"Clouds also don't save you (unless you have two thick layers to fly through) because this technique is even easier with satellites."

This is incorrect. A typical satellite will orbit once every 100 minutes or so (military spy satellites more often because they fly lower, but that only makes the next part even worse). To have any kind of resolution the swath it can scan is very narrow. It'll pass from horizon to horizon in some 10-14 minutes or so, if if passes reasonably overhead (which it'll do once, the next orbit it'll be far from overhead or not seen at all, depending on your latitude).

For a satellite to spot an airplane you need to be in luck. A coincidence. It's not something you can use for spotting airplanes. The harder you look (the more you increase resolution) the more narrow the swath gets. You can have more satellites. There's still no chance of actively detecting airplanes on a regular basis. And this doesn't even take into consideration that the data must be processed after having been dumped from the satellite. The satellite is by then elsewhere.

You could use a geostationary satellite, to monitor a good third of the planet at once. But then you're nearly 36000km above equator and you can't see any details. So, not that either.

Satellites are great for scanning the surface of the planet. And for that we're now at a stage where it's hard to hide anything, for very long at least. But moving airplanes is something entirely different.

(My job is about processing data from satellites).

> Most camera sensors already detect in the IR range, you just need to remove the filter.

Yes, but no. "Infrared" is a very wide range, and you're talking about different things here.

"Just off the red end of the visible spectrum", aka near-IR is typically considered 700-1400nm, which is what your normal visible-spectrum camera becomes sensitive to when you pop off the filter. That's fun, and you'll find lots of cool things there. Remote controls use near-IR typically in 850nm, flowers often reflect vividly in this range too. Notably, near-IR passes through most materials that're clear in the visible spectrum, which is how bugs are able to have eyes that see it to locate those flowers. Also, plastics and glasses, so NIR-capable optics are cheap. (NIR windows are often dyed to look dark-purple or black to the visible spectrum, because these dyes are transparent to IR.)

However, "thermal infrared" is much longer -- rocket exhaust can be seen with a mediumwave-IR sensor in the 3000-8000nm range, but warm bodies only start to show up in longwave-IR, 8000-15000nm. Sensors for those are mindbogglingly harder to make than near-IR. And these wavelengths don't pass through normal materials. Plain old glass, for instance, is totally opaque to thermal wavelengths, so if you take a thermal picture (a thermograph) of a window, you don't see the warm bodies inside, but rather the temperature of the glass itself, combined with whatever outside objects are reflected off its surface -- it acts like a mirror, not a window.

This means making lenses for thermal cameras is also difficult and expensive. The materials are awful -- Zinc Selenide is one of the most common, despite being expensive and toxic so it's difficult to machine. Pure Germanium works, but it's even more expensive. Sodium Chloride is amusingly transparent to LWIR but it tends to dislike getting rained on.

Removing the hot-mirror from a visible-light camera is a neat party trick and does legitimately see into "the infrared", but that's not the same as thermal infrared, which does still require specialized equipment.

Curious if you saw any difference in far IR vs near IR on cameras with removed filters. When I tried camera without filter or camera with IR filter (one that only lets IR through and look redish black to naked eye) attached I had nice b/w images of trees on long exposures or better pictures of tv remote leds. But if I make a picture of an iron or pan they look the same when either cold or hot. This is very different from thermal cameras I used where you can clearly see unevennes of the pan heat, dark image on tshirt being warmer on the sun, person at night etc. I can speculate that thermal exhaust falls into the second category.

> Most camera sensors already detect in the IR range, you just need to remove the filter. I've done it, works great and pretty fun

It's not the same IR you'd need for military purposes, you're seeing a tiny percentage of the IR spectrum, and it's the most useless. That's why any semi decent military tier system costs 50k+ a pop. Civilian tier scopes are 1k+ and pretty shit, you won't see anything smaller than a human more than 1km away.

Does a consumer-grade 8K camera sensor detect a 20°C difference in a few pixels? Cameras have to aggressively fight against thermal noise. Visible light is much more energetic than IR; detecting a faint IR signal with a sensor which is about as hot as the object it's looking at is hard. You need to cool the sensor down; old military aircraft even carried a tank of liquid nitrogen for that.

I mean, yes, you can take a low-noise sensor, add cooling, add a telescope lens so that you'd see the shape more readily, put a bunch of these telescopes on a rotating platform to scan the sky, etc. This is doable, but the thread started with an idea that it's doable with consumer-grade ("cheap") tech. I doubt that.

While at it, even if we assume that stealth does not exist for fast and heavy aircraft, it seems to effectively exist for slow, lighter-weight drones. Ukrainian drones, built from ultralight aircraft like Aeroprakt A-20, somehow penetrate 700 miles into Russian territory to burn refineries. With a cruise speed of 70 mph (sic), it should take them 10 hours to fly this distance. Were they detected efficiently, that would be enough time to scramble an interceptor a hundred times. Apparently this does not happen.

In visible range, would. The technique shown in the video work not perfectly clear sky conditions? Clouds are not uniform layer that moves with exactly the same speed in each point.

Are there reliable ways to figure out which pixels on subtracted frames are cloud movement and which are the few ones that are an aircraft?

Correct me if I'm wrong, but adding extra cameras doesn't seem to solve this problem, each source reporting "almost everything moves" would make solving intersections and tracking them impossible, because each target candidate can be assigned to many changes pixels in consecutive frames. Unless some additional pattern detection is done, but again it's hard for very small objects.