I have searched the forum and read an interesting post about height of flight. That answered some of my questions but I have a few more.
I do not have a drone yet but am considering the DJI mini 2 SE (I have quite a few unrelated questions but will make a different thread for each one unless advised otherwise).
I see that a drone cannot have any knowledge of how high it is from the ground beneath it which is a shame as that is the height that must not exceed 120m. But it does know what it’s height is compared to it’s take off point. I presume this height is displayed on the screen of the mobile app attached to the controller?
How does it measure the height at take off and at the present time? Presumably with some device inside it? What is this measuring to determine height - air pressure? I don’t /need/ to know the answer to this - it is just idle curiosity.
Thank you for reading and answering this newbie question.
There are LIDAR sensors on the bottom which tell the height at low height (< 10m or so), which help with hitting ground and landings. Over that I believe it’s simply GPS, esp when up at the 100m!
There are two black round things on the bottom that are working as LIDAR sensors and I think are used when landing. I think one projects an infrared pattern and the other detects it.
It has also a barometric pressure sensor, which measures air pressure to estimate altitude. The principle is straightforward: as the drone ascends, the air pressure decreases, which the drone interprets as an increase in altitude. This takes a pressure reading at the takeoff point and then determines the relative altitude from there.
I wouldn’t be surprised if there some DJI magic going on, using data from IMU and GPS too. But barometric pressure is the primary method.
This - though GPS absolute height is recorded in EXIF data if you take a pic, it is very inaccurate. I’ve seen it where it thinks absolute height at takeoff is 50m below sea level ( I was actually 7m above sea level )
Almost all drones (and aircraft in general) use barometric pressure as their main altimeter, with other systems (eg optical or radar) used when nearer the ground for more accuracy when landing.
So when you turn the drone on, it makes a note of the air pressure at that point. Then as it climbs the pressure reduces and it can derive its altitude to within a couple of metres accuracy. That’s good enough.
You’re correct in your statement that a drone never truly knows its height above ground, only above (or below) where it took off from. So if you’re in a hilly area you may need to adjust your RTH height for example to make allowances. And yes, you might have to adjust the limit to allow you to fly to 120m altitude - though it has to be said, most of the time I’ve never really needed my Mini 1 or 3 Pro much higher than 60m. Above 100m the wind speed can often be challenging for the Minis
It’s a MEMS device. They are tiny! A diaphragm on an IC that acts as a capacitor, and its capacitive properties change as atmospheric pressure changes.
The spec sheet of one I found claims it can measure the pressure change writhing. 9cm column of air.
They’re small enough, Apple Watch uses one to count how many flights of stairs climbed, or height climbed when hiking.
Some used for paragliding, no readout just an audible feedback for going up/down, were minute 25 years ago.
I very much doubt the ones in drones are calibrated to provide millibars. The logs don’t record “pressure”, so chances are it’s only accurate for pressure change.
You know some phones/watches particularly sports tracker watches have barometers?
Drones also have 3 axis gyros … a bit smaller than those we had as kids.
I must confess that my original drone (Phantom 2) was bought as much (perhaps a geeky bit more than “as much”) to marvel at its ability to hover in one place without any pilot input as it was to take pics/video with my GoPro.
I still marvel at it - and it’s still a significant part of the pleasure of any flight.
The zero pilot input to hover is still something that I believe many new to the hobby often don’t fully appreciate before their first flight.
I did manage to fly my Phantom 2 indoors - and that’s a bit large with strong props!
It was in the early days of this house renovation when the room was totally empty with nothing to break or damage … other than the Phantom.
The only bit that was tricky was when it reached the ceiling and “stuck to it” … and I knew that reducing the power it would suddenly “un-stick” without enough power to recovery before it hit the floor. So I grabbed it.
Heh. I have a habit of testing all my quads indoors for their “maiden” shakedown test… Angle mode of course, the 7 inch was scary… touch wood not hit anything (yet) and always checked the prop direction on right!..
You forget the down draft and all that dust and crap from under the sofas fly around. Actually when hovering that FPV the other day, my small corsair warbird plane went flying across the room, as I did fly it around the lounge hah! Love that you can fly the DJI FPV the same as mini, etc or in manual! Makes landing simples…
