Gimbal Lock and Derotation in FLIR/ATFLIR systems

Mick West

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Staff member
Four-axis gimbaled airborne sensor having a second coelostat mirror to rotate about a third axis substantially perpendicular to both first and second axesSource: 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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Mick West

Administrator
Staff member
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 "a reflective derotation mechanism 25" (and another for the visible light at 35).
View attachment 30892
 

Mick West

Administrator
Staff member
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

Administrator
Staff 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.
I don't follow, where's the lock?
 

Mick West

Administrator
Staff member
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

Administrator
Staff member
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

Administrator
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

Administrator
Staff member
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
 
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