Barometer way off

[Tricky]

So flew my new bird (P3P, sent for repair, ended up being replaced) last night. Everything except for one thing seems perfect. The barometer. I flew at my usual park, no wind. Level ground. Just flew it back and forth, up to 150' and back down. I burned a whole battery literally just flying to and fro.

When I came back to land...I noticed the bird, at eye level, was reading -7' altitude. That is no bueno.

Question - does the VPS play a role in determining altitude? I think I had it on...when usually on my old bird I had it off. Otherwise...what gives? I never had a negative altitude reading before this...what else could cause the reading to be so off?

thanks everyone for your suggestions and insight!!

 

[msinger]

does the VPS play a role in determining altitude?

No.

 

[Tricky]

Thx msinger, how about I try an imu calibration...i thought dji calibrated imu before shipping birds, I remember reading that somewhere...but would that possibly be a reason for bad reading?
Thx!!

 

[sar104]

So flew my new bird (P3P, sent for repair, ended up being replaced) last night. Everything except for one thing seems perfect. The barometer. I flew at my usual park, no wind. Level ground. Just flew it back and forth, up to 150' and back down. I burned a whole battery literally just flying to and fro.

When I came back to land...I noticed the bird, at eye level, was reading -7' altitude. That is no bueno.

Question - does the VPS play a role in determining altitude? I think I had it on...when usually on my old bird I had it off. Otherwise...what gives? I never had a negative altitude reading before this...what else could cause the reading to be so off?

thanks everyone for your suggestions and insight!!

Atmospheric pressure was rising in NY last night - averaging about 0.01 in Hg per hour. That's equivalent to a decrease of around 10 ft per hour in barometric altitude. It doesn't fully account for a 10 ft or so offset in 20 - 30 minutes, but the rate of change is not constant and you may have been flying when it was changing even more rapidly. Additionally, the barometric altitude is only accurate to a few feet. That doesn't seem like a problem.

 

[msinger]

how about I try an imu calibration

Yes, that would be the best thing to try.

 

[sar104]

Yes, that would be the best thing to try.

What does the IMU have to do with the barometric reading? The OP's observations are completely consistent with the atmospheric conditions last night combined with the accuracy of the onboard barometer.

 

[msinger]

What does the IMU have to do with the barometric reading?

If the OP is able to reproduce that behavior day after day, then that would be an indication that the IMU might need to be calibrated.

 

[Tricky]

Atmospheric pressure was rising in NY last night - That doesn't seem like a problem.

I agree with your theory...as it was humid last night...bordering on misting. However my experience with my own bird has been, never have I had a negative altitude reading.

I am not debunking your theory, I fully accept that it is a possibility...I just am surprised that I haven't experienced this before...in over 400 flights and 2 years of flying. New bird, new hiccups I guess.

I would normally not be concerned about 5 feet here or there, but I have a job this weekend, and 5 feet when flying waypoints could mean the difference between clearing an obstacle and...well, not clearing the obstacle.

I am going to IMU calibrate tonight. I get another crack at test flying tomorrow before my gig Saturday. Frankly, I should have calibrated IMU...that was rookie of me not to.

Thx everyone!

 

[sar104]

If the OP is able to reproduce that behavior day after day, then that would be an indication that the IMU might need to be calibrated.

I don't follow that argument. Calibrating the IMU does not calibrate the pressure sensor, and inertial data are not used in calculating altitude, so I cannot see what that will achieve. What am I missing?

 

[msinger]

I don't follow that argument.

I'm not arguing -- just sharing common knowledge. Calibrating the IMU sometimes resolves altitude inconsistencies. Why? I'm not sure really. I don't have intimate knowledge of the IMU's internal makeup.

As for your theory, it seems it could be easily debunked by running the same test on a different day. If the same thing happens, then would you recommend the OP toss the IMU and replace it?

 

[sar104]

I'm not arguing -- just sharing common knowledge. Calibrating the IMU sometimes resolves altitude inconsistencies. Why? I'm not sure really. I don't have intimate knowledge of the IMU's internal makeup.

As for your theory, it seems it could be easily debunked by running the same test on a different day. If the same thing happens, then would you recommend the OP toss the IMU and replace it?

I don't understand what you are asking. Running which same test? The OP already stated that he has not seen negative altitudes before. But the point remains - those observations are not inconsistent with the prevailing atmospheric trend, or are you disagreeing with that?

What is there to debunk? Barometric altitude at a fixed point varies with prevailing atmospheric pressure. That's not a theory - it a basic fact. The variation is also trivially calculated from the variation in atmospheric pressure, which I did, for last night, for NY. Now that does not prove that the OP does not have a faulty or uncalibrated IMU or barometer - it just means that neither of those is required to explain his observations.

 

[msinger]

I don't understand what you are asking. Running which same test?

The OP needs to fly some more to try to narrow down the problem. At this point, the problem could have been caused by any of the following:

• A bad IMU/barometer
• Bad IMU calibration
• Weather conditions

Without further testing, the OP is not going to have enough information to be able to resolve this issue.

But the point remains - those observations are not inconsistent with the prevailing atmospheric trend, or are you disagreeing with that?

Yes, sir. It's nothing but an assumption at this point.

 

[tcope]

I've not seen the OP mention if that was the altitude or VPS reading.

There is a reason DJI installed the ultrasonic sensors.... they can be more accurate at much lower levels.

However, I also don't see the big issue with the altitude being off by 15' (though that does seem a little high).

 

[sar104]

The OP needs to fly some more to try to narrow down the problem. At this point, the problem could have been caused by any of the following:

• A bad IMU/barometer
• Bad IMU calibration
• Weather conditions

Without further testing, the OP is not going to have enough information to be able to resolve this issue.


Yes, sir. It's nothing but an assumption at this point.

No - you don't understand. Stating that his observations are consistent with other factors is not an assumption. Stating that they were definitely caused by those factors would have been an assumption - one that I did not make.

Further, stating that they could be caused by an out-of-calibration or bad IMU appears to be an unreasonable assumption on your part unless the IMU has an influence on the barometer reading.

 

[Meta4]

stating that they could be caused by an out-of-calibration or bad IMU appears to be an unreasonable assumption on your part unless the IMU has an influence on the barometer reading.

Given the number of times I've seen DJI people suggest recalibrating the IMU for bad barometer readings, and the number of times this seems to have worked, it could be that the barometer is also calibrated when the IMU is?

 

[Tricky]

Boy I am getting all the heavy hitters to come out for this one. Thanks again everyone. I will be re-calibrating IMU tonight. Will advise. Unfortunately, my bird is not in the freezer, and its ~60F most of the day today...unsure whether the "cold" IMU calibration is still a thing...but, the temp is what it is.

Will report back! Thx again.

 

[sar104]

Given the number of times I've seen DJI people suggest recalibrating the IMU for bad barometer readings, and the number of times this seems to have worked, it could be that the barometer is also calibrated when the IMU is?

It's possible, although DJI support appear to suggest all kinds of random things to try to fix problems. But that was the reason for my question.

The broader issue, however, is how a barometer is calibrated. The basic calibration of a regular pressure altimeter is simply to tell it its actual altitude, so that it has an accurate starting point. That doesn't happen on this equipment - it doesn't care about MSL. Calibration of its measurement of pressure change, from which it calculates altitude change, requires subjecting it to known, varying external pressues - something that cannot be done by the user. The only other calibration that might apply is variation of measured pressure with temperature, and that could be done in situ during an IMU calibration as the aircraft warms up.

So, to be clear, there is no harm in doing an IMU calibration. But the point I was trying to make is that the OP's observations do not actually indicate a problem with the equipment, and not that those observations prove that the equipment does not have any problems.

 

[RodPad]

Hum, I have seen many negative altitudes on my flights and many others.
This ones shows he is not taking off (up), he is taking down.
Phantom Log Viewer - PhantomHelp.com

Rod

 

[sar104]

Hum, I have seen many negative altitudes on my flights and many others.
This ones shows he is not taking off (up), he is taking down.
Phantom Log Viewer - PhantomHelp.com

Rod

The start of that record, when the reported altitude is dropping, looks like barometer stabilization after motor start. PhantomHelp doesn't report stick input but, judging from the battery voltages, actual takeoff looks to be at 16 s, and the altitude rises appropriately from there.

 

[Mike2015]

Tricky I have experienced similar problems with my P3A in recent days. It was fine until my RC went in for repair. When I got my RC back I took her out for a test flight and she was at about 6ft but DJI Go App was showing 13ft. I did an IMU calibration and RC Calibration and it is fine now. Not sure what caused it it it seems fine now.