Consistent False Altitude Readings

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I don't know if anybody else is been having this problem but it seems that when I fly up to 400 feet or a little more and then come back down to about 10 feet off the ground whether it's in front of me or down the road within sight, my screen shows that I'm between 20 to 30 feet above what I am actually at . I have noticed this on practically every flight of high-altitude when returning home and it makes me question... if I ever flying to different locations after a high-altitude flight because if I'm not looking at my screen to get an idea of how high I am when you coming near the ground ....I'll probably crash this thing! Any help or suggestions would be greatly appreciated.
 
The barometric altimeter was designed to maintain a constant, stable height by detecting small changes in static pressure. It's not calibrated or compensated and fluctuations in the reported height are common. Even manned aircraft have to deal with similar effects. That's why pilots are trained to 'fly the aircraft' all the way to the ground.
 
Yep I think the Richard explained it pretty good I think a lot of people have the same thing going on what their Phantoms it's not going to depict height exact. All I ask is one traveling that it maintains height. and it does
 
If it was just using gps it wouldnt be know where as accurate. Granted the GPS is quite good in this quadcopter it also uses glonass but as far as barometer I think it also uses that when Landing also
 
GPS altitude can be off 10's of meters and it is reported as height above mean sea level (msl). Phantoms report height above the home or takeoff point (not the distance to the ground directly below the bird). For that they use a pressure sensitive component. My guess would be a pressure variable resister. A simple bridge circuit could be incorporated and the controller would balance or zero the bridge as part of the initialization. After that, voltage out of the bridge would change as changing height varied the resistance. Simple look up type table or equation would transform the measured voltage to height. All of the components in such a circuit would have tolerances and fluxuate some with temp, humidity, etc. giving different readings between birds and over time in the same bird. When landing, the bird may know when it is getting close, but waits until it senses that the bird is no longer decending before the motors are turned off. Same as when you hand catch it.
 
Pilots I believe recalibrate their altimeter by contacting tower for ground barometric pressure and ground sea level.
GPS altitude based on sea level if accurate would still be useful, just take sea level altitude at takeoff and report the difference.


Sent from my HTC 10 using PhantomPilots mobile app
 
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Phantoms report height above the home or takeoff point (not the distance to the ground directly below the bird). For that they use a pressure sensitive component. My guess would be a pressure variable resister. A simple bridge circuit could be incorporated and the controller would balance or zero the bridge as part of the initialization. After that, voltage out of the bridge would change as changing height varied the resistance. Simple look up type table or equation would transform the measured voltage to height. All of the components in such a circuit would have tolerances and fluxuate some with temp, humidity, etc. giving different readings between birds and over time in the same bird. When landing, the bird may know when it is getting close, but waits until it senses that the bird is no longer decending before the motors are turned off. Same as when you hand catch it.

Wouldn't it just be simpler to use GPS to record the take-off height in MSL and convert that to AGL? I'm just surprised because this is the first time I've heard of a barometer altimeter installed. You would have to update the pressure every time you take off.


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As mentioned earlier in this thread, altitude via GPS is highly inaccurate. Altitude by barometer is much more accurate. On takeoff, AC takes current barometer reading as ground zero and calculates from there. You're only flying a max of 20-30 minutes where ambient barometric pressure shouldn't change that much and if it were, that usually means a drastic weather change that you shouldn't be flying in anyway.
If you enable captions for video recording and play your video on a player capable of including a separate caption file, it will show you the X,Y,Z GPS coordinates and the barometer reading relative to launch. You'll note the Z GPS reading is useless.

Sent from my HTC 10 using PhantomPilots mobile app
 
Wouldn't it just be simpler to use GPS to record the take-off height in MSL and convert that to AGL? I'm just surprised because this is the first time I've heard of a barometer altimeter installed. You would have to update the pressure every time you take off.


Sent from my iPhone using PhantomPilots
Which is exactly what the circuit like I described would be doing everytime you started up.
 
Your VPS (Visual Positioning System) altitude reading will give you a more accurate altitude reading when you're below 50ft or so. It uses a combination of optical and sonar data to give you a more accurate altitude reading when flying near ground or obstacles. If VPS is giving you bad data, try calibrating your VPS sensors.
 
If you're curious about the barometer readings, turn on caption on your video settings. That will generate a SRT file accompanying the video file. You'll need a video player that supports using a caption file.
Instead of altitude, barometer reading is reported in the caption. At least that's how it is on the P3.
The caption file is plain text so you can read it with wordpad or similar.
 

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