Altitude zones: How are they enforced by the drone?

[Droning on and on...]

My understanding is that altitude zones are MSL, not AGL. Lacking an altimeter and only having relative height above takeoff, how does the drone know you are approaching a ceiling?

I suppose it could be using the un-corrected barometric pressure, but the error range on this with weather pressure changes is huge.

Anyone know?

 

[anotherlab]

It uses barometric pressure sensors to measure altitude.

 

[AnzacJack]

It is the PIC responsibility to be aware of the position of his drone. The drone will only give you its height relative to takeoff point

 

[Kilrah]

My understanding is that altitude zones are MSL, not AGL.

Those that matter for drones are AGL.

 

[AeroJ]

Has anyone yet come up with a reasonable answer for how we are supposed to real-time calculate the variation in ground level under the craft whilst in flight ?

You know, like if we launch from the top of a huge hill, rise to 120m above it, and then fly away from the hill, and across a valley to (let's say) another hill on the other side of it. Sure, we are required to drop level as we cross the valley, but how the hell are we meant to judge how much and when ?

 

[twickers14]

Has anyone yet come up with a reasonable answer for how we are supposed to real-time calculate the variation in ground level under the craft whilst in flight ?

You know, like if we launch from the top of a huge hill, rise to 120m above it, and then fly away from the hill, and across a valley to (let's say) another hill on the other side of it. Sure, we are required to drop level as we cross the valley, but how the hell are we meant to judge how much and when ?

This is a regular Forum topic and there will be folk here who will show you how it works.

 

[Vic Moss]

Has anyone yet come up with a reasonable answer for how we are supposed to real-time calculate the variation in ground level under the craft whilst in flight ?

You know, like if we launch from the top of a huge hill, rise to 120m above it, and then fly away from the hill, and across a valley to (let's say) another hill on the other side of it. Sure, we are required to drop level as we cross the valley, but how the hell are we meant to judge how much and when ?

You just need to "do the best you can". It's not an exact science.

 

[Kilrah]

You'd look up the heights on e.g. Google earth during your flight preparation.

 

[Galopin]

When you start your drone and your RC, the home point is being updated by the GPS for Lat. Long. and the altitude above the ground (AGL) by the barometer of the drone. But like an airplane, the barometer works with the actual atmospheric pressure indicating an altitude that you have to adjust at take off on the altitude of the airport. With a drone, your RC indicates 0 meter. Then during your flight, if the pressure changes, the altimeter will work at the inverse of the barometric pressure. If the atmospheric pressure is increasing during your flight, the altimeter will decrease. Once, on a 22 minutes flight with my Mini 3 Pro, I took off at 0 meter and when I came back to land, my RC altimeter was indicating minus 36 meters. The reason: a catabatic cold air flow had come down the mountain side during my flight increasing the atmospheric pressure of the barometer and decreasing my altitimeter.

 

[Droning on and on...]

Those that matter for drones are AGL.

Also irrelevant to the question asked. I'm thinking I didn't word it well.

In an altitude zone, particularly near an airport, it is possible to have your drone hit a ceiling and refuse to ascend further.

This has caused problems for some during RTH, causing the drone to stop short of its configured RTH homepoint-relative altitude, and hover in place until critical battery forces a landing, rather than flying home at the ceiling altitude.

These altitude limits, as defined by the FAA, are MSL. How is the drone detecting this altitude?

 

[AeroJ]

You'd look up the heights on e.g. Google earth during your flight preparation.

Yeah but even if we know the height of the valley and the 2 hills that doesn't help us with how to move between them.,

You just need to "do the best you can". It's not an exact science.

That seems achievable at least ! I will continue to vaguely guess !

 

[Droning on and on...]

It uses barometric pressure sensors to measure altitude.

Thank you, the only responsive answer so far.

Do you know this from some sort of independent, authoritative source from DJI, or is this educated speculation on your part?

AFAIK, there are only two sources of altitude information available in the drone: The uncalibrated barometric altimeter, which can be off more than 1000ft, is obviously useless for absolute altitude measurements.

GPS typically has a vertical error about 2x that of horizontal accuracy. While there are occasional transitory bad satellite configurations that result in larger than usual errors, the vast majority of the time GPS horizontal accuracy is within 15ft, putting altitude accuracy within the range of 30ft.

FAA requires barometric altimeters set at the airport to the current baro pressure to be accurate to 75ft or less.

Conclusion: The drone is using GPS altitude to monitor and enforce altitude restrictions in the FlySafe database.

So, I've answered my own question, but with speculation. I'd like to know the facts, if anyone does.

 

[Droning on and on...]

Please reread the OP and see if you have anything directly to add.

BTW, it's the driver's responsibility to follow speed limits on public roads. I decided you needed to know that.

 

[anotherlab]

Thank you, the only responsive answer so far.

Do you know this from some sort of independent, authoritative source from DJI, or is this educated speculation on your part?

AFAIK, there are only two sources of altitude information available in the drone: The uncalibrated barometric altimeter, which can be off more than 1000ft, is obviously useless.

GPS typically has a vertical error about 2x that of horizontal accuracy. While there are occasional transitory bad satellite configurations that result in larger than usual errors, the vast majority of the time GPS horizontal accuracy is within 15ft, putting altitude accuracy within the range of 30ft.

FAA requires barometric altimeters set at the airport to the current baro pressure to be accurate to 75ft or less.

Conclusion: The drone is using GPS altitude to monitor and enforce altitude restrictions in the FlySafe database.

Just a quick search on the Internet on the phrase "how do mavic drones measure altitude" will come up with various version of how the barometric sensor is used to measure altitude. You wont find any reference that states that the drones are using GPS for altitude.

"Huge" changes in barometric pressure would occur during a storm and you probably wouldn't be flying into a storm.

The barametric sensor resets to 0ft when the drone takes off. That becomes the reference height when you start off.

 

[Droning on and on...]

Just a quick search on the Internet on the phrase "how do mavic drones measure altitude" will come up with various version of how the barometric sensor is used to measure altitude. You wont find any reference that states that the drones are using GPS for altitude.

"Huge" changes in barometric pressure would occur during a storm and you probably wouldn't be flying into a storm.

The barametric sensor resets to 0ft when the drone takes off. That becomes the reference height when you start off.

Do you understand what altitude zones are, and that somehow DJI drones all by themselves are able to fairly accurately detect when you are going to exceed this regulatory altitude and stop the drone?

An uncalibrated barometric altimeter is simply not capable of this level of accuracy. It is impossible that this is how the FlySafe altitude restrictions are being automatically enforced by the drone itself. Otherwise, a pilot could easily climb hundreds of feet into an airport runway approach, even 1000+ft on a nice warm clear high pressure sunny day.

Yet, DJI drones do respect these ceilings, and with quite good accuracy. How, when according to you the only altitude information they have is height over takeoff?

BTW, surely you don't think that if it can't be googled, it doesn't exist. Despite what you've failed to find googling, DJI drones do capture and record GPS altitude, it's there in the logs. Don't know if it's used for anything, but I strongly suspect this is the source for enforcing FlySafe altitude restrictions, just as GPS horizontal position is used to enforce restricted regions in FlySafe.

 

[Droning on and on...]

"Huge" changes in barometric pressure would occur during a storm and you probably wouldn't be flying into a storm.

Huge changes in barometric pressure occur over days between having a low pressure center over your area, and days later a high pressure cell move in. This can easily result in 1000' difference in measured "altitude" one day to the next, without storms.

 

[Droning on and on...]

Leave the 500m and 1000m DJI "altitude zones" out of the discussion... these are not regulatory, but rather arbitrary limits DJI has created, and those are clearly AGL and enforced by the system based on homepoint-relative height.

I'm interested in the restrictions in FlySafe, that ultimately come from the FAA in the US, are MSL, and somehow pretty accurately enforced by the drone.

How?

 

[Kilrah]

Likely GPS alt since that's what would technically make sense... but since it's not documented nobody can tell you.

 

[anotherlab]

Huge changes in barometric pressure occur over days between having a low pressure center over your area, and days later a high pressure cell move in. This can easily result in 1000' difference in measured "altitude" one day to the next, without storms.

While you are flying for 20 to 40 minutes, you are not going to see a 1000' difference in measured altitude due to changes in barometric pressure.

 

[anotherlab]

Likely GPS alt since that's what would technically make sense... but since it's not documented nobody can tell you.

It would make less sense to use GPS. GPS vertical accuracy has always been much worse than horizontal accuracy.

If you lost the GPS lock, you would lose that information.

Just ask on the DJI forums and an actual DJI team member will tell you that their drones use a barometric sensor to measure altitude.

This response for example:

....With regards to this matter the DJI Mavic Mini's altitude measured using a barometer device on the drone. The barometer is zeroed at launch, so the display simply shows the drone's altitude above the point at which you launched. In addition, the altitude is calculated by an on board barometer that detects pressure and measures the difference from when it was launched to where it is now....