Altitude

john.a.quist · Jul 24, 2023

I just started practicing with my new-to-me P4P v1.0. I noticed that when hovering the altitude drops and yet the altitude on the screen stays at the same numerical height. I looked through the manual and searched within the forum. Am I doing something wrong? Thanks.

Meta4 · Jul 24, 2023

john.a.quist said:
I just started practicing with my new-to-me P4P v1.0. I noticed that when hovering the altitude drops and yet the altitude on the screen stays at the same numerical height. I looked through the manual and searched within the forum. Am I doing something wrong? Thanks.

Can you give a little more information do describe what you are seeing?
The altitude shown on the screen comes form a sensor that measures air pressure.
It's quite sensitive and should show small (less than one foot) changes in height.

john.a.quist · Jul 24, 2023

Meta4 said:
Can you give a little more information do describe what you are seeing?
The altitude shown on the screen comes form a sensor that measures air pressure.
It's quite sensitive and should show small (less than one foot) changes in height.

Thanks for responding. I set the height for say 4 meters. When I either hover or practice maneuvers slowly, the actual height of the drone drops 25% or more. When I check the screen to see what the height is reading, it still says 4 meters. In GPS mode, 32 C, 29.90Hg, 28% humidity, clear skies.

Pygar70 · Jul 24, 2023

I often find that my launch and landing elevations differ - sometimes by as much as 2-3 metres. This is evidence that altimeter 'drift' does occur. I've always assumed it was gradual, but it can be hard to tell over the course of a 15 min+ flight.

Martin Healy · Jul 25, 2023

john.a.quist said:
I just started practicing with my new-to-me P4P v1.0. I noticed that when hovering the altitude drops and yet the altitude on the screen stays at the same numerical height. I looked through the manual and searched within the forum. Am I doing something wrong? Thanks.

Hi John,
I have made a lot of repetitive low level flights (2-4M agl) to monitor wheat and maize growth on a farm that conducts seed trials. My P4 often sank down slowly but I don't recall it flying higher than the programmed height. I thought that this height change could be caused by wind gusts confusing the air pressure sensor in the altimeter. I also assumed that the incompressible ground beneath the P4 effectively amplified the pressure changes caused by wind gusts. It would be interesting to experiment by hovering, in gusty conditions, at say 1,2,3 to 10M and see if the altitude changes only occur close to the ground. But I would be happy to learn more from someone more knowledgeable.
All the best, Martin

john.a.quist · Jul 25, 2023

Martin Healy said:
Hi John,
I have made a lot of repetitive low level flights (2-4M agl) to monitor wheat and maize growth on a farm that conducts seed trials. My P4 often sank down slowly but I don't recall it flying higher than the programmed height. I thought that this height change could be caused by wind gusts confusing the air pressure sensor in the altimeter. I also assumed that the incompressible ground beneath the P4 effectively amplified the pressure changes caused by wind gusts. It would be interesting to experiment by hovering, in gusty conditions, at say 1,2,3 to 10M and see if the altitude changes only occur close to the ground. But I would be happy to learn more from someone more knowledgeable.
All the best, Martin

Thanks. I am wondering if that pressure-based altimeter could use a cleaning. The drone has been in storage unused for years.

Meta4 · Jul 25, 2023

john.a.quist said:
Thanks for responding. I set the height for say 4 meters. When I either hover or practice maneuvers slowly, the actual height of the drone drops 25% or more. When I check the screen to see what the height is reading, it still says 4 meters. In GPS mode, 32 C, 29.90Hg, 28% humidity, clear skies.

A few metres +/- drift over the duration of a flight is common and normal.
That's what you get when the altitude comes from measuring air pressure.
If you are wanting to fly at low level, you need to be able to see the drone and how far it is from the ground, rather than relying on the number on the screen.

Andy9 · Aug 3, 2023

I think that here is an another issue. If you fly less than 10m high the altitude is measured by the UZ sensor which measures the distance from the ground, thus the measured hight is the same all the time but the drone follows the UZ values so if the ground goes down the same will do the drone. Try to fly over 10m and you'll see.

Pygar70 · Aug 3, 2023

Andy9 said:
I think that here is an another issue. If you fly less than 10m high the altitude is measured by the UZ sensor which measures the distance from the ground, thus the measured hight is the same all the time but the drone follows the UZ values so if the ground goes down the same will do the drone. Try to fly over 10m and you'll see.

What you're describing is the Terrain Follow Mode. In normal flight mode the downward vision positioning system maintains horizontal positioning only. If it were otherwise, maintaining stable flight below 10 metres would indeed be a challenge.

Andy9 · Aug 7, 2023

I agree, but I've often experienced such drone behaviour if the terrain is not flat.
So a slight corrections in altitude are sometimes necessary. It happens in normal flight mode, not in 'terrain follow'.

Pygar70 · Aug 21, 2023

I've just been testing a replacement drone that I received from DJI after sending my old one in for repair. It has trouble maintaining elevation - bobbing up and down. I've had P4s before that have done this some of the time, but this one seems worse - or at least more consistently bad.
I recorded the movement - if anyone's interested. There was only the slightest of breezes at the time.

MapMaker53 · Aug 21, 2023

Two suggestions... If you have the downward facing sensors turn on, turn them off. They can be very glitchy when flying over water where the reflective water surface messes up what they are seeing. The sensors will be especially problematic if flying over rapidly moving water as the sensor signal reflection is constantly changing. Have you tested the drone out over land to see if you experience the same issue? Also, the vertical up and down drifting will be much less noticeable if you play the video at normal speed.

BigL · Aug 21, 2023

MapMaker53 said:
Have you tested the drone out over land to see if you experience the same issue?

The answer to this should prove very interesting.

Pygar70 · Aug 21, 2023

MapMaker53 said:
Two suggestions... If you have the downward facing sensors turn on, turn them off. They can be very glitchy when flying over water where the reflective water surface messes up what they are seeing. The sensors will be especially problematic if flying over rapidly moving water as the sensor signal reflection is constantly changing. Have you tested the drone out over land to see if you experience the same issue? Also, the vertical up and down drifting will be much less noticeable if you play the video at normal speed.

Good point about the downward sensors. I should have mentioned that they were turned off - something I always do when flying low over water. Over solid ground the instability was still there. With those sensors turned off, it shouldn't make any difference what surface I'm flying over.
Of course the movement is less noticeable with the video at normal speed!

I sped it up deliberately to highlight the movement. When reviewing all the video the instability was most apparent at hover or slower speeds, and when lower to the ground. I don't recall my other P4 units (& I've had a few) being this bad.

Meta4 · Aug 21, 2023

Pygar70 said:
I've just been testing a replacement drone that I received from DJI after sending my old one in for repair. It has trouble maintaining elevation - bobbing up and down

How much was the drone moving vertically?
According to the specs, it's vertical hovering stability is ±0.5 m.

Pygar70 · Aug 21, 2023

Meta4 said:
How much was the drone moving vertically?
According to the specs, it's vertical hovering stability is ±0.5 m.

I would say occasionally outside that range, but mostly within it.
It's not just the range that's a factor, that drone wouldn't stay still. I've sent up drones before and had them hover right in front of me while I fiddled with settings, and they've barely moved.
Do you know if it just the barometer that controls vertical stability, or does the accelerometer also come in to play?

BigL · Aug 22, 2023

You can always fix the vertical drift in video post.

Meta4 · Aug 22, 2023

Pygar70 said:
I would say occasionally outside that range, but mostly within it.
It's not just the range that's a factor, that drone wouldn't stay still. I've sent up drones before and had them hover right in front of me while I fiddled with settings, and they've barely moved.
Do you know if it just the barometer that controls vertical stability, or does the accelerometer also come in to play?

You could try recalibrating the IMU as that contains both the gyro sensors and the barometric sensor.
There's nothing else that would affect vertical position holding.