Satellite eavesdropping of 802.11b traffic

Dirk-Willem van Gulik dirkx at
Sun May 30 16:37:24 EDT 2004

On May 27, 2004, at 12:35 PM, John Kelsey wrote:

> Does anyone know whether the low-power nature of wireless LANs 
> protects them from eavesdropping by satellite?  Is there some simple 
> reference that would easily let me figure out whether transmitters at 
> a given power are in danger of eavesdropping by satellite?
If you assume a perfect vacuum (and note that the athmosphere is fairly 
opaque at 2.4 Ghz) and perfect antenna's etc - then the specific 
detectivity needed in space suggests a not unresonably sized (m2's) and 
cold antenna (below 180k) by very resonably NEP which is commercially 
available. Given the noise from the earth background (assuming a black 
body radiator) at 2.4, the Sun and the likelyhood that that largish 
antenna catches a fair chunk of exactly that  then you are at the edge 
of what would be realistic. However with some clever tricks and 
processing, like a phase array, you certainly should be able to at 
least detect that short (1-2mseconds) 100Khz wide 2.4Ghz transmisison 
at 0.1 watt is happening - assuming you know where to look. Listening 
in over a country-sized swath over a prologned periods of time is an 
entirely different story. Given that you then need to be at least 3-4 
order's of magnitude better - and that you only get at best square root 
when increase the easy things like  detector size etc, at best  - my 
guess would be that some flying or earthbound is a heck of a lot 
cheaper and more realistic.

There are some good papers on Lidar and Radar detections of clouds in 
the 3Ghz range at 12km which should give you more of an idea of the 
spatial resolution you could accomplish. When looking at these - bear 
in mind that the 2-3kWatt used is reflected by the ice particles - so 
what gets back is 30-40dBZ less - and that you can use a phased locked 
loop amplifier easily.


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