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101
Wild guesses here. I've done work in optical product identification, but I don't know how well those challenges translate. Also, it's an obvious enough idea that I expect there are teams working on it.
Lens and CCD technology is not trivial at those speeds and insane angular resolution. It's not just about counting pixels, it's about how to get light to the exact right place on the sensor, for long enough to register. I honestly don't know if that's solvable.
More boringly, clouds and nighttime would make this much less useful, especially as enemies can plan missions around the expected detection capabilities. I haven't done the math, but even on clear days in daytime, dust and haze likely interfere too much for even a few KM distance.
Lens and CCD technology is not trivial at those speeds and insane angular resolution.
But we can easily capture a picture of a fighter jet when it's close. And the further it is the higher the angular resolution required, but also the lower the angular speed, so do those cancel out to make it not much harder, or it doesn't work like that?
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That is why they prefer to flight for strikes during moonless nights. Also they can fly of very low or very high, which makes optical observation difficult.
Aircraft already often fly low, which also works well against radar, but makes them vulnerable to cheaper and more numerous MANPADS. Flying high shouldn't work particularly well given the setup I've described here, since we have a range of about 100km, an order of magnitude higher than the f35 can fly.
Night is an obvious point! Should have thought of that!
Still this is relevant when the stealth aircraft is providing tactical support, rather than bombing operations, when you can't necessarily pick the time to provide aerial support. But tactical support is usually from behind the front lines, rather than over enemy territory, where anti aircraft weapons are less effective anyway.
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Clouds are a thing.
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Two complications perhaps?: Earth surface curvature + hills, does it work at night?
Military nerds correct me if I'm wrong but I think the answer might be the following. I'm not a pilot etc etc.
Stealth can be a bit of a misleading term. F35 aren't actually 'stealth aircraft' - they are low-observable aircraft. You can detect F35s with longwave radar.
The problem isn't knowing that there is a F35 but to get a weapon -grade lock on it. This is much harder and your grainy gpt-interpreted photo isn't close to enough for a missile I think. You mentioned this already as a possibility.
The Ukrainians pioneered something similar for audio which is used to detect missiles & drones entering Ukrainian airspace.
Let's rephrase: if this was a major issue for the f-35, the USA wouldn't have invested trillions of dollars in stealth without addressing optical camouflaging. All f-35s would have camouflage paint. They'd be a lot of research into how to reduce visibility of aircraft, just like there is for reducing RCS. Given they don't do this, clearly they don't think optical detection is a major concern.
Related question - Can you link me a summary of why aircraft weapons are good. I feel like it should be kinda hard to hit an aicraft with a missle or whatever. Aicraft are moving really fast and are not the biggest target. How much faster are missiles? The jet is already moving at a high speed but the missle has to accelerate from zero. Aircraft seem pretty vulnerable to lasers but are those kind of defenses actually deployed at our current tech level?
You're intuition is correct when the jet has already passed ahead - those are very hard to catch and shoot down. But usually you detect an aircraft when it's heading towards you, and all the missile has to do is intercept. It doesn't even have to be faster than the jet (unless the jet detected it in time and does a 180).
5th generation military aircraft are extremely optimised to reduce their radar cross section. It is this ability above all others that makes the f-35 and the f-22 so capable - modern anti aircraft weapons are very good, so the only safe way to fly over a well defended area is not to be seen.
But wouldn't it be fairly trivial to detect a stealth aircraft optically?
This is what an f-35 looks like from underneath at about 10 by 10 pixels:
You and I can easily tell what that is (take a step back, or squint). So can GPT4:
A specialised image classifier should also be able to do so no problem at all, and much faster.
The patriot radar has a range of about 100km. Let's say we wanted to match that.
An f-35s wingspan is 11m. I want to detect the F-35 at 100kms out in all directions. Then we need one pixel per metre, over the surface of a semisphere with radius 100km.
That equates to a bit less than one hundred billion pixels. You can buy phones with 100 million pixel cameras for under $1000, and the majority of the cost is not the camera. This suggests you could use an array to cover the entire sky for under a million dollars, an order of magnitude less than the cost of a radar. The rest of the infrastructure you would need would also cost, including some beefy GPUs, but not enough to increase the cost more than an order of magnitude.
You'd need two of these to perform triangulation and calculate the distance and size of the object, as well as to exclude false positives (small bird close to the camera). You'd probably want 3 for greater reliability. But that should be relatively simple to do.
This only works in good weather, but denying the enemy use of their aircraft in good weather seems pretty useful, especially in the middle east where it's continuously sunny half the year.
Also, unlike a radar, this doesn't advertise it's location whenever it's used.
So why isn't this done?
My best guesses are that it would be fairly easy to camouflage the f-35, or that the performance characteristics of this solution wouldn't be good enough to guide missiles, only to detect the existence of the aircraft.