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Finding the B-21's hangar location from the stars in its press image

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Re: Finding the B-21's hangar location from the stars in its press image

#121

Post by [deleted] »

runlaszlorun wrote: post #109
runjake wrote: post #70

Earlier quoted context omitted.

It's pretty clearly an exercise in astronavigation. The livestream event itself mentioned it was taking place at the Northrop facility in Palmdale. This author is not responsible for what clickbait farms do. Aside: I'm not even sure this plane will end up doing flight testing somewhere secret in Nevada. They may just do it out of Edwards South Base, which is an "interesting" location not many in the public know about…

I hadn’t realized that much of the public doesn’t know Edwards. Then again, I’m a SoCal native with a father who was an aerospace engineer…

[deleted]

Re: Finding the B-21's hangar location from the stars in its press image

#122

Post by postingawayonhn »

supernova87a wrote: post #98

He seems to use the hour estimate of 1:30am being a time when very few satellites are likely overhead as a sort of confirming factor. (I guess because many satellites are sun-synchronous around noon, and the USAF would purposely time the photo to not expose the aircraft to observation). But is that a very strong confirming factor? Is it likely that NG / USAF would be actively avoiding the potential for satellite imag…

> Is it likely that NG / USAF would be actively avoiding the potential for satellite imagery?

Yes, we still haven't seen the back of the B-21 (those at the unveiling could only view it from the front). There was also an incident when the B-2 was unveiled and a photographer flew over and got pictures of it.

Re: Finding the B-21's hangar location from the stars in its press image

#123

Post by mvnuweucxqokii »

vilhelm_s wrote: post #57
lambdasquirrel wrote: post #49

Earlier quoted context omitted.

I don't know if I entirely buy the line on "34-35 degrees off the horizon." Unless we knew the focal length of the lens it was taken with, and which camera it was taken from, you don't actually know that. A wide-angle lens is going to have a much larger field of view than, say, a 100mm macro. And different camera systems have different angle of view for the same focal length.

You know what the angle between the stars in the constellation are, you don't need to know the field of view of the camera.

I think what you're suggesting is that because we know the angle between a pair of stars, we can use that to infer the angle to the horizon by measuring the distance between each in pixels.

I don't think it's a safe assumption that there is a linear relationship between the angles. Like, two pairs of objects in the image, in which the angles subtended by each pair relative to the viewer are the same in real life, may not be separated by the same number of pixels in image space depending on the lens used.

Furthermore, this could be thrown off by even a small hill or mountain obscuring the astronomical horizon.

Re: Finding the B-21's hangar location from the stars in its press image

#124

Post by TheOtherHobbes »

vilhelm_s wrote: post #57
lambdasquirrel wrote: post #49

Earlier quoted context omitted.

I don't know if I entirely buy the line on "34-35 degrees off the horizon." Unless we knew the focal length of the lens it was taken with, and which camera it was taken from, you don't actually know that. A wide-angle lens is going to have a much larger field of view than, say, a 100mm macro. And different camera systems have different angle of view for the same focal length.

You know what the angle between the stars in the constellation are, you don't need to know the field of view of the camera.

If you were doing this professionally you would, because lenses introduce distortions of various kinds - typically barrel or pincushion depending on the focal length and/or the lens.

So any view of the sky is an approximation, and will introduce subtle errors that make it harder to pin down an exact location.

They probably won't be huge areas, but this would be a harder problem if the time and longitude had to be exact with no existing guesses.

Re: Finding the B-21's hangar location from the stars in its press image

#125

Post by protastus »

4gotunameagain wrote: post #88
pifm_guy wrote: post #75

Earlier quoted context omitted.

In my case, I had nighttime images from the same camera, so all the calibration was already done... I was just looking for a rotated version of the nighttime image in the daytime image. But even in the general case, smallish image patches don't have enough distortion to matter, while still having plenty of oversampling, and only have 3 unknowns. You can probably do some kind of fft trickery to reduce it to 1 unknown.…

What does an optimization algorithm have to do with a brute force search? Even if your image patch is small, without previous images to compare, you need to brute force the entire sky? And how can "fft trickery" reduce the attitude state to 1 unknown ? 1 unknown with what units ? Sorry but I suspect you are making all these up.

The claims from pifm_guy are reasonable.

This is a scenario with extremely high coding gain[1]. The DC value of the sky can be removed by high pass filtering. Finding the rigid transform (rotation+translation) between two images captured by the same camera, assuming no distortion[2], assuming the image pair has substantial intersection, doesn't require calibration.

Dropping the "same camera" and "substantial intersection" assumptions requires searching the entire sky and also finding an unknown scaling factor. It all comes down to the images having low enough noise and high enough resolution, relative to the search space. The math is sound.

You're not entitled to a tutorial in signal processing, though.

[1] https://en.wikipedia.org/wiki/Coding_gain

[2] Astronomy lenses have very small FOV and very small distortion.

Re: Finding the B-21's hangar location from the stars in its press image

#126

Post by Sporktacular »

stuff4ben wrote: post #80
Sporktacular wrote: post #54

Earlier quoted context omitted.

Sure, but the reason you didn't lead with that, and also the reason you did this for a secretive aircraft and not a new campervan is clicks, no?

Even if he did do this for the clicks, so what?

So it's manipulative. Like a sensational headline that buries the mundanity at the end. Lead with "this is already publicly known" so people can make an informed choice about where to direct their attention.

Re: Finding the B-21's hangar location from the stars in its press image

#127

Post by breck »

dylan604 wrote: post #113
mk_stjames wrote: post #24

I feel like the entire thread should have led off with "This is in fact a publicly known hangar in Palmdale and this was known as public information as soon as the unveiling event occurred, and thus this is just an exercise to show how locating the spot could have been possible from just this photo as a technical demonstration." Because I guarantee there are going to be a hundred clickbait articles by various 'news'…

People have been doing photo analysis for a long long time, and have gotten quite good at doing geolocation based on details in the background that the average viewer ignores. Spy agencies are especially good at it. There's many a spy movie that offers glimpses into this, but The Good Shepard comes to mind. Even as a joking kind of thing, the opposite has been done. There's a famous exchange between Neil DeGrasse Tys…

> There's a famous exchange between Neil DeGrasse Tyson and James Cameron about how the sky in the Titanic was wrong. Cameron accepted the challenge, and on a remastered version had the sky replaced to be historically accurate.

Very interesting! https://www.youtube.com/watch?v=8B6jSfRuptY&t=110s

(Interesting comment for a number of reasons. Thanks!)

Re: Finding the B-21's hangar location from the stars in its press image

#128

Post by pigbearpig »

cossatot wrote: post #89
johnmcelhone wrote: post #17

Earlier quoted context omitted.

You're right, it wasn't too difficult to guess the location without the stars. The base did happen to be right in-between the 34 and 35 degree lines (34.6 lat), and you can estimate pretty well with only one line

Little Rock AFB is a few tens of km north (34.9 deg lat) for what it's worth. There may be some others as well. The chances of getting a picture of the stars that clear in Arkansas is way lower than in the Mojave however...

Considering Little Rock is a C130 airlift base it's pretty easy to rule that out without looking at stars.

Re: Finding the B-21's hangar location from the stars in its press image

#130

Post by gmiller123456 »

tetris11 wrote: post #43
mierle wrote: post #26

Wow, it's cool to see astrometry.net get put to use! I worked on astrometry.net back in undergrad and grad school. If you have questions about how it works, I can answer them. How does it work? astrometry.net uses 4-star combinations to define codes, then indexes the codes on the celestial sphere. The particulars of each of these phases matter, but that's the basic idea.

is A,B,C,D in the database the same as D,C,B,A? Or does order infer orientation?

It uses a hash which is scale and orientation independent. A lot of the details are described in this paper:

https://arxiv.org/abs/0910.2233

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