Drone to Controller communication

[Baine]

Hi all - newbie pilot here, happy to join the forum!

If you want to cut straight to my question: What are the technical methods by which a mavic controller communicates to the DJI MAVIC drones? Is this via radio signal, satellite comm, etc.? Also, from your experiences, how reliable is this signal and what can interfere with the signal's communication range?



If you're curious on how I got here: Bought my first Mavic Mini Saturday morning, crashed/lost it Saturday night. I am 100% willing to chalk this up to my own fault but now I am focused on understanding the true technical capabilities of controller communication before flying another drone. I live in a downtown city location and I was flying my Mini atop a 10-story building. I had visual line of sight of the drone for most of my flight but lost visual line when piloting behind a building at least 20stories tall. I believed I would be fine as I was still close to the drone and still had a visual on the phone/camera display. Later in flight my drone gave me the 20% low battery warning and I immediately called for the drone to Return to Home.

Maybe 3 seconds after initiating RTH, my controller lost communication with the drone and the video signal stopped. I could not send signal to the drone and rushed to regain visual sight. At that point my drone was no longer in the sky and had disappeared. Using the flight path recording I've learned that the drone may have crashed into the large building after losing signal to my remote. Then, based on the increasing descension rate recorded, I believe it fell to the ground and has since been lost.

And that's my long story . Once more, I accept responsibility for not having constant visual line of sight, but I am also confident that had I not lost signal on the controller, I could have piloted the drone back to me safely. Looking for any insight on this situation and the questions posted above. Thank you greatly for your expertise.

 

[KingRat]

Watching the replies to this, so I can learn from your mistake!

 

[Paul2660]

Mini uses WiFi and this does not have a very powerful signal compared to other DJI drones.

When you flew behind the building odds are you lost the signal just like you would with any WiFi.

However might have happened with Occusync 2 also. Flying between tall structures is a easy way to loose signal.

Why RTH did not occur correctly would need flight logs. There is a sticky post on how to get them in the crash sub forum.

Paul C

 

[raven swe]

its radio not sat comms and wifi is a radio frequency, even if it was sat comms it would still use a radio frequency

 

[Paul2660]

Here is a link.

Mavic Flight Log Retrieval and Analysis Guide

With the introduction of the Mavic Mini there has been a noticeable uptick in the number of posts requesting help with lost or crashed aircraft. While there is plenty of help and advice available on this forum, and detailed guidance and options on several other websites, I thought that perhaps a...

mavicpilots.com

 

[old man mavic]

the Rc when first turned on links to the app in the device and then when you turn on on the AC the RC and AC establish a radio link between each other this link has nothing to do with GPS
after you have taken off, then the RC sends the instructions that you the pilot give to the control sticks and it responds accordingly,the same when you tell it to start video or move the gimbal this signal can not turn corners or pass through trees especially when they are in leaf as the water in the trees absorbs and disrupts the signal if you go behind a building that is between you and the drone then depending on the setting you have chosen, for loss of signal ,such as RTH, or hover, or land, then after a preset time it will try to do what you have chosen
the problem is if the obstruction you have flown behind is higher than the RTH height you have set then it will just fly into the structure and crash if you are lucky and have a M2P or zoom ,the drone will retrace its last recorded flight path for a given distance and then return, often this will mean it will clear the obstruction and then be able to fly home,and it will reconnect
to the RC as well

 

[BadWolf1]

the problem is if the obstruction you have flown behind is higher than the RTH height you have set then it will just fly into the structure and crash

My thought on reading the OP.

I was flying my Mini atop a 10-story building. I had visual line of sight of the drone for most of my flight but lost visual line when piloting behind a building at least 20stories tall.

The profile is pretty square - up to RTH height, straight line home, down to land. I read that and thought "If the stories above 10 are more than the RTH height, the point of impact will be somewhere on the far side of the building. I'd know where to look for the wreckage and see if the memory card survived with more data after contact was lost.

 

[sar104]

Hi all - newbie pilot here, happy to join the forum!

If you want to cut straight to my question: What are the technical methods by which a mavic controller communicates to the DJI MAVIC drones? Is this via radio signal, satellite comm, etc.? Also, from your experiences, how reliable is this signal and what can interfere with the signal's communication range?



If you're curious on how I got here: Bought my first Mavic Mini Saturday morning, crashed/lost it Saturday night. I am 100% willing to chalk this up to my own fault but now I am focused on understanding the true technical capabilities of controller communication before flying another drone. I live in a downtown city location and I was flying my Mini atop a 10-story building. I had visual line of sight of the drone for most of my flight but lost visual line when piloting behind a building at least 20stories tall. I believed I would be fine as I was still close to the drone and still had a visual on the phone/camera display. Later in flight my drone gave me the 20% low battery warning and I immediately called for the drone to Return to Home.

Maybe 3 seconds after initiating RTH, my controller lost communication with the drone and the video signal stopped. I could not send signal to the drone and rushed to regain visual sight. At that point my drone was no longer in the sky and had disappeared. Using the flight path recording I've learned that the drone may have crashed into the large building after losing signal to my remote. Then, based on the increasing descension rate recorded, I believe it fell to the ground and has since been lost.

And that's my long story . Once more, I accept responsibility for not having constant visual line of sight, but I am also confident that had I not lost signal on the controller, I could have piloted the drone back to me safely. Looking for any insight on this situation and the questions posted above. Thank you greatly for your expertise.

If you want insight then I recommend that you post the flight log.

 

[Baine]

Thank you all for your replies and knowledge. I'm looking forward to reading up on the Flight Log retrieval posted by Paul2660. I will happily upload the log for review once I am able.

Some additional information: No memory card inserted during the flight, didn't even record the flight to phone. Additionally, as people have noticed, the RTH altitude was set at the default 30m whereas the building I was navigating around was at least 50m tall. I am confident my little guy crashed into the building on his return (no obstacle avoidance on the Minis) and fell onto the street.

I've attached a screenshot of the last image I received from the drone. At this point, the drone is Returning to Home automatically and you can see the drone was dangerously close to the building. I tried finding the drone on the street, checked the neighboring buildings, put out word to the neighbors, etc. Still no luck.


Lastly, if anyone knows how to download the flight path recording, I can upload that as well for anyone interested. Thanks again!

 

[Baine]

Thank you all for your replies and knowledge. I'm looking forward to reading up on the Flight Log retrieval posted by Paul2660. I will happily upload the log for review once I am able.

Some additional information: No memory card inserted during the flight, didn't even record the flight to phone. Additionally, as people have noticed, the RTH altitude was set at the default 30m whereas the building I was navigating around was at least 50m tall. I am confident my little guy crashed into the building on his return (no obstacle avoidance on the Minis) and fell onto the street.

I've attached a screenshot of the last image I received from the drone. At this point, the drone is Returning to Home automatically and you can see the drone was dangerously close to the building. I tried finding the drone on the street, checked the neighboring buildings, put out word to the neighbors, etc. Still no luck.


Lastly, if anyone knows how to download the flight path recording, I can upload that as well for anyone interested. Thanks again!View attachment 103390

Got the logs! I used the phantomhelp site. Here is the link for anyone interested in viewing them: DJI Flight Log Viewer - PhantomHelp.com

 

[RadioFlyerMan]

the Rc when first turned on links to the app in the device and then when you turn on on the AC the RC and AC establish a radio link between each other this link has nothing to do with GPS
after you have taken off, then the RC sends the instructions that you the pilot give to the control sticks and it responds accordingly,the same when you tell it to start video or move the gimbal this signal can not turn corners or pass through trees especially when they are in leaf as the water in the trees absorbs and disrupts the signal if you go behind a building that is between you and the drone then depending on the setting you have chosen, for loss of signal ,such as RTH, or hover, or land, then after a preset time it will try to do what you have chosen
the problem is if the obstruction you have flown behind is higher than the RTH height you have set then it will just fly into the structure and crash if you are lucky and have a M2P or zoom ,the drone will retrace its last recorded flight path for a given distance and then return, often this will mean it will clear the obstruction and then be able to fly home,and it will reconnect
to the RC as well

I don't know what we would do without your thorough and ever present assistance!

 

[old man mavic]

I don't know what we would do without your thorough and ever present assistance!

@RadioFlyerMan thanks my friend, looking forwards to your next masterpiece of filming and editing wizardry

 

[BadWolf1]

On the screen shot vertical speed -12.8 m/s is not flying, but crashing. It tumbles (pitch / roll / yaw values going all over the place) about ~ 45m in ~ 4 seconds.
The plot shows you went out over the parking lot, turned and went along Jackson street, behind the tall building, turned further away from yourself, and then hit RTH
The data says the drone started to turn at 637 seconds and started to fly home at 640 seconds. It flew pretty much south, and was still accelerating to cruise speed (red line) at about 649.8 when seems to have hit what google maps calls the Dallas Power and Light flats near the South east corner about 130 feet up. I think the picture is before the impact and you got data but not video after that. GPS wasn't great among those buildings (green stepped line). The other green line (east-west speed) is pretty much zero until the impact, and the drone tumbles South and East - so it wasn't a head-on impact . It probably landed on your side of Jackson Street, though depending on what traffic was like ... who knows.
Given what we said about the signal range there was a lot of building between you and the controller continued to get telemetry until very close to impact.

 

[Baine]

On the screen shot vertical speed -12.8 m/s is not flying, but crashing. It tumbles (pitch / roll / yaw values going all over the place) about ~ 45m in ~ 4 seconds.
The plot shows you went out over the parking lot, turned and went along Jackson street, behind the tall building, turned further away from yourself, and then hit RTH
The data says the drone started to turn at 637 seconds and started to fly home at 640 seconds. It flew pretty much south, and was still accelerating to cruise speed (red line) at about 649.8 when seems to have hit what google maps calls the Dallas Power and Light flats near the South east corner about 130 feet up. I think the picture is before the impact and you got data but not video after that. GPS wasn't great among those buildings (green stepped line). The other green line (east-west speed) is pretty much zero until the impact, and the drone tumbles South and East - so it wasn't a head-on impact . It probably landed on your side of Jackson Street, though depending on what traffic was like ... who knows.
Given what we said about the signal range there was a lot of building between you and the controller continued to get telemetry until very close to impact.

View attachment 103394

Awesome breakdown! Much appreciated.

One small thing to note about the screen shot - the video image from the Mini showed the still image of the flight with the building to the right, and stayed still for the remainder of the transmission. Although the image was still, the metric data was still transmitting and changing on screen up until the screen went black/white on me. My screenshot captures the data at the high descension rate. Doesn't change the rest of the story, but figured I'd mention it. Regarding your head-on impact theory, I agree. You can't see it on the image, but there are cables dangling off the building from the window washer system. I believe the drone didn't hit the building itself, but maybe got caught in those cables that hang a few feet away from the building.

The crash occurred around 11p CST with limited pedestrian traffic, and the street has very very limited vehicle traffic as the street is blocked off. However, this occurred on the night of the George Floyd protests here in the US (more foot traffic on the ground) so it is possible that the drone fell on the ground, and by the time I made it to the ground floor it had already been picked up by a pedestrian.

Still, completely my fault and inexperience but happy to learn from the mistake.

 

[sar104]

That was unfortunate. You didn't lose the control link, but pressing the RTH button on the RC was the problem with the RTH height set to 30 meters. You almost got away with it though. The actual flight track looks like this:



The impact was on the corner of the building - it just clipped it on the direct line home:

 

[KingRat]

How do you do that? How do you know which building it was and the flight details from that graph? Very, very clever.

 

[sar104]

How do you do that? How do you know which building it was and the flight details from that graph? Very, very clever.

The GNSS position data are in the txt log, together with the height relative to the takeoff point. All that is needed in addition to those data to create a kml file of the flight track is the takeoff point elevation MSL, which is found using Google Earth. The impact point is easily identified in the attitude and velocity data, although it is pretty clear from the position data anyway.

 

[RadioFlyerMan]

@RadioFlyerMan thanks my friend, looking forwards to your next masterpiece of filming and editing wizardry

Actually, my next theme video is now formulating in my head... something to do with the coming of Summer. I really like it when all the plants, etc. start growing around here, and it's all due to my wife's handy work.

 

[KingRat]

The GNSS position data are in the txt log, together with the height relative to the takeoff point. All that is needed in addition to those data to create a kml file of the flight track is the takeoff point elevation MSL, which is found using Google Earth. The impact point is easily identified in the attitude and velocity data, although it is pretty clear from the position data anyway.

View attachment 103410

View attachment 103411

Ok. I don't actually understand your reply - it is way above my understanding. I do appreciate your effort to explain though.
GNSS?
KML file?
MSL?

I still navigate by the seat of my pants (land in sight, you have landmarks and buoyage to use), chart and compass. I still use my M73... yes I am a bit of a dinosaur. Everything about these drones, the stuff that drives them - it is mind blowing. I haven't really flown mine yet, 20ft max.... I dare not just yet. I don't want to end up on these pages asking questions like the first one here!

It is another world

 

[Baine]

Ok. I don't actually understand your reply - it is way above my understanding. I do appreciate your effort to explain though.
GNSS?
KML file?
MSL?

I still navigate by the seat of my pants (land in sight, you have landmarks and buoyage to use), chart and compass. I still use my M73... yes I am a bit of a dinosaur. Everything about these drones, the stuff that drives them - it is mind blowing. I haven't really flown mine yet, 20ft max.... I dare not just yet. I don't want to end up on these pages asking questions like the first one here!

It is another world

Haha agreed! But man that's how you learn. Ended up buying another Mavic Mini the next day to find the lost mavic. Between my last mistake, a few practice runs, actually learning what the heck RTH does, and the info from these forums I feel so much more confident when flying around. Even took her up for an aerial view of the city the next day: 2020_05_31_21_00_39_Cache.mp4

I will admit that my heart was in my stomach the whole time though, haha!