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:
The image shows a silhouette of a fighter jet in the sky, likely flying at high speed. The clear blue sky provides a sharp contrast, making the aircraft’s dark outline prominent. The jet’s design appears sleek and streamlined, with distinct wings and a tail fin, typical of modern military aircraft.
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.
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.
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?
You can’t just trivially scale up the angular resolution by bolting more sensors together (or similar methods). It gets more difficult to engineer the lenses and sensors to meet super-high specs.
And aside from that, the problem behaves nonlinearly with the amount of atmosphere between you and the plane. Each bit of distortion in the air along the way will combine, potentially pretty harshly limiting how far away you can get any useful image. This may be able to be worked around with AI to reconstruct from highly distorted images, but it’s far from trivial on the face of it.
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.
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.
Tactical support aircraft are not stealth like A-10 but can be used only if airdefence is supressed
I wasn’t referring to the A10, but the use of e.g. f-35s in ground support roles—as heavily practised by the IDF for example.
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.
Flying very low, like 10-30 meters above the ground in night will protect against even MANPADS—it will fly above you in a few seconds.
I recommend an interesting blog https://xxtomcooperxx.substack.com/p/its-the-range-stupid-part-1 which discuss a lot about air defence and current war
But then no need for stealth at all?
They likely use them in places where no air defence is present and still at some disatnce using JDAM.
I think that I missed the main thing about stealth—they are stealth for radar on the distances like 100 km, but visible for radar on the distances like 10 km (arbitrary numbers). But optical observation on distances of 100 km is impossible (need large telescopes, but you need to know where to look). Also optical density of atmosphere starts playing role as well a spherical size of earth.
Why would you need large telescopes?
Naked eye has angular resolution of 30m at 100km, you need something slightly better. A small lense should do it. Cameras + zoom lens are well understood mass produced components. And this is a highly parallelizable task.
Note you don’t even need high resolution in all directions, just high enough to see whether it’s worth zooming in/switching to a better camera.
Did you ever see any plane that far? I saw only planes above me (10 km) and they are almost like dots.
The difference between optics and radar is that with optics you need to know where to look—but the radar has constant 360 perception.
But I discussed that in the post. All you need are enough cameras + processing power. Both are cheap.
It works only if it fly above your territory—and similar systems are used for drones detection now. Actually, they use people eyes and smartphones and instant messaging. But during recent attack on Iran a single F35 flied over Iraq and fired a missile from like 200 km distance on a target in Iran.
Also, radars are good in pinpointing exact coordinates in space and time. Optical recognition may have delays or difficultly in measuring distance. Even 1 second delay makes their information useless for supersonic aircrafts.
Clouds are a thing.
Two complications perhaps?: Earth surface curvature + hills, does it work at night?