I'd like to open a new topic regarding repetitive programmed flights in Waypoints.

Two weeks ago, one of my DJI Inspire 3 did not stay above the ground during a repeat of a programmed route in 3D dolly mode, but, when he has been looking for first position, hit the ground with the gimbal and kept trying to descend. I managed to turn it off, it was thankfully over grass and only two propellers has been damaged.

   This incident led me to question on what basis Inspire 3 determines its altitude in repeat mission mode and with what accuracy. If, like me, you work in the film business then you know that directors and cinematographers often push us to the limits of the technical capabilities of the drone. It's okay. It's their job.

But my local dealer was unable to answer me how and with what accuracy the drone determines his height in the repeat mission mode.

It uses a barometer, GPS/Glonass phase shift signal;  ultrasonic sensors, optical altitude determination based on 3D optical warning analysis, or...????

Anybody knows???

How important is the location of the RTK station and its height and how important is it to have the same drone starting position in case we return to the same location, let's say after a week /same relative height difference/  

What accuracy can we count on????

Thank you very much for any explanation

Best Jaromir

I've also had a lot of issues with altitude and repeatable routes, where everything about the flight is the exact same, yet the altitude wildly differs to the point of unsafe flight. I have repeatedly tested w/ and w/out D-RTK-2, making sure the takeoff point of the drone is in the exact same place, yet I constantly get this annoying altitude issue. I was going to post the exact same thread but glad you did. Hopefully we can get some further guidance from a DJI official

ski Posted at 12-15 22:10
I've also had a lot of issues with altitude and repeatable routes, where everything about the flight is the exact same, yet the altitude wildly differs to the point of unsafe flight. I have repeatedly tested w/ and w/out D-RTK-2, making sure the takeoff point of the drone is in the exact same place, yet I constantly get this annoying altitude issue. I was going to post the exact same thread but glad you did. Hopefully we can get some further guidance from a DJI official

I sent the drone to the DJI service center to read the logs from internal memory in servis mode at the same time I sent DJI a list of questions about the altitude determination and its accuracy.

I'll add the answers from DJI here as soon as I receive them.

Questions for DJI

1/ How does the drone determine its altitude in programmed mission mode? (In general and also depending on the surrounding terrain)
2/ Which sensors does it use to determine its altitude /Ultrasonic, Barometer, or optical sensors/?
3/ What altitude accuracy can we count on for programmed missions (+/- cm; +/-0,5m; more? How much more???
4/ If it uses the altitude assigned by GPS coordinates, what if the terrain elevation profile has changed?
5/ How does it determine its altitude in RTK mode?
6/ How does the drone determine its altitude relative to the RTK base station?
7/ At what height should the RTK station be located? / Should it touch the ground with the monopod spike? Does it mandatory? Does it need to be positioned in the same place and at the same height when the mission is repeated or not/?
8/ Does the drone need to be launched from the same position /at the same height/ when repeating missions?
9/ If so what solution to choose when the altitude profile at the original launch site has changed? (I.e., the terrain has been modified and the original site is either higher or lower /in relation to the surrounding terrain/)

This is most likely because you do not know how to install your GNSS receiver.  

This is the big problem with the DRTK-2, its use is not ergonomic.  The important thing is to enter the exact coordinates (WGS84 only!) of the antenna in Pilot 2.

To know them, the ideal is to take a known geodetic reference, or to measure it, then always install the antenna at this location and record coordinate in Pilot2.  

After that you have to choose ASL (EGM96) flight plans to have a constant altitude, but on this point I have no idea how the I3 works.

If the antenna is not properly configured, there is no advantage to using RTK for your use.
It's clearly written in the manual, page 41.

LV_Forestry Posted at 12-16 02:12
This is most likely because you do not know how to install your GNSS receiver.  

This is the big problem with the DRTK-2, its use is not ergonomic.  The important thing is to enter the exact coordinates (WGS84 only!) of the antenna in Pilot 2.

Thank you very much for the important point.  Appreciate this very important information.
This will allow us to accurately position the DJI RTK2 and determine the DJI RTK2 altitude using ASL.
But how does the drone know the relative difference of his take off altitude to the DJI RTK2 station?
And what if we don't program the mission, but want to repeat the recorded flight, or enter Way points using the buttons on the RC controller? And what if we are not using the DJI RTK 2, but a network RTK service from a national provider?
Would you please have some explanation how the drone will determine its altitude in this case and with what accuracy
I would be grateful for any information

jaromir5.0 Posted at 12-16 05:18
Thank you very much for the important point.  Appreciate this very important information.
This will allow us to accurately position the DJI RTK2 and determine the DJI RTK2 altitude using ASL.
But how does the drone know the relative difference of his take off altitude to the DJI RTK2 station?

No one really knows how the DRTK-2 works because DJI has locked it for single use with its drones. But the basic principle of what we call RTK is the correction of Geographic coordinates using “RTCM” message transmission. Publications (rtcm.org)

The principle is as follows:
-The antenna (base station) knows its coordinates (hence the importance of always installing it in the same place, at least with the same reference coordinates)
-It receives signals from satellites, deduces coordinates (like any random GNSS receiver from consumer drones, phones, etc.)
-It calculates the difference between these coordinates from the satellite signals, and the true coordinates that you indicated to it (reference coordinates). This difference is the error that we want to correct. It is due to the disruption of satellite signals as they pass through the atmosphere (to do it simply).

-This error is considered to be the same within a radius of up to 35km* around the antenna. *(+/-, be careful because this depends on the quality of the antennas)
-As a result, the principle of the antenna (base station) is to inform the drone (rover) of this error so that it can make the correction. And applied it (base station correction) to its own (rover) calculation of the satellite signals received and thus correct its position (rover).

There are 3 possible levels when using RTK.
-Single: not connected to the NTRIP network (the base station).
-Float: Connects but the correction is not stable (see tips below)
-Fix: connect and fixed, you must target this position to operate with complete peace of mind.

-How does the drone know the relative difference of his take off altitude to the DJI RTK2 station?


The relative difference between the antenna and the drone doesn't matter. But the drone knows it because among the messages that the antenna sends, there is one which contains its coordinates.

-what if we don't program the mission, but want to repeat the recorded flight, or enter Way points using the buttons on the RC controller?

On this point I have no experience with the inspire 3, so I'm not going to get too far ahead. I would say that the important thing is that the flight plan is recorded containing waypoints with geographic coordinates XY and Z. Versus flight plan with Z relative to the takeoff point. So if you have the choice to put "ASL" do it !

-what if we are not using the DJI RTK 2, but a network RTK service from a national provider?

I strongly recommand. Popis sítě (cuzk.cz)
In this case you rule out all the problems of the DRTK-2, because the national provider sends the famous message containing the coordinates of the base antenna, as well as the correction messages based on the differential calculation. An internet connection is required.

-how the drone will determine its altitude in this case and with what accuracy


Condition: The RTK status must be FIX to ensure the following
-The drone will find its geographic position using satellite signals.
-It will carry out the correction using the messages received from the base station.
-It will thus know its geographical coordinates with a precision of the order of a centimeter. +/- 5cm it's reasonable to say that yes you will have them. Below, good luck possible but not guaranteed..
We must not forget that it is a drone which is held in the air with 4 propellers, which is subject to wind and other atmospheric conditions. The coordinates can certainly be known with an accuracy of 1cm, but the drone will never be that stable. Stability is what matters when using the Inspire 3.


Warning !


The antenna (base station) must have as wide a clear view of the sky as possible, no trees, no buildings, etc.
For the drone it's the same, if you pass it under the trees, next to a building, near a high voltage line, expect to have RTK disconnections. If the drone has a critical flight plan where every centimeter counts, it is recommended to activate the Flight plan pause function if the RTK signal is lost. No idea how to do it on Inspire3.


Tips


Sometimes before takeoff the drone can remains in Float for a very long time. This is very often due to signal disturbances due to the environment. in this case :
-Disables RTK,
-Takes off the drone,
-Activates RTK once the drone is in the air free of obstructions and wait for a Fix.
This procedure will have no impact on the accuracy of the altitude once you get a Fix.


Hope this helps!
The DJI moderators are nice here, but they don't use the products. As a result, there may be discrepancies between what they announce (From DJI official and commercial documentation) and reality.
In my opinion, using the DRTK2 for non-surveyor professionals, to do what the Inspire 3 promises, is total nonsense and even a danger.



National NTRIP network is your immediate solution!

I re-read the manual and watch the DJI tutorial video. As usual with this kind of somewhat "pro" function, the instructions are almost non-existent. The only thing that reassures me that it works like the enterprise range drones is the screenshot below where it is clearly marked that the ALT mode is relative to the starting altitude. So I suppose that in the menu there is the ASL mode relating to the difference between WGS84 and EGM96.

You can also use ALT but in this case take care to choose the same take off point. And RTK is mandatory because the barometer can cause surprises, and I think that's what happened to you.



Page 41 of the manual:


It means everything and nothing. What I understand is that you should not use a mobile receiver (DRTK-2), but fixed receivers (National NTRIP).
Or not switch between different modes of RTK, SITE, iMAX, VRS... but once again there are no details.


Funny post 2#:
https://forum.dji.com/forum.php? ... =288557&pid=3023370

LV_Forestry Posted at 12-16 09:54
I re-read the manual and watch the DJI tutorial video. As usual with this kind of somewhat "pro" function, the instructions are almost non-existent. The only thing that reassures me that it works like the enterprise range drones is the screenshot below where it is clearly marked that the ALT mode is relative to the starting altitude. So I suppose that in the menu there is the ASL mode relating to the difference between WGS84 and EGM96.

You can also use ALT but in this case take care to choose the same take off point. And RTK is mandatory because the barometer can cause surprises, and I think that's what happened to you.

Thank you very much for the perfect and detailed analysis /your information help a lot!!! Your true Captain/.
Results of your analysis are consistent with the empirical tests we have conducted. We had the most consistent results using the national network RTK service.
The Inspire 3 control system is clearly adopted from the Enterprise series.
In the case of using the national network RTK services we moved with a +/- 10cm X;Y accuracy /measured from a fixed tripod with a Leica Rangemaster CRF 2400-R rangefinder/ and in the Z axis about 20cm /measured from reference points marked in the field with a  rangefinder Hilti PD-E/
Let's wait for the response from DJI and we will see.....
I wish nothing less than to set the drone first position one foot above Bugatti Veyron hood with a possible deviation of two feet (((;
Thank you once again for very helpful information

I use RTK Inspire 3 in Las Vegas..having mixed results.  Glad to read this thread..everyone is very helpful.  Until another firmware update, will only use RTK well above the ground.  I’m have similar results with repeatable missions day to night videos. ,

For what its worth, Ive had similar issues with the 3d dolly function.

If you want to fly the drone anywhere close to the ground (say maybe with 3m), I would rate the function as not great.

There is also a fairly big issue with the drones route being much different depending on the speed at which you fly when it comes to the 3D dolly function. If you go slowly youll hit all the points, but as soon as you pick up speed it will start going to "approximate" points and have far greater curves in the route.

It might be worth having a mode in 3D dolly that limits your speed to suit the route you are flying to make sure you hit all the points consistently + have a guide when making the route to suggest that points/turns are too close together and will limit your speed to "X" value, depending on what you've tried to set up to be able to repeat the route reliably.

I came across this today for the first time!

Now all of DJI's waypoint planning needs to have the ability to fly a series of straight lines, not the bezier curves that they currently do.  And when it does fly bezier curves, I need that visible on the map screen so I can plan accurately where the drone will be in space.  There absolutely must be an option to select from straight lines or curves, because I need to be able to do both, and I need to know exactly what my drone is programmed to do before I start the mission.  As it currently functions, if you plan a straight line with three waypoints, 100'/400'/100'.  It will fly up to 100' then to 400' back down the 100', the drone will start a bezier curve at 400', exceeding that height (potentially violating FAA regulations), and then back to the third waypoint.  There is nothing in the planning that shows that the drone will exceed the max waypoint height.  The same happens between any two points.  I have currently blown past my max height, and narrowly avoided hitting obstacles that were nowhere near where my programmed flight path was indicated in the app's map.  This needs fixing ASAP.  This is also the case with the Mavic 3 Pro/Cine and the Inspire 3.  It seems a big oversight on the part of the folks making the waypoint apps.

Now all of DJI's waypoint planning needs to have the ability to fly a series of straight lines, not the bezier curves that they currently do.  And when it does fly bezier curves, I need that visible on the map screen so I can plan accurately where the drone will be in space.  There absolutely must be an option to select from straight lines or curves, because I need to be able to do both, and I need to know exactly what my drone is programmed to do before I start the mission.  As it currently functions, if you plan a straight line with three waypoints, 100'/400'/100'.  It will fly up to 100' then to 400' back down the 100', the drone will start a bezier curve at 400', exceeding that height (potentially violating FAA regulations), and then back to the third waypoint.  There is nothing in the planning that shows that the drone will exceed the max waypoint height.  The same happens between any two points.  I have currently blown past my max height, and narrowly avoided hitting obstacles that were nowhere near where my programmed flight path was indicated in the app's map.  This needs fixing ASAP.  This is also the case with the Mavic 3 Pro/Cine and the Inspire 3.  It seems a big oversight on the part of the folks making the waypoint apps.

Just drop the SDK already and let us use Litchi, which is a far better waypoint planner that is drone platform agnostic so all your compatible drones can fly the same mission and has a web interface so you can plan flights from your desktop in the studio.