Altitude

BenDover · Dec 15, 2015

Hi. I've been flying my p3p every day now since I got it last week and I am noticing that the altitude is not very accurate. Is this normal? I take off and as I ascend it seems to be ok but I move down range, ascend and descend and when I bring the drone back and have it hovering in front of me about 10 feet high, my altitude is reading like 39 feet.

Now if I
Have it in my sight, I am visually flying not instrument flying, but I have been testing limits and if i fly it out 2000 feet or more at an altitude of 150 feet and then lower down to below 100 feet to "look" around and the drone is not in my field of view, a difference of 30 to 40 feet vertically could be disastrous.

Thoughts? Opinions? Advice?

Goggles Pisano · Dec 15, 2015

Try IMU and compass calibration, maybe try shutting VPS also.

Goggles

ParsnipHysorter · Dec 15, 2015

BenDover said:
Hi. I've been flying my p3p every day now since I got it last week and I am noticing that the altitude is not very accurate. Is this normal? I take off and as I ascend it seems to be ok but I move down range, ascend and descend and when I bring the drone back and have it hovering in front of me about 10 feet high, my altitude is reading like 39 feet.

Now if I
Have it in my sight, I am visually flying not instrument flying, but I have been testing limits and if i fly it out 2000 feet or more at an altitude of 150 feet and then lower down to below 100 feet to "look" around and the drone is not in my field of view, a difference of 30 to 40 feet vertically could be disastrous.

Thoughts? Opinions? Advice?

This doesn't seem unusual. When landing back at the home point I often get an altitude of 25-30 feet (when it should be zero). I believe this has been discussed in other threads and is caused by the temperature changes in the barometric pressure sensor over the course of the flight. If you are really concerned about it you can hover for a couple minutes to let things warm up and reach equilibrium, then land so it resets the altitude, then proceed with the flight. But there is still going to be drift in the altitude measurement over the course of the flight as the temperature will keep changing.

BenDover · Dec 15, 2015

Not sure about temperature changing. I live in Miami Florida and it is constantly hot. Today for example it is 82° and I'm in rural farmland ( Redlands). Flat as it comes.

eBird · Dec 15, 2015

If plus or minus 30 to 40 feet is the generally accepted altitude accuracy of the P3 then we all need to be sure to take this into account when setting RTH altitude. If you're thinking the highest trees are 60 feet and set your RTH at 100, you really should set for 140.

Meta4 · Dec 15, 2015

BenDover said:
Not sure about temperature changing. I live in Miami Florida and it is constantly hot. Today for example it is 82° and I'm in rural farmland ( Redlands). Flat as it comes.

That was referring to the temperature of the inside of your Phantom where the sensor is.

Meta4 · Dec 15, 2015

Goggles Pisano said:
Try IMU and compass calibration

Compass ?? There's no connection between the compass and altitude sensor..

Fat City · Dec 16, 2015

From a quick look at the specs of a typical barometric pressure sensor, temperature clearly has a big influence on the "raw" reading, but I think most of this include a temperature sensor to largely compensate for that effect. If anybody has identified the chip used by the P3 , it should be easy to find the expected temperature dependence. I think in some forum someone said it was 1 cm/sec for the P3.
Even if the sensor has zero drift itself, atmospheric pressure is always changing and this could also contribute to the error. The bottom line is that longer your bird has been flying, the greater the altitude safety margin you should allow.

phantom13flyer · Dec 16, 2015

Pretty normal

BenDover · Dec 16, 2015

I agree Fat City. And my concern was is this normal? Apparently it is so I will compensate and be cautious accordingly.
I am mastering looking down while descending then leveling out my gimbal on the horizontal, doing a 360° and seeing where I am height wise in relation to nearby objects. And I am going to take this margin of error into account for fail safe return.
I know this type of flying is unorthodox for photography, true but it is not just a platform for a camera, this this is fun as **** to fly. I have a boat, a Harley and this beats them all to hell right now in fun factor. Loving my P3P

WetDog · Dec 16, 2015

Definitely give your self a nice margin on RTH altitude - it is extremely hard to judge the height of a tree from the ground. Best way is to fly to the vicinity and look at your altitude and give yourself a comfortable margin while realizing that DJI secretly believes in the concept of a flat earth. It is absolute altitude from your starting point, not relative altitude over terrain.

The aircraft people euphemistically refer to this as 'controlled descent into ground' - you don't want to to that.

Wibble · Dec 16, 2015

The ONLY accurate measure of height would be a radar or laser altimeter. GPS isn't very accurate for height and a barometric altimeter needs constant recalibration due to changing weather.
I a hoping they will soon start using some sort of radar altimeter - we could have ground following capability then!

kphantom · Dec 17, 2015

Interesting posts regarding usage of the barometer sensor (only) for P3 height measurement. Why wouldn't GPS/Glonass be used in GPS-P mode ?

Meta4 · Dec 17, 2015

kphantom said:
Interesting posts regarding usage of the barometer sensor (only) for P3 height measurement. Why wouldn't GPS/Glonass be used in GPS-P mode ?

As Wibble said above ..GPS isn't very accurate for height.
He's correct but GPS accuracy for altitude is much worse than not very accurate.
Despite giving pretty good accuracy in the horizontal plane - it's woefully inaccurate for altitude with errors of 100-200 ft commonly observed.

John Locke · Dec 17, 2015

lenmcgilvary · Dec 17, 2015

Does your phone or tablet have a barometer, I read this causes inaccurate altitude readings if it's not there

Noodle · Dec 17, 2015

Wibble said:
The ONLY accurate measure of height would be a radar or laser altimeter.

Hmm I am wondering how big this radar module will be and how much battery juice is it going to suck when in usage?
Personally I have noticed a few feet change in altitude in my P3A and I am suspecting it is quite normal. 25-30 feet change will be a bit too much for my comfort.

Wibble · Dec 17, 2015

Noodle said:
Hmm I am wondering how big this radar module will be and how much battery juice is it going to suck when in usage?
Personally I have noticed a few feet change in altitude in my P3A and I am suspecting it is quite normal. 25-30 feet change will be a bit too much for my comfort.

It need be no bigger than the ultrasonic module already attached. I don't know why they don't use that for altitude reading it works in a similar way but range would be very restricted.

kphantom · Dec 17, 2015

I think a laser range sensor would be much more accurate.

kphantom · Dec 17, 2015

Meta4 said:
As Wibble said above ..GPS isn't very accurate for height.
He's correct but GPS accuracy for altitude is much worse than not very accurate.
Despite giving pretty good accuracy in the horizontal plane - it's woefully inaccurate for altitude with errors of 100-200 ft commonly observed.

I think it depends how it is used. With 15+ sats, there has to be a good least squares 3D centroid where the Phantom resides. The processing capability to correlate the position might be lacking, I would think that is a possibility. There are a lot of methods to calculate and improve the accuracy. Laser range finding might be used to get better elevation instead of a barometer. Modern manned commercial aircraft use ADS-B, based on GPS. So do JDAM smart bombs and Tomahawk missiles. Those are very accurate in X, Y, and Z 3D coordinates and based upon GPS.