Incorrect altitude

[edonovanl]

My altitude reading shows negative 32+ meters -I have calibrated the compass and advanced IMU calibration to get the mods value to about 1400. Only thing I haven't tried is de-gaussing and recalibrate...

Any thoughts on why the altitude won't reset correctly?

 

[N017RW]

The altimeter is based on barometric pressure. There are a number of threads regarding its quality and errors like your seeing. There's no calibration or fix you can do unless it is non-functional. Stabilizing the temperature before flying helps.

 

[edonovanl]

There must be a sensor of some type so maybe it can be replaced....I have researched but will dig deeper....

 

[AmosMoses]

The altimeter is based on barometric pressure. There are a number of threads regarding its quality and errors like your seeing. There's no calibration or fix you can do unless it is non-functional. Stabilizing the temperature before flying helps.

This is 100% correct. What I will do is start the Phantom and let it sit for about 45sec to a minute. Then power cycle just the quad. Sometimes it will take a couple cycles to get a good accurate reading.

 

[jumanoc]

Mine shows +7 mts and other times +15 mts or any rand values (less than 20 mts always). The warning here is when you use WayPoints GS and the FC may "think" that it is at 20 mts and really it is flying at 1 mt. or viceversa Crash on the way
I personally set my waypoints over 25 mts always.

 

[bbfpv]

mine's usually +13-17 ft. I just do the math to know where I am.

 

[N017RW]

There must be a sensor of some type so maybe it can be replaced....I have researched but will dig deeper....

The IMU in the Naza contains the barometer.

http://wiki.dji.com/en/index.php/IMU

I believe it's proprietary and thus unavailable but please let us know if you are able to replace it somehow.

 

[jasonnelson007]

Nice thing about the DJI Ultimate app. It zeros as soon as you take off.

 

[GaryDZ]

Nice thing about the DJI Ultimate app. It zeros as soon as you take off.

Yes, but I think it only zeros (corrects) the display in the app. It does not actually change anything in the Phantom so you would still have to be careful of how the erroneous reading would affect RTH altitude. Please correct me if I am wrong about this.

 

[jasonnelson007]

I'll check tonight.

 

[cahutch]

The altimeter is not meant show actual altitude to that accuracy. It's really only there to sense changes in altitude and was never meant to be accurate down to the foot.
As long as it goes up and down as the phantom does, it's working.

All barometric altimeters are this way. They are affected by temperature and they are never accurate down to the foot or even tens of feet. They are only accurate to hundreds of feet.
Even in real airplanes, the pilot has to calibrate the altimeter before takeoff. The phantom has no way to do this because it doesn't matter.

Just stop worrying about it and it will be fine.
If it really bugs you, let the Phantom warm up on the pad, and then cycle power so the altimeter re-sets to zero at the warmed up temp.
Know this though, as soon as you take off, the Phantom will cool off and the altitude reading will be off by a few meters again.
If you leave it be, when it cools off after takeoff it'll probably be more accurate.

 

[BlackTracer]

Nice thing about the DJI Ultimate app. It zeros as soon as you take off.

Yes, but I think it only zeros (corrects) the display in the app. It does not actually change anything in the Phantom so you would still have to be careful of how the erroneous reading would affect RTH altitude. Please correct me if I am wrong about this.

Zeroing the display and not actually zeroing the barometer would be a useless and dangerous bug/feature. In fact I would not want that for the reasons you mention. I certainly hope it doesn't do that.

 

[BlackTracer]

Nice thing about the DJI Ultimate app. It zeros as soon as you take off.

Yes, but I think it only zeros (corrects) the display in the app. It does not actually change anything in the Phantom so you would still have to be careful of how the erroneous reading would affect RTH altitude. Please correct me if I am wrong about this.

Zeroing the display and not actually zeroing the barometer would be a useless and dangerous bug/feature. In fact I would not want that for the reasons you mention. I certainly hope it doesn't do that.

 

[cahutch]

Zeroing the display and not actually zeroing the barometer would be a useless and dangerous bug/feature. In fact I would not want that for the reasons you mention. I certainly hope it doesn't do that.

How would it be dangerous?
The altitude reading isn't used for landing. All that matters during landing is that the altitude is changing, not what the actual attitude is. When it stops changing, it's on the ground.
As for RTH altitude, just know that the altimeter is not accurate down to tens of feet and add some safety margin to your RTH altitude.

Also, see above. The altimeter is sensitive to temperature. Temperature is not a constant.
If you re-set the altitude on the ground after the Phantom has warmed up, once you take off it will cool down again and your altitude will be off again.
The best thing to do is leave it be, and when it cools off again after takeoff it will be more accurate.

But again, accuracy down to tens of feet (or meters) is not possible and not necessary in this application. Let it go and you'll be a much happier flyer.

 

[BlackTracer]

Zeroing the display and not actually zeroing the barometer would be a useless and dangerous bug/feature. In fact I would not want that for the reasons you mention. I certainly hope it doesn't do that.

How would it be dangerous?
The altitude reading isn't used for landing. All that matters during landing is that the altitude is changing, not what the actual attitude is. When it stops changing, it's on the ground.
As for RTH altitude, just know that the altimeter is not accurate down to tens of feet and add some safety margin to your RTH altitude.

Also, see above. The altimeter is sensitive to temperature. Temperature is not a constant.
If you re-set the altitude on the ground after the Phantom has warmed up, once you take off it will cool down again and your altitude will be off again.
The best thing to do is leave it be, and when it cools off again after takeoff it will be more accurate.

But again, accuracy down to tens of feet (or meters) is not possible and not necessary in this application. Let it go and you'll be a much happier flyer.

You answered yourself why it is dangerous. Not necessarily for you or me but for others who may not be so cognizant of how the altimeter is used for RTH. Imagine someone putting in a RTH height of 100 feet when their altimeter is off by -100 feet. Is it really that hard to see the danger of crashing? I am not having angst over this. My altimeter is off by less than 10 ft after it is warmed up.

 

[cahutch]

It shouldn't be off by 100 feet. I've never seen it drift that far out. 10-20 feet maybe or 5-10 meters.
But I suppose it could be that far off if it was siting in direct sunlight and got really really hot.
It will cool off after takeoff though. I would check the altitude after motor start with the props idling. With the props spinning the temp should be cooler and closer to operating temp.

Also note, Unless I'm mistaken, + temp = + altitude, - temp = - altitude because cooler air is denser.
In winter climates, if you bring your Phantom out of a warm house or car, it can cool down significantly after turning it on causing the altitude reading to go negative and It may cool down even more after takeoff. This would give you extra height margin during RTH since the phantom would think it's lower than it is.

I have my RTH altitude set to 100 meters (328ft) because I have 60 meter trees near my house. If it was off by +40 meters (130ft) I might end up hitting a tree during RTH.

DJI probably should update the firmware to reset the altimeter to zero at takeoff. It should be a simple thing to do.

 

[N017RW]

The way I always remembered the effects of temperature change on a baro altimeter enroute was:

When the temperature lowers enroute, you are lower than the altimeter indicates.
(and vice verse)

 

[bbfpv]

The way I always remembered the effects of temperature change on a baro altimeter in-route was:

When the temperature lowers in-route, you are lower than the altimeter indicates.
(and vice verse)

Which makes sense... when I take the bird out of my warm car and fly it in 20 degree weather, it reports its alt as 17ft when I'm hovering 3 ft off the ground out in front of me. Thanks for that tip, easy to remember!

 

[cahutch]

The way I always remembered the effects of temperature change on a baro altimeter enroute was:

When the temperature lowers enroute, you are lower than the altimeter indicates.
(and vice verse)

That's backwards from what I thought. Cold temps cause the altimeter to read higher and warm temps cause it to read lower?
So sitting on the ground warming up the altitude reads negative?

 

[N017RW]

Remember this is relative to MSL not terrain!

If you fly into an area or altitude that is warmer than you left (assuming no thermal compensation), thus the air is less dense, you'll read higher than you actually are.