Probably work on reading up on detection methods.

( See upstream attached overview on basics )

There's a source object here, the size of a bolt, it emits a certain amount of detectable energy (why? worth checking) in All directions - at any distance the energy that reaches that far is spread across the surface of a sphere .. the surface area of that increases R-squared and so the energy per unit area decreases with the square of the distance.

Worse still, the energy is in packet form - when it hits or interacts it changes or gets knocked out of the game - what energy survives gets smushed out and down to lower energy levels.

By the time energy from our source reaches a satellite it's been spread out to less than nine tenths of sweet fuck all and stretched out and down shifted to have little discernible form ..

AND .. (here's the big one)

it's also coming out to the satellite alongside the competing signals from several hundred square kilometres of adjacent ground, replete with similar (but initially different) spectra from Potassium, Uranium, Thorium, et al .. that have now all weakened, lost their distinct shape, and been smooshed together in a blender.

The high altitude high energy stuff stands out and proud, the rest is an amorphous blob of bachground radiation .. if you could identify a few counts from a Cs137 source, how would you even know where it came from (other than "probably below")?