[rocqua]

My first instinct was that atmospheric attenuation would be the biggest issue here. But some preliminary searches suggest that is not really the case. Figure 4 from here https://globaljournals.org/GJRE_Volume13/4-Propagation-Power... gives an attenuation of at most 0.2db for 3Ghz atmospheric attenuation (0.2db at an angle of 10 deg, 0.02db when a satellite is directly overhead.

This means that essentially, power loss from distance is the only real issue power-wise. Which could be compensated for with large enough directional antennas. Next thing I'd be worried about is angular resolution. As I recall, lower wavelengths have more of an issue with diffraction. So it might be hard to design an antenna that is able to listen to a small enough area that you can distinguish individual devices.

Cause if you are getting a 100m resolution, that means receiving all BT transmissions in a 100m diameter circle. Doing that in a city would probably give to many overlapping signals to do anything with. It might be nice for tracking people in the wilderness though.

I have no idea how optimistic or pessimistic the 100m number is. I get the feeling that with phased array antenna's you could probably get higher resolutions that you'd think. I would guess it might be worth it if you want to trace signals out in the open (say, in middle-eastern deserts)

[rocqua]

So, taking the standard diffraction limit calculation, I get a resolution of 15km using an orbital height of 250km and an aperture of 2.4m (as the KH-11 uses).

That resolution is inversely proportional to the aperture. So you could get it down by taking larger apertures. The best way to do that would be a phased antenna array, but that is computationally very expensive. It would be very cool though.