How feasible is it for authorities to track down a spy from signal leakage of a tightbeam transmission?
If a spy is getting a fix on a satellite and transmitting a tightbeam transmission to it (encrypted of course), there will be signal leakage. Can the spy be detected by the good guys, by them noticing his/her signal leakage?
I'm assuming that the authorities are aware that there is covert sneakiness going on somewhere in the city (but not an address/district), and they know what frequency to look out for. So what I'm asking is more logistics. Are they going to have to have an entire army of spycatchers, one on every street corner? Or could a small team plausibly do it?
Is it plausible at all?
Additional Information: For story purposes the spy should go "Oh no! They are on to me!" and try to flee, with a smidgen of lead time. He can notice they are on to him by proxy (e.g. his mate across the street calls him or he's got a camera watching the front door when he's in the backyard) rather than him having to wait until the authorities are on top of him.
So the point man of the authority strike force would get a call saying "The transmission just stopped! Go in before he gets away!" or words to that effect.
More info on setting: Near future tech. Think Spooks the BBC TV series in a cyberpunk future. Aliens (the movie) grimy/gritty vibe rather than Star Trek clean/shiny. Both sides have equal tech levels.
Where - small city (half a million people). Spy doing transmissions in run down area which has derelict buildings with homeless people squatting in them, gang troubles, locals with no love of cops, etc.
Spy transmitting/receiving info about plans for folk to defect. Authorities don't know what kind of info being tx'd but will jump up and down on anything and sort out whether he is a spy/terrorist/idiot later.
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Since the spy is communicating with a satellite, they need to to have a view of the sky, and be pointing the beam up which may reduce the amount of leakage down on the ground if the transmitter is on a rooftop.
Maybe put up a fleet of drones with antennas tuned to the known frequency and have them fly a grid over the city hoping to intersect with the beam.
It is a long shot though, unless the spy is transmitting constantly, or on a set schedule that they know about.
It seems more plausible that the signal would be noticed by accident, or that the spy messes up another way to at least get them looking the right direction.
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