Height telementry is off

[slothead]

The barometric sensor in the Phantom sees rising temperature as an increase in altitude. There is a lot of heat generated inside the body just sitting there.

Mine always thinks it's 10 feet up after 3 minutes on the ground the first time it's powered on. If you've been messing around setting waypoints for much longer than that I wouldn't be surprised if the bird thinks it's already at 15 feet when you hit "Go". Just watch your camera screen for a few minutes on the ground at first power-on to see what I'm talking about.

Interesting! I wonder if better ventilation is necessary for more accurate height determination? I would want to terminally alter the vents on the bird just to find out I was wrong (although a temporary mod might be feasible - not sure how at the moment). An opening top and bottom letting prop wash through could help I suppose (but again, that sounds fairly permanent). The only real value in that kind of surgery of course is having the altitude accuracy to fly very close to the ground without crashing - not sure if that is worth it.

And by the way, any drift that you guys see like this could either be gain or offset error because both types of affected electronic parts (resistors or op amps) have temperature coefficients, and the barometric sensors are probably much worse (but would cause only offset error).

 

[Sasquatch]

Thanks again guys for getting this discussion going more and with more input from various and more seasoned flyers than me.

I wasn't able to get my bird out this weekend so far...too much stuff to do around the house and even though the weather has been good the last two days.

I'm still going to try manually resetting my home position for next flight and see of that helps. However and after reading the many comments, it seems like that won't help. It sounds like it's not that uncommon and a science that hasn't been perfected apparently.

We will see and I'll let y'all know.

 

[iswimmer]

Thanks again guys for getting this discussion going more and with more input from various and more seasoned flyers than me.

I wasn't able to get my bird out this weekend so far...too much stuff to do around the house and even though the weather has been good the last two days.

I'm still going to try manually resetting my home position for next flight and see of that helps. However and after reading the many comments, it seems like that won't help. It sounds like it's not that uncommon and a science that hasn't been perfected apparently.

We will see and I'll let y'all know.

I got curious so just tried this. Resetting home position made no difference in altitude. Compass calibration didn't either. Only turning it on and off would reset the altitude back to zero.

First try it drifted up to 30 feet in about five minutes of sitting.

Then restarted and it went back to zero and drifted up to 15' in about five minutes.

Third try I let it sit 20 minutes and it drifted only up to seven feet.

Didn't start motors during any of this time.

Fourth try I restarted again and let it sit five minutes...it stayed at minus one to zero that whole time. Then I started the motors and flew 10 minutes. Went up to 400' and back down to the ground three times and did some hovering. Each time back to the ground and at the end of the flight the altitude on the ground was minus 1 to zero.

This was my Saturday night entertainment.

 

[M.Tigelaar]

Definitely the temperature.

First in the oven before flying :lol: :lol:

 

[Sasquatch]

I got curious so just tried this. Resetting home position made no difference in altitude. Compass calibration didn't either. Only turning it on and off would reset the altitude back to zero.

I was reluctant to think it would...thanks for experimenting though. What do you mean by "turning it on and off" was the only way to reset it back to zero?

 

[N017RW]

What is the expectation here?

It seems many expect wayyyyy too much in terms of accuracy and repeatability from this device (Naza-M V2).

Look at all the functionality you get from a $200 FC. There's going to be a trade-off.

 

[Mori55]

Thanks again guys for getting this discussion going more and with more input from various and more seasoned flyers than me.

I wasn't able to get my bird out this weekend so far...too much stuff to do around the house and even though the weather has been good the last two days.

I'm still going to try manually resetting my home position for next flight and see of that helps. However and after reading the many comments, it seems like that won't help. It sounds like it's not that uncommon and a science that hasn't been perfected apparently.

We will see and I'll let y'all know.

I got curious so just tried this. Resetting home position made no difference in altitude. Compass calibration didn't either. Only turning it on and off would reset the altitude back to zero.

First try it drifted up to 30 feet in about five minutes of sitting.

Then restarted and it went back to zero and drifted up to 15' in about five minutes.

Third try I let it sit 20 minutes and it drifted only up to seven feet.

Didn't start motors during any of this time.

Fourth try I restarted again and let it sit five minutes...it stayed at minus one to zero that whole time. Then I started the motors and flew 10 minutes. Went up to 400' and back down to the ground three times and did some hovering. Each time back to the ground and at the end of the flight the altitude on the ground was minus 1 to zero.

This was my Saturday night entertainment.

so what was wrong with this ? Seems pretty close on the ground ? Gps isn't perfect , this isn't military grade.

 

[iswimmer]

I got curious so just tried this. Resetting home position made no difference in altitude. Compass calibration didn't either. Only turning it on and off would reset the altitude back to zero.

I was reluctant to think it would...thanks for experimenting though. What do you mean by "turning it on and off" was the only way to reset it back to zero?

When I turned the phantom battery off and back on the altitude would go back to zero. After things equilibrated - the temperature I assume - it held constant altitude.

 

[Sasquatch]

When I turned the phantom battery off and back on the altitude would go back to zero.

This is what I thought you meant. However, my bird has been turned off and on numerous times and the height telemetry is consistently off about 15-20' at the start of each flight and even before take off.

I realize that the science isn't perfect on a consumer based product like this. The main reason I asked this question was to try to understand why this was happening AND if this discrepancy in height telemetry could be a cause for concern when flying. It does bug me that it's off, but I guess I won't worry about it too much anymore.


Thanks

 

[bizrank]

The barometer isn't accurate enough to be sure of where the ground is and neither is GPS altitude.
GPS altitude accuracy is +/- 15 meters best case. A barometer will vary as the weather changes, temperature, wind, etc, so it can't be trusted not to change during flight.

Cahutch, I was also interested in the effect of barometric changes in the weather on the altitude readout, and this makes sense. But am I reading you correctly--altitude accuracy is at best +/- 15 meters? That means that the -20 or -30 or -40 feet reading I get sometimes at startup (other times it's near the mark, within a foot or two of 0) is within the tolerances. Did you mean to have a decimal point in there? If there's a 15 meter tolerance, what does that mean for the RTH height of 20 meters? How do you think that could that be specified by DJI with such accuracy? Is the app readout wrong and not just somewhat erratic but HIGHLY erratic, and therefore superfluous?

 

[rrmccabe]

This thread had jumped around a little for me and doesn't exactly follow how I thought the Phantom worked. Of course pressure changes with weather but not usually enough during the 15 or 20 minute flight so the pressure sensor is just for short term changes.

I know the height is set at sometime before home lock too. Resetting home lock does not change the elevation.

It was my understanding that it should zero sometime after power on and the difference in barometric pressure set the elevation.

I also thought there should be no error at ground "zero" because that is where it zeros from and any error would grow as you gained elevation and possibly as things warmed up and might show different when you set it back down.

Is this not the case?