Gimbal Lock and Derotation in FLIR/ATFLIR systems

Mick West

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Source: https://www.youtube.com/watch?v=4X1PRDbtiF0


The above video is an attempt to demonstrate how a system mounted like the ATFLIR targeting pod:
A) requires a derotation mechanism
B) require major camera movements around 0°

It's a tricky thing to explain, and I anticipate this thread getting somewhat detailed and technical, but unless Raytheon wants to weigh in then some digging will be required to figure out exactly what is going on with this rotating glare.

What I'd like to do is collect as many references as possible that address this issue, to help paint a better picture.

One interesting patent I found is US9121758 - "Four-axis gimbaled airborne sensor having a second coelostat mirror to rotate about a third axis substantially perpendicular to both first and second axes" held by Raytheon, which has this interesting discussion on the need for derotation:
https://patents.google.com/patent/US9121758
And also the issue near the 0° position, referred to to as "gimbal lock" or "gimbal singularity"

The mention of +/- 3° is particularly interesting, as all the major apparent motion of the object happens between -3° and +4°
 
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jarlrmai

Active Member
Is it worth mentioning that the jet can also rotate and the the camera has to adjust for that as well?
 

Mick West

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Copied this from my post here: https://www.metabunk.org/posts/217222/


Another source of info for the ATFLIR obsessed is Raytheon's patents.
https://patents.google.com/?q=gimbal&q=IR&q=rotation&q=visible&assignee=raytheon


https://patents.google.com/patent/US6288381B1/
The following diagram is a schematic of an ATFLIR, with the nose (and hence the windows) on the left. It explains that the image is "derotated" by "a reflective derotation mechanism 25" (and another for the visible light at 35).
 
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Mick West

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An interesting analysis along these lines on YCombinator from colanderman, who discusses the gimbal lock issue, and hypothesizes it's some warm bird poop on the camera

https://news.ycombinator.com/item?id=20019375

While the gimbal lock analysis is fine, the bird poop explanation does not really hold up, as there's a distinct size change, which seems to indicate the object is getting closer and/or brighter
 
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Agent K

Active Member
I demonstrate gimbal lock by making a "thumbs up" gesture and showing how I can "pan" left and right and roll the first, but not tilt up and down. To point it down, I'd have to roll 90 degrees first.
 

Mick West

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I demonstrate gimbal lock by making a "thumbs up" gesture and showing how I can "pan" left and right and roll the first, but not tilt up and down. To point it down, I'd have to roll 90 degrees first.
I don't follow, where's the lock?
 

Mick West

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The mention of +/- 3° is particularly interesting, as all the major apparent motion of the object happens between -3° and +4°
An interesting thing with that range is that it's a good chunk of range that the Nimitz/FLIR/Tic-Tac goes over, from 4° right to 6° Left, and as it goes past 2° left there's a fairly distinct rotation of a light area in the background, then a slightly less distinct one at
f4-2007-field-rotation.gif
Which all is very consistent with the idea of a glare being rotated by the camera.
 

Mick West

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It's the same thing you show in the video, just using a fist instead of sticks.
But a fist has more degrees of freedom. With the thumb up I can rotate around all three axes. The gimbal setup can only rotate around two.
IMG_5632.gif
 

Agent K

Active Member
But a fist has more degrees of freedom. With the thumb up I can rotate around all three axes. The gimbal setup can only rotate around two.
View attachment 37557
I know, but pretend it can't tilt up and down.
I also demonstrate the North-East-Down axes with the left hand by extending the index finger forward (North), thumb to the right (East), and middle finger down. Then, I can show the yaw-pitch-roll rotation from NED into image coordinates Depth-Column-Row.
 

Candy-O

New Member
How would FLIR footage of a disc shaped craft that rotates on it's side DIFFER from what we see in the gimball video? Or an actual tic-tac shaped object differ from what we see on the Nimitz footage? I'm not going down any logical path here, I'm just curious how you feel footage of objects as described would differ from what we see.
 

Mick West

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Staff member
How would FLIR footage of a disc shaped craft that rotates on it's side DIFFER from what we see in the gimball video?
I'd expect it to remain the same profile through the video, whereas it actually changes shape quite a lot.

Metabunk 2019-06-29 05-19-25.jpg Metabunk 2019-06-29 05-20-13.jpg

Or an actual tic-tac shaped object differ from what we see on the Nimitz footage?
Similar here, the shape of the "object" changes in both visible light and infrared. which suggests something more like an irregular object.
Metabunk 2019-06-29 05-17-11.jpg
 

Gerard

Member
Similar here, the shape of the "object" changes in both visible light and infrared. which suggests something more like an irregular object.
Can you describe what this image is showing. Is the upper image visible and the lower IR ? If so why aren't the engines showing as hot spots ?
 

Mick West

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This patent is interesting, in that it focuses largely on derotation, and discusses the corrections for gimbal lock.
https://patents.google.com/patent/EP2525235B1/
Metabunk 2019-07-13 06-32-38.jpg


Discussing different methods of derotation:
Discussing gimbal lock
 

Mick West

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This A10 targetting pod footage is interesting.
Source: https://youtu.be/If53pV3ibo0?t=130


It shows derotation, possibly in software, and you can actually see the frame rotate - i.e. there are dark areas of no image that rotate around.
Metabunk 2020-08-20 08-12-57.jpg

There's also a display that pops up "GIMB ROLL" just before it does a major gimbal roll. Unfortunately, the video fades to a different shot at the same time.

 

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Ravi

Member
It shows derotation, possibly in software, and you can actually see the frame rotate
I think it actually is caused by the vignetting occurring at the de-rotator part. A square mirror design of this part would result in a square (blocking) frame.

It could of course be software, but I doubt it as it, is prone to errors etc. A hardware solution is likely preferred.

Here is a link to a similar de-rotator design, but for an instrument used on a large telescopes (VLT). VLT telescope design (altazimuth mount) causes the FoV to rotate as Earth rotates.

https://www.researchgate.net/figure/Optical-concept-for-the-ESO-MCAO-Demonstrator_fig1_41194920

As can be seen it involves 3 mirrors, that rotate radially along the optical axis.
 
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Mick West

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It could of course be software, but I doubt it as it is prone to errors etc
Rotating an image isn't really a complex bit of software, so long as the hardware can supply the correct angle, there would be zero problems.
 

Ravi

Member
Rotating an image isn't really a complex bit of software, so long as the hardware can supply the correct angle, there would be zero problems.
True, I can imagine..

But, why then have a de-rotator in the design... hmm...
 

Max Phalange

New Member
It shows derotation, possibly in software, and you can actually see the frame rotate - i.e. there are dark areas of no image that rotate around.
There's some quite noticeable circular blur around 0:39, which suggests that the sensor is static relative to the gimbal rotation. If the de-rotation happened optically before hitting the sensor, this shouldn't happen.
 

Amber Robot

Active Member
I think it actually is caused by the vignetting occurring at the de-rotator part. A square mirror design of this part would result in a square (blocking) frame.

It could of course be software, but I doubt it as it, is prone to errors etc. A hardware solution is likely preferred.

Here is a link to a similar de-rotator design, but for an instrument used on a large telescopes (VLT). VLT telescope design (altazimuth mount) causes the FoV to rotate as Earth rotates.

https://www.researchgate.net/figure/Optical-concept-for-the-ESO-MCAO-Demonstrator_fig1_41194920

As can be seen it involves 3 mirrors, that rotate radially along the optical axis.
In astronomy, I have heard this called a "K Mirror". I gave a presentation once on how a telescope might be retrofitted with a K Mirror and had to look into this a bit.

Also, a similar situation to the "gimbal lock" happens with alt-az telescopes when tracking objects that pass through or near the zenith. This is usually not allowed because the azimuth speed wants to go to infinity when an object passes directly overhead.
 

Ravi

Member
In astronomy, I have heard this called a "K Mirror". I gave a presentation once on how a telescope might be retrofitted with a K Mirror and had to look into this a bit.
Indeed! I forgot about this term. I worked for ESO in the past, on adaptive optics instruments.
 

gtoffo

Member
I think this theory makes a lot of sense. But I have a couple of questions:
  1. If I understand this correctly we are assuming the rotation is caused by the gimbal crossing the centerline and having to make a rotation to keep tracking the object without locking correct?
  2. Shouldn't then the rotation be a full 180° rotation given the flat plane (-2 degrees vertical) as your video shows? The rotation seems to be much less.
  3. Also the movement does not seem in a uniform single motion contrary to the "GIMB ROLL" videos posted (although the systems are different and in different modes). Is this expected? I see intermittent rotation at 13°L, 6°L, 3°L, 0, 4°R

    Thanks
 

Mick West

Administrator
Staff member
  1. If I understand this correctly we are assuming the rotation is caused by the gimbal crossing the centerline and having to make a rotation to keep tracking the object without locking correct?
  2. Shouldn't then the rotation be a full 180° rotation given the flat plane (-2 degrees vertical) as your video shows? The rotation seems to be much less.
  3. Also the movement does not seem in a uniform single motion contrary to the "GIMB ROLL" videos posted (although the systems are different and in different modes). Is this expected? I see intermittent rotation at 13°L, 6°L, 3°L, 0, 4°R
The gimbal roll is an adjustment to allow for continued tracking using the internal mirrors and possibly the azimuth rotation. The system will seek to minimize such movements, as they are coarse, and can result in jitter and/or loss of lock. The algorithm it uses to decide when to rotate and by how much is unclear.

In the older 2004 FLIR1 footage, you see two gimbal rolls. In the larger roll tracking is lost.

So it might be that newer systems try to apply the gimbal roll corrections in a more intelligent way to minimize this.

 

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gtoffo

Member
The gimbal roll is an adjustment to allow for continued tracking using the internal mirrors and possibly the azimuth rotation. The system will seek to minimize such movements, as they are coarse, and can result in jitter and/or loss of lock. The algorithm it uses to decide when to rotate and by how much is unclear.

In the older 2004 FLIR1 footage, you see two gimbal rolls. In the larger roll tracking is lost.

So it might be that newer systems try to apply the gimbal roll corrections in a more intelligent way to minimize this.
FLIR1 shows exactly what I would expect. A rapid full 180° rotation if I'm not mistaken. That is not what gimbal shows.

From what we know of the physical structure of the ATFLIR (a head that swivels ~90° mounted on a platform that can rotate 360°. That's also what I see you reproduced in the video above also) the 180° rotation of the external window is necessary to track the object across the horizon or isn't it?

Nothing I can think of can prevent this unless the head can swivel more than 90° and that would negate gimbal lock across the 0° and the whole theory that what we are looking at is the ATFLIR rotating to avoid lock. Maybe the new versions have a head that is able to do this and gimbal lock is a non issue?

Or am I missing something? Thanks
 

Mick West

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FLIR1 shows exactly what I would expect. A rapid full 180° rotation if I'm not mistaken. That is not what gimbal shows.
That difference is what I was explaining. FLIR1 nearly fails to reacquire tracking with its single big roll. The newer GIMBAL video does smaller rolls, and very little recentering is required.

It's 11 years later, seems entirely reasonable that the algorithm and maybe the hardware had improved.
 

gtoffo

Member
That difference is what I was explaining. FLIR1 nearly fails to reacquire tracking with its single big roll. The newer GIMBAL video does smaller rolls, and very little recentering is required.

It's 11 years later, seems entirely reasonable that the algorithm and maybe the hardware had improved.
Yes but it can't "make smaller rolls". The full 180° roll is required given the physical constraints of a 90° swivelling head. Or NO rotation is needed if the head swivels 180°.

There is no way of getting past this limit that I am aware of. Do you have any ideas to get past this problem?
 

Mick West

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There is no way of getting past this limit that I am aware of. Do you have any ideas to get past this problem?
Sure, if you combine it with a pointing system inside the head (the mirrors), you get much more leeway regarding how much you need to roll and when. It's not just a 2-axis camera + derotation. It's 2 coarse axes, and 1-3 fine mirror axes, plus derotation.
 

gtoffo

Member
Sure, if you combine it with a pointing system inside the head (the mirrors), you get much more leeway regarding how much you need to roll and when. It's not just a 2-axis camera + derotation. It's 2 coarse axes, and 1-3 fine mirror axes, plus derotation.
You mean within the system there are mirrors that "point" the camera without moving the external window? So for example it may stay fixed externally but internally move and track an object making small movements (that would make sense)? I wasn't aware of that. Even so with large movements across the centerline the full 180° rotation is still always needed as the external structure must also move. Right?

I might have missed something here but I'm asking those questions because aren't we theorising that the glare is generated by the external window and follows its rotation? Or is there something else that might cause the glare+rotation within the internal components? Or do we just don't know exactly what might cause this.

Thanks
 

Mick West

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You mean within the system there are mirrors that "point" the camera without moving the external window?
Yes, this patent describes them as "coelostat mirrors", also discusses not wanting to use the main roll axis.
https://patents.google.com/patent/US9121758

Or do we just don't know exactly what might cause this.
This, basically, because the patent are both not easy to read, and often have various possible embodiments.
 
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