IMU Calibration while Sailing

[Sail4Epilepsy]

Hi there:

In March the non-profit I work for purchased 2 DJI Phantom 4 Pro+ Version 2.0 Quadcopters. We wanted to use them while sailing across the Atlantic Ocean. Got them set up and tested on land, good to go. Once on the boat (and they had been stored in cases upright) got the IMU calibration error. Tried to fly with no luck - but did manage to keep the drone aboard. Couldn't recalibrate on the boat while underway. Recalibrated IMU again in the Azores, tested, good to go. Stored them flat this time (read somewhere that could have caused the need for calibration). Tested again on land in Portugal (Porto), good to go. Sailed for about 24 hours south towards Lisbon, got the IMU calibration error and couldn't recalibrate. Do anyone know of any way to stop this need for calibration? Turn some sensors off?? Or can the only way be to calibrate right before leaving the dock and use soon after? I know I've seen sailing YouTube channels were people use Phantoms...any advice appreciated!

Thanks,
Lauren

 

[Andy9]

Not need to calibrate every time before the flight even at remote locaton.

 

[Sail4Epilepsy]

@Andy9: Thanks for the reply. Unfortunately when I turn the drone on, when sailing, I get the IMU calibration error and when trying to fly with that error active, the drone is not able to be controlled.

 

[Meta4]

Thanks for the reply. Unfortunately when I turn the drone on, when sailing, I get the IMU calibration error and when trying to fly with that error active, the drone is not able to be controlled.

It's not normal to have to recalibrate the IMU unless the drone has been crashed.
There are two possible causes of your issue.

The drone has a genuine IMU problem.
Or the IMU does not like the way you are launching it.

The drone doesn't need to be stored level when switched off, but it does have to be close to level when launching.
What speed is the boat sailing when you attempt to launch?
The IMU will sense motion and there could be a problem if the boat is moving too fast when you launch.

How are you trying to launch ... are you hand launching and retrieving?
Anything else could be a problem.

 

[Sail4Epilepsy]

The boat was probably sailing at 6kts at most, so no more than 7mph. And likely less - I'm not likely to fly in high wind or rough seas.

What happened was that I turned the drone on, calibrated the compass, set it down on deck again, and then got the IMU calibration error. I tried starting the blades but the drone basically tried to fly backwards and took out a chunk of my leg. I then had someone hold it while I started the blades, it was pulling against their grip and when I tried to fly it, it just went backwards and I had to emergency stop the blades to keep it on the boat.

After I had recalibrated the IMU on land and brought back to the boat to try again, I didn't try to start blades or even launch once the IMU error showed up again. This happened two identical drones (in case I dunked one) so I don't think its an actual problem with the IMU.

I think once the IMU calibration error shows up, I can't fly it. It does seem to go between "okay to fly" and IMU calibration error, so there must be something about speed / motion??

BUT I have seen plenty of sailing YouTube channels using Phantoms at sea, I just don't see how they are doing it.

 

[JerryG]

Surely it’s to do with the fact that your takeoff point is moving?

When the IMU tries to calibrate it will be assuming that your takeoff point is static, but it finds that it’s moving at 6 knots and concludes “I’ve got a problem”. (I THINK you should be able to fly despite that warning)

Also, were you standing astern of the drone (with regard to the boat) when it bit your leg? The moment it takes off it will try to maintain a GEOGRAPHICAL hover, and not a hover in relationship to the boat – so it will immediately shoot towards the stern of the yacht at the speed that the yacht is moving.

 

[Andy9]

Did you try to launch it when the boat was still - not moving?

 

[Meta4]

The boat was probably sailing at 6kts at most, so no more than 7mph.

What happened was that I turned the drone on, calibrated the compass

There's no need to calibrate the compass .. that's completely unnecessary.

set it down on deck again, and then got the IMU calibration error.

The drone needs to be level for launching.
The deck of a sailboat while underway isn't going to be level very often.
You should be hand launching and catch landing if flying from a sailboat.

That's best learned on dry land before trying it out at sea.

I tried starting the blades but the drone basically tried to fly backwards.

As someone else mentioned. the drone will try to hover in place when you launch it, but if the boat is moving, that will look like the drone is flying backwards.
You need to launch from a position where the space behind the drone is clear of rigging, lifelines, people etc, so that it has a clear passage

After I had recalibrated the IMU on land and brought back to the boat to try again, I didn't try to start blades or even launch once the IMU error showed up again. This happened two identical drones (in case I dunked one) so I don't think its an actual problem with the IMU.

Think of it as the IMU warning of a problem, rather than a problem in the IMU.

I think once the IMU calibration error shows up, I can't fly it. It does seem to go between "okay to fly" and IMU calibration error, so there must be something about speed / motion??
BUT I have seen plenty of sailing YouTube channels using Phantoms at sea, I just don't see how they are doing it.

Having an assistant helps.
You get the assistant to hold the lower part of the landing gear semi-loosely above their face level and trying to keep the drone level.
They need to instantly release as you give it some throttle to lift off.
It helps to practise on dry land before trying on a boat.

Your recorded flight data might help confirm whether the speed or non-levelness is causing the problem.
Go to DJI Flight Log Viewer | Phantom Help
Follow the instructions there to upload your flight record from your phone or tablet.
That will give you a detailed report of the flight.
Come back and post a link to the report it gives you.
Or .. just post the txt file here.

I'll be able to go through the data and see what it says about the speed and the tilt & roll angles of the drone at launch time.

Flying a drone is quite easy.
Flying from a moving boat is much more complicated than landlubbers can imagine.
Ideally you are familiar with all aspects of drone operation and have practised every technique you will use at sea before trying on the boat.
At sea is the worst place to be learning new tricks and techniques.

Or another way to confirm the issue would be to try launching from a level surface when back on dry land after having the issue on the boat.
If it launches without complaining, you know it has to do with the launch environment rather than the drone.

 

[Meta4]

In case you aren't aware of it, I'll point out one potentially big problem you could encounter flying at sea.
It's something that most land-based flyers will never encounter.

The drone will record its home point (a GPS location) a little time after powering up.
But as the boat is moving, even a few seconds after launch, that home point will be somewhere astern of the vessel.
As you sail further, you could get to a point where the flight controller calculates that it's getting close to a point where it only just has ebough battery to get back to the home point (somewhere way back in your wake).
And then the drone zooms off towards the homepoint in the distance.
You don't want this to happen to you.

To prevent it, you should reset the homepoint every 5 mins or so to keep the home point close to where the boat is.
To do that, go to this screen in the settings and tap either of the icons indicated.
One resets the home point to the drone's current location, the other resets to the current location of the controller.
Either one is fine as whichever you set, the boat will have moved away in just seconds.
That will bring up one of these two screens:

Click OK and keep flying for another 5 mins or so and do it again to avoid having the drone think its home point is back somewhere over the horizon.

The other important thing is to bring the drone back well before you get into a low battery situation.
You need to allow time for making several attempts at bringing the drone in as it can be tricky from a moving boat.
It helps to have the boat going as slow as possible.
Bring the drone in towards the stern, with your catcher ready.
Rotate the drone so you are looking at the battery (back of the drone).
Try to match speed with the boat and gently reverse it towards the boat.
It's a good idea to do this backwards to avoid confusion on the controls ... left on the stick will move teh drone left etc.
Bring the drone towards the catcher up above face level.
The catcher reaches up and grabs the bottom of the landing gear to hold the drone level.
** Important** As soon as the catcher has the drone, you need to flick the flight mode switch all the way to the left (A or Atti Mode) and hold the left stick hard down for a couple of seconds to stop the motors.
This stops the drone from using GPS and that prevents the drone from fighting the catcher to try to get back to the position where it was caught.

This all takes coordination and practice between so the flyer and catcher each know what will happen and what they have to do.
And don't be surprised if it takes 3 or more attempts to get it right (that's why you need to start bringing it in earlier than you would on land).

ps .. for the Flight Mode Switch to work and to be able to switch to Atti Mode, you need to have enabled this setting in the app.

Any extra questions ... just ask.

 

[JerryG]

Well done "Meta4". I meant to mention that point and then forgot to! Several navies have lost quite expensive drones by forgetting the issue that "HOME" is no longer where you are, it's way back down the track.

 

[Andy9]

Isn't much easier to fly all the time in ATTI mode, avoiding then all GPS generated problems.
As asore there is nothing but the sea to film I suppose that you want to shoot the boat from above. If so, this is much easier to do by using the follow me function or flying around by point of interest function.

 

[CoreySnipes]

Good advice in this thread. I have flown from a boat a few times with P3 and I believe these two items (also suggested above) will get things working for you:

• Calibrate compass on shore before you leave, well away from metal pier etc. Find some grass. Don't calibrate compass again on the boat.
• Hand launch and hand catch.

Before launching from a boat I always check my return-to-home settings, disable vision positioning, and disable obstacle avoidance. I also aim to come back with at least 50% battery, so I have plenty of time for landing.

 

[Meta4]

Good advice in this thread. I have flown from a boat a few times with P3 and I believe these two items (also suggested above) will get things working for you:

• Calibrate compass on shore before you leave, well away from metal pier etc. Find some grass. Don't calibrate compass again on the boat.

There's really no need at all to be calibrating the compass.
The idea that you need to is a myth.
Just leave it alone.

 

[CoreySnipes]

There's really no need at all to be calibrating the compass.
The idea that you need to is a myth.
Just leave it alone.

Very good point. A better instruction in my list would have been "calibrate compass on shore once, to make sure it wasn't mis-calibrated on a boat - then leave it alone".

 

[nickaa]

IMU calibration should only be done with the drone on a level surface or the ground and at rest. Otherwise you will induce errors into the gyroscope, possibly resulting in an incident.

 

[Andy9]

Calibrating the compass on a boat might be wrong.
In particular when on the sea. There are many devices on the boat that can have an impact on the compass.
And at the moment when the drone takes off all these disturbances are gone and the compass is calibrated unproperly.

 

[Meta4]

Very good point. A better instruction in my list would have been "calibrate compass on shore once, to make sure it wasn't mis-calibrated on a boat - then leave it alone".

Even better would be to have no mention of recalibrating the compass.
There's no reason to do that ever, unless you modify, add or remove accessories or rebuild the drone.

 

[DC Steve]

On the “never calibrate the compass” advice, this goes against best practices, less so in open airspace perhaps, than in high density, urban settings where precise control is a must-have. It takes one minute to complete and significantly mitigates a major risk.

 

[Meta4]

On the “never calibrate the compass” advice, this goes against best practices, less so in open airspace perhaps, than in high density, urban settings where precise control is a must-have. It takes one minute to complete and significantly mitigates a major risk.

Actually, my advice is best practice.
There are lots of myths and misunderstanding around the drone's compass.
Unnecessary compass recalibration doesn't mitigate any risk.
Your comment indicates that you don't understand what compass calibration actually does.

The first post in this thread gives a good summary of what compass calibration actually does and dispels some of the myths around the topic.

A short explanation of compass function, calibration and errors.

BACKGROUND The aircraft compass is often misunderstood, both in terms of function and purpose, but it is important to understand what the compass measures, why, and what happens to that information. At the most basic level the purpose of the compass is to measure the earth’s magnetic field in...

mavicpilots.com

 

[Oso]

Actually, my advice is best practice.
There are lots of myths and misunderstanding around the drone's compass.
Unnecessary compass recalibration doesn't mitigate any risk.
Your comment indicates that you don't understand what compass calibration actually does.

The first post in this thread gives a good summary of what compass calibration actually does and dispels some of the myths around the topic.

A short explanation of compass function, calibration and errors.

BACKGROUND The aircraft compass is often misunderstood, both in terms of function and purpose, but it is important to understand what the compass measures, why, and what happens to that information. At the most basic level the purpose of the compass is to measure the earth’s magnetic field in...

mavicpilots.com

Looks like the “must always calibrate compass” myth still lives on despite our best efforts over many years. It feels like we’re back in 2016.

While we’re back in 2016, maybe I’ll buy lots of Bitcoin. Maybe plenty of toilet paper also.