RTH Question ?

[old man mavic]

@Droning on and on... the manual states max range 14m so you are correct ,of course this is dependant on the surface below and available light

 

[MS Coast]

Maybe you need to read the excerpt from the manual again. It mentions altitude twice, and how it adjusts.

RTH altitude setting once and altitude once. But that's not the point.

It "adjusts the altitude according to environmental factors, such as obstacles and transmission signals." So, if there's poor signal strength, it would climb. And if it detected an obstacle in its flight path, it would fly around or over the obstacle. But there's no way for the drone to sense altitude above ground.

As @Droning on and on... points out, when it descends low enough (30 or 40? feet) it can use the IR sensors to determine altitude above. How far ahead of a moving drone do those sensors cover? Is that adequate to allow the drone in RTH mode flying at 20 mph or so to detect obstacles below and ahead of and alter it's speed and altitude to clear them?

"if bypass is active and optimal is selected as the RTH feature does indeed return following the contours of the ground beneath itself,"

I just can't see how that's possible, especially when that ground is more than 45 feet below and has tall trees with bare branches and buildings on it.

 

[Photo Beaty]

I was flying a DJI drone (not sure which one) several years ago, I flew from a high point on a mountain but there were still higher peaks near by. I flew below me in a valley following the contours of the valley. When I hit return home the drone went up to the height I had set and headed straight for home. The problem was that there was a hill still higher between home point and the drone. As a result the drone smashed into some oak brush. I located it using a Marco Polo finder and the only damage was to the propells.

 

[Droning on and on...]

@Droning on and on... the manual states max range 14m so you are correct ,of course this is dependant on the surface below and available light

Actually, in the case of the IR sensors available light is irrelevant... The sensor emits a pulse of IR light, and measures the "time of flight" of the pulse down and back to the IR receiver. Using the speed of light the drone calculates the distance.

This sensing system replaced the ultrasonic system that was on the Phantoms. It's more accurate and has a much narrower, focused aperture than the ultrasonic sensors do.

The "adequate lighting" thing is for the VPS cameras... sufficient light is necessary to discriminate detail in the image so that optical flow algorithms can see movement and hold position.

 

[Ken-Mav-Mtl]

If you apply the manual quoted by MS_Coast, then the drone's RTH behavior when the lighting is insufficient and the vision sensors are unable to see obstacles, would be to ascend straight up to the set RTH altitude before moving to the Home Point?
It seems to me that the only safe way for the drone to return to home (in darkness) would be to ascend first to RTH altitude before moving to the home point.

 

[Droning on and on...]

If you apply the manual quoted by MS_Coast, then the drone's RTH behavior when the lighting is insufficient and the vision sensors are unable to see obstacles, would be to ascend straight up to the set RTH altitude before moving to the Home Point?
It seems to me that the only safe way for the drone to return to home (in darkness) would be to ascend first to RTH altitude before moving to the home point.

Yes. Even people stumble into trees walking around in the forest in pitch black dark.

 

[Droning on and on...]

As @Droning on and on... points out, when it descends low enough (30 or 40? feet) it can use the IR sensors to determine altitude above. How far ahead of a moving drone do those sensors cover? Is that adequate to allow the drone in RTH mode flying at 20 mph or so to detect obstacles below and ahead of and alter it's speed and altitude to clear them?

Turns out the forward OA sensors do that job, and do it pretty well. Based on the front camera images, with the exception of fine, hard to discriminate objects like bare tree twig/branches in the winter, OA can handle ground obstacles the drone is approaching.

This actually has allowed the engineers to greatly narrow the aperture of the downward AGL sensors, which is highly desirable. The IR ranging sensors give a much more accurate measure of the distance straight down from the drone, and aren't defeated as easily by a "soft" variable surface, like bushes.

Skip if not interested in technical explanation

The size of something that can be "seen" by some reflection based detector is related to the wavelength of the incident signal. Sound waves compared to light have a very long wavelength, so the size of the "spot" the transmitter is shining on is comparatively very large.

An ultrasonic detector is "illuminating" a large patch below the drone, say 15ft dia for sake of argument. This is due to the long wavelength of the transmission, as well as the nature of the medium – air – which disperses sound energy in every direction, smearing out the signal.

By contrast, light isn't the oscillation of some medium (sorry cave-dwellers, there is no "ether" ), and stays tightly confined in a beam, especially in a vacuum. This is because it is a stream of uncharged particles, photons, that have momentum, so unless acted upon by a force, will just sail on in the same direction forever if undisturbed. See "laser".

So the smeared out sound pulse, if the character of the surface is compatible, will return more or less an average of the distance to that 15ft dia patch, if it gets a strong enough peak in the return signal at all to calculate any distance. The higher the drone, the bigger the error range on any signal it does get.

Like a laser, a lens on both the transmitter and receiver on the IR range sensor can "paint" a much smaller spot directly under the drone, and similarly "look at" just that smaller area.

As such, the IR system is much much more reliable and accurate, and to about twice the usable AGL.

The next step would be to go to an actual laser, LIDAR, which can detect AGL up to several hundred feet. Small enough low power modules are available on the market. Problem is, they cost as much as the drone

Shame, cause it would be incredibly useful in consumer drones. There just isn't the demand to achieve the economies of scale to make these modules as cheap as GPS hardware.

 

[old man mavic]

to all who posted in this thread, in responce to my posts #13 and 15# ,firstly let me apologise ,the flight i described that day ,was not actually to test the RTH feature ,but purely the OA
i had a senior moment and got confused , i am only human, and although it would have been easy to just delete those posts ,it would have messed up the whole thread ,but moving on,so today it was bright and sunny, which meant i could try the Advanced RTH as DJI call it ,flew out to 1,243 ft distance, and down into a valley to an idicated minus 50.2 ft below the home point after rising 20.4 ft above the launch pad to activate precision landing,took a deep breath and pressed the RTH button on the controller the drone appeared to climb then came back quite fast and upon reaching the home point it was showing 66ft altitude before it decended down for the landing
it was quite frankly rather an anti climax moment ,the only difference i could really say, between that and the basic RTH, was that it did not rise up to my preset RTH height that i had selected in the settings that being 120 ft
so once again sorry for my confusion

 

[MS Coast]

Turns out the forward OA sensors do that job, and do it pretty well.

I agree with what you said. I was thinking about the drone's full sensor set rather than just the downward sensors. I can't see that the drone is capable of terrain following flight. I do understand that it can avoid obstacles in its flight path during RTH operation.

Rather than rely on the drone to use OA to steer around obstacles like think leafless branches and wires, I prefer to use Preset RTH and set the RTH altitude based on my knowledge of the operating area.

I use Preset RTH exclusively. I understand that the initial climb during Preset RTH consumes battery power that might be avoided with Optimal RTH. I've not been convinced that there's enough advantage to the Optimal mode. The risk doesn't seem worth the reward.

Are there other advantages to Optimal RTH mode that suggest that it should be used?

 

[EssenYVR]

I lose sight of the drone and press RTH ! Does it climb vertically 2150 feet hence busting the 400 foot limit or what does it do ?

The only time you're forced to RTH is when you loose connection to the drone. And in that case you're likely to reacquire connection shortly after the drone starts to climb, at which time you can cancel it and then maneuver the drone back toward the slope in order to stay within the 400' AGL requirement.

If you merely loose sight of the drone, you can turn it to face the slope and then manually ascend up the slope back toward the launch point while using the view on the remote to visually keep reasonably close to terrain. If you're unsure of the exact direction to the launch point you can refer to the radar map on the controller.

 

[Yorkshire_Pud]

An ultrasonic detector is "illuminating" a large patch below the drone, say 15ft dia for sake of argument. This is due to the long wavelength of the transmission, as well as the nature of the medium – air – which disperses sound energy in every direction, smearing out the signal.

Why do you refer to sound waves?
Ignoring noise, as produced by motors, props and any attached speakers etc., as far as know there are no sound transmitters or receivers in any DJI drone
I may have seen reference to ultra sonic in one or more dji manuals but assumed that that was a translation error.

By contrast, light isn't the oscillation of some medium (sorry cave-dwellers, there is no "ether" ), and stays tightly confined in a beam, especially in a vacuum. This is because it is a stream of uncharged particles, electrons, that have momentum, so unless acted upon by a force, will just sail on in the same direction forever if undisturbed. See "laser".

With regards to EM radiation, the particles are photons not electrons.

As far as I know, I think it is flawed to imply that every EM source intrinsically produces a beam.
Whilst it is true to say that once photons exist they do tend to move in straight lines,
the production of a beam requires the 'emitter' to constrain the path of emitted photons in some way e.g. through the use of lenses and reflectors etc., otherwise the initial radiation tends to spherical.
Radio emitters may well be directional in their emissions and produce lobes but they need specially designed antennae to produces 'beams'.
I think lasers are something different but they are beyond my understanding.

 

[Droning on and on...]

I agree with what you said. I was thinking about the drone's full sensor set rather than just the downward sensors. I can't see that the drone is capable of terrain following flight. I do understand that it can avoid obstacles in its flight path during RTH operation.

It all depends on what sort of surface it is ranging. I've had my Mini 2 (now sold) terrain-follow up a gentle dirt slope just by flying slowly and let the Landing Protection keep the drone 2ft above the ground. No OA at all.

It's interesting to do this across a curb... It will "terrain follow", popping up the height of the curb as it goes over it. It doesn't go down, however, passing the other way.

And it has to be as low as it will let you get without initiating the Landing process.

 

[Droning on and on...]

Why do you refer to sound waves?

Uhhh... because I'm talking about sound?

Ignoring noise, as produced by motors, props and any attached speakers etc., as far as know there are no sound transmitters or receivers in any DJI drone

Okay. What do you think those large round things are on the bottom of the Phantom 4 between the two VPS cameras?

I eagerly await your answer

 

[Droning on and on...]

With regards to EM radiation, the particles are photons not electrons.

Quite right, and man am I embarrassed about that error.

I have a Bachelor's in physics (and EECS).

 

[Droning on and on...]

As far as I know, I think it is flawed to imply that every EM source intrinsically produces a beam.

I didn't.

However, focusing photons with a lens can. The IR emitter and detector in an IR "3D" () ToF ranging module have typical dome-shaped LEDs, which act as a lens and focus the light, making them directional.

Whilst it is true to say that once photons exist they do tend to move in straight lines,

Rather, they do, not tend to. A photon will follow the local spacetime metric unless exposed to an electric field. As photons are the carriers for the electromagnetic force, they are influenced by electric fields.

the production of a beam requires the 'emitter' to constrain the path of emitted photons in some way e.g. through the use of lenses and reflectors etc., otherwise the initial radiation tends to spherical.

Yes. The dome lens on the emitter and detector LEDs in the IR sensor.

Radio emitters may well be directional in their emissions and produce lobes but they need specially designed antennae to produces 'beams'.

Yeah... so? That's radio. Way way way way way way longer wavelength than light, way lower energy photons.

Not seeing the relevance to Infrared ranging systems.

I think lasers are something different but they are beyond my understanding.

Lasers are just photons like any other light. The chief distinction is all the photons are coherent – i.e. the quantum mechanical "wave" characteristic of the photons is aligned so they're all "wiggling" in synchrony. Normal light is not synchronized like this, with some photons at crests, troughs, zero crossing, and everything in between if you could measure this at some point in space.

 

[MS Coast]

It all depends on what sort of surface it is ranging. I've had my Mini 2 (now sold) terrain-follow up a gentle dirt slope just by flying slowly and let the Landing Protection keep the drone 2ft above the ground. No OA at all.

It's interesting to do this across a curb... It will "terrain follow", popping up the height of the curb as it goes over it. It doesn't go down, however, passing the other way.

And it has to be as low as it will let you get without initiating the Landing process.

Yes, I've seen the same thing with very slow flight. But I can't see it happening with the current model DJI drones flying at 20+ mph during an auto RTH.

 

[Yorkshire_Pud]

I didn't.

We will have to differ in that, I think you did. If you wished to described what I was taught to think of as a 'light ray' then perhaps " EM emitted as a beam stays tightly confined in a beam" would have been better.

A photon will follow the local spacetime metric unless exposed to an electric field. As photons are the carriers for the electromagnetic force, they are influenced by electric fields.

If by "spacetime " you mean "gravitational lensing" etc. then I was aware that gravitational lensing has been observed.
Out of curiosity does "spacetime" cover refraction?
Can you cite some articles, written in layman's terminology, detailing that EM is deflected by electric fields because what I have found so far suggests otherwise.

Not seeing the relevance to Infrared ranging systems.

It is still an EM radiation and emitted more spherically than beam like, besides I suspected that if I didn't mention it you would have cited the lobes as indications of beam like emission of EM.

I eagerly await your answer

I don't know and I can't check if they are sources and receivers of ultrasound but I have not seen their like on any Mavic style drone.

 

[Pacefast]

I am sitting on a mountain top at 6000 feet set a RTH at 150 feet so it should climb to 6150 feet to return to me !
I descent following the mountain contour to 4000 feet ASL while keeping below 400 ft AGL following the contour.
I lose sight of the drone and press RTH ! Does it climb vertically 2150 feet hence busting the 400 foot limit or what does it do ?
This is aldo relevant flying up a Skyscraper which is legal to 120 feet ??? above the Skyscraper top
With all the confusing regs and programming to stop the drones going high ????

Thanks for replies I was indeed talking about busting the 400 feet Agl limits in the drone climbing vertically to the HP plus the HP plus RTH selected .
Flying visually to can contour fly so just wondered how the drone would handle such an extreme situation ! Thanks

 

[Droning on and on...]

I don't know and I can't check if they are sources and receivers of ultrasound but I have not seen their like on any Mavic style drone.

Changing the goal posts will get you ignored.

 

[Yorkshire_Pud]

Changing the goal posts will get you ignored.

Oh goody, please do, many thanks! I will reinstate the ignoring for you too!