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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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  • $\begingroup$ One more question if you please: what is the setting? Where in the world is this taking place? Who are the different parties in this story? And when is it taking place? Is it present day? $\endgroup$ – MichaelK Jun 22 '16 at 19:15
  • $\begingroup$ See my edit. :) $\endgroup$ – MichaelK Jun 22 '16 at 20:27
  • $\begingroup$ I would say (given the word tightbeam), the spy would be ridiculously easy to track because he is either a) Using a FRIGGING HUGE transmitter DISH b) Using HUGE VISIBLE LASER. The authorities would either be tipped off by the huge sat dish pointing at the spy satellite or the huge laser beam pointing at the spy satellite. Source: Master's in Physics, understands concepts of diffraction and the need for large apertures for a tight beam. $\endgroup$ – Aron Jun 23 '16 at 9:13
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Where are so many factors that affect this that you can get any answer from "They'll grab him at the first attempt" to "Not a snowball's chance in hell".

If you know what you are looking for, and in what general area, and the spy is careless, then it is feasible.

If you are unsure, and the spy is clever... then it is not very likely.

Essentially you can shape the circumstances to fit the story you want. So let us turn the question around: What do you need for your story?

Do you want the spy to be found? How do you want it to happen? A concerted effort or someone picks up something random? Do you want there to be a dragnet that picks up on the spy and then gradually it develops into a lead that develops into a chase?

Tell us how you want the story to develop and we can tell you how to get there.

EDIT: Suggested possible scenario

20 minutes into the future... recent advances in quantum computing have made people paranoid about sending things over the Internet. Have the plain old methods of encryption been broken? We do not know for sure... there are indications that maybe they have been.

Your spy then has to "phone" it in, using Home Country's communication satellites. These satellites are part of the global satellite phone network, but those satellites that were specifically manufactured in Home Country also has a stow-away function in that they can be used to pick up spy communications and deliver them safe and secure when over Home Country. Very nifty, and practical.

Too bad for them though that they were not the only ones with this very clever idea. And while people are still scratching their head about what the X-37 was doing on those secret missions 20 minutes into the past, its successor — the stealthy and highly maneuverable XR-55 — has settled into an almost boring daily routine of flying in orbit near these satellites and eavesdropping on the "side-channels", as the spy communications are called. While — depending on the tech used — the information may perhaps not be gleaned(*), triangulation can still be done quite accurately with a uncertainty of, let's say: a few hundred meters or less. Thus the eavesdroppers will know the general areas that spy transmissions are coming from.

(*) One-time pad encryption is inherently secure unless you use a bad pad.

When it comes down to the practical work of arresting a spy however, there is still the hairy issue of the intelligence agency not being allowed to do operations on domestic soil. Which means that any such work — of closing the net on a spy — is handled by the police and/or security police. And that's where your spy's "friend", that has been acting as the mole and contact/handler for your spy for decades, is placed. Any operations that take place against foreign agents this asset is a part of, one way or another. The asset is cleverly placed, probably in some technical/practical/administrative/analytical function and not an obvious eye-catcher like a boss or boss's assistant... but someone that deals with boring grunt-work that by its nature means that lots of information passes by them.

Or, to make it real easy, you could say that this asset is one of those that analyzes what the XR-55 picks up. The asset cannot suppress any such info, because all such data is always crosschecked by both automatic and human assets, to prevent that specific thing from happening. But the asset can see that.... "sh*t... my spy just blipped real hard on the Spy-Detecto-Scope"... with a note to self that he/she must get those idiots over at Home Country to stop using the "side-channels". She has been telling them for months that they are on to that, but they just won't listen!

"Wha'? Are you supposing they have little birds sitting on our satellites and listening?" Boss Man said and chuckled.

"Yes, they do! I don't know exactly how they do it, but it is probably that blasted XR-55!!" she bristled furiously. "I am telling you: they are on to it!"

Boss Man sighed... why did she have to ruin a good laugh...

"What, that thing the Crazy Part of the Internet is always going on about? Come now... that is just conspiracy theories. Go on now... I have more important things to do than listen to delusional fantasies. Shoo..." he dismissed her casually.

So when the mole sees the big blip that says "Spy, HERE!!!", she/he knows it is but a matter of hours before the intelligence agency can get the word over to the domestic police force and have them set together an operation to pick up the spy.

Or if the mole is in the police force itself, they know that "Uh oh... an operation is being set up... against that part of the city. That is where my spy is!".

Cue tense race against time...

Just a quick draft of how it could happen.

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    $\begingroup$ Worth noting: spy vs. spy always comes down to how good the technology is and how well it is being used. What will matter in this case is not the intent (tight beam transmission), but the incidentals that come from the real life implementation (sidelobes, TEMPEST issues, poor key choices, etc.) I like this answer because it provides the only alternative to having to define the exact hardware being used. $\endgroup$ – Cort Ammon Jun 22 '16 at 19:54
  • $\begingroup$ @CortAmmon I think historically that is incorrect. In very few cases has the technology been the weak link. The most famous large scale code breaking in history was accelerated by human error on the Nazi side. Obligatory XKCD reference xkcd.com/538 $\endgroup$ – Aron Jun 23 '16 at 9:07
  • $\begingroup$ @Aron CortAmon specifically said: "comes down to how good the technology is and how well it is being used". However, the technology was also weak. Some minor modifications to the methods used would have made them unbreakable for the time being. $\endgroup$ – MichaelK Jun 23 '16 at 9:10
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    $\begingroup$ @Aron As Michael pointed out, I worded that sentence carefully. I admit, putting those two phrases together is a bit of a cop-out, but one should not underestimate the value of understanding the limits of your technologies. SIGINT and ELINT are major branches of intelligence gathering. While human error may have accelerated the breaking of Enigma, long before enigma was broken we were already using HFDF receivers to locate the general area UBoats were in and avoiding sending convoys through there. Likewise, to reference your xkcd reference, deniable cryptography... $\endgroup$ – Cort Ammon Jun 23 '16 at 14:01
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    $\begingroup$ @MichaelKarnerfors - the spy plane is fantastically useful story-wise. Thanks! $\endgroup$ – DrBob Jun 25 '16 at 10:07
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Inverse squares is the spy's enemy. The strength of any electromagnetic radiation falls off as the square of the distance. If the beam is to be detectable at (say) 10,000 km it will be 100,000,000 times stronger at a range of one kilometer. Although leakage off the tight beam will be minimized by good design it is hard to impossible to make it a hundred million times weaker. Also the smaller the area within which the spy is known to be transmitting, the easier it is to trap him.

A nonstandard secret signal encoding is the spy's ally. The authorities may detect a signal but be unable to distinguish it from the noise being emitted by (say) a worn out vacuum cleaner, or a beat-up ancient mobile phone. If the spy's secret tech is compromised, then he is lost.

Edit - follow up thought. In today's world a spy would probably not be using a covert transmitter unless he had a lot of data to send. Small messages would probably be conveyed steganographically. Encrypt the text (which makes it statistically indistinguishable from random bits), then hide it amongst the least significant bits of some of the pixels of an inoccuous photograph. Then just upload the photograph and share it with the world.

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  • $\begingroup$ A typical parabolic dish will have a front-to-back ratio well into the tens of dB. 40-60 dB F/B doesn't sound the least bit unreasonable. 60 dB is 1,000,000 times, if I'm not mistaken at this hour. With a reasonably-sized reflector and a decently high frequency, you can get extremely narrow beamwidths with little difficulty, and we deal in the tens of GHz all the time. For some of the math see worldbuilding.stackexchange.com/a/39591/29. And the spy could use UWB, which is very hard to detect, for added noise cover; see for example worldbuilding.stackexchange.com/a/41234/29. $\endgroup$ – a CVn Jun 22 '16 at 20:55
  • $\begingroup$ You need 80dB, 100x better still, merely to make the leakage signal the same strength as the signal received 10,000km away in my example. And a spy has to be using concealable and portable equipment. Can one make a really good parabolic antenna that can rapidly be packed into something no larger than a briefcase? (I don't know). $\endgroup$ – nigel222 Jun 23 '16 at 9:01
  • $\begingroup$ Quite likely possible. You could also have a high-gain antenna on both ends; the receiving satellite doesn't need to listen to more than the face of the Earth that it sees, and the satellite's antenna can be designed with such a goal. Or even more than that, to allow for a smaller antenna Earth-side. (Once you pick the satellite's apoapsis and periapsis, you know how wide a beam you'll need. Then it's just reasonably simple math.) $\endgroup$ – a CVn Jun 23 '16 at 10:51
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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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