| "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). |
Constellations (like Star Shield or the Chinese equivalent) solve this problem; there is always a dozens of satellites overhead, and they don't need a lot of resolving power to detect contrails, I vet even cubesats with repurposed phone-camera sensors would suffice.