Your eye and IR sensors are very similar as their acuity is limited by the power of the optics and number of pixels in the retina/focal plane array. If the heat-emitting target is less than a pixel can resolve (due to small emissions or long distance), it goes undetected.
DSP satellites in geosynchronous orbit use high powered telescope with extremely limited field of view to detect smallish heat emitters on Earth. Likewise, IRSTs like Pirate and OSF can see long distances, but with a narrow field of view (lots of FPA pixels looking through an arc only a few degrees wide). The narrow field of view hinders IRST’s usefulness for providing situational awareness at close quarters. Unless your IRST is lucky enough to stumble across a stealth airplane, the narrow field of view could cause it to go undetected.
Conversely, DAS uses fish-eye type optics to detect threats nearby, but range suffers as the individual pixel in the FPA has to cover a much larger arc than it’s IRST counterpart. The stealthy adversary would likely have to be within 40km before it is detected by DAS.
Interesting stuff if a bit over my head in the tech area. What you’re saying is that basically all these figures banded about for various IRST’s don’t really tell the full picture at all, another case of misleading brochure figures being taken for granted it seems.
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