.
My P3 is stored in a 15c spare room.

 

[SGs]

I do know that when I first got my P3P I did the IMU calibration in my house at room temperature. I never noticed any altitude drop until I read about a cold IMU calibration. I too was waiting 2 or 3 minutes to fly so I figured I'd give it a shot. It got me flying quickly, but I have noticed altitude reading differences since doing that. I didn't really have enough experience before hand to notice much until I read this thread.

 

[ZeusDog]

nhoover -

First, my compliments on your choice of JGB for the music track.

Now, regarding your altitude loss issue, I think the earlier comment about temperature affecting the barometer reading is pretty close to the mark. However, recall that our Phantoms measure altitude above the home point. To do that, the firmware is essentially doing a 1-point altitude calibration (similar to setting the altimeter of a conventional aircraft). If, as noted in a previous comment, there is an air chamber (as in a conventional aircraft altimeter), then this 1-point calibration is only valid as long as the air chamber temperature is equal to that when the firmware started up. If the air chamber gets warmer after that time, the indicated altitude will be overstated (and the true altitude will decrease as the firmware maintains a constant - although inaccurate - indicated altitude).

So if, for instance, your Phantom was colder than ambient temperature before you launched and then warmed up in flight, this might produce just the effect you have encountered.

If I'm correct, an IMU calibration (at any temp) will not help because it is the temperature change after startup that matters. What will help will be to let your Phantom equilibrate to the ambient temperature at your flight location before takeoff.

If my hypothesis is correct, you would probably not see the altitude drop if you do a second flight at the same location immediately after the first flight and a battery changeout.