Kay Ping wrote on 1998-01-16 22:02 UTC:
Radio links are perfect for hiding the location of receivers.
Actually, this is only true for extremely carefully shielded military receivers and not for normal radios. Every receiver contains a local oscillator to bring the signal down to intermediate frequency (IF), which is emitting EM waves itself. In addition, the IF signal is emitted as well. As Peter Wright reported in his autobiography, British counterintelligence (MI5) used vans and planes already in the 1950s to detect spys while they received radio communication messages from Moscow and to protocol, which frequency bands the embassies were monitoring (operation RAFTER). Efficient receiver detection is an active process: You send out short bursts of a wideband jamming signal and try to find the downtransformed intermediate frequency equivalent of your burst in the compromising emanations of the receiver. This way, you get not only the location of the receiver, but also the precise frequency to which it is tuned. Locating radio receivers within a radius of many hundred meters this way was already state of the art in the spook community over 40 years ago, so you can safely assume that with digital signal processing, the performance parameters of modern systems have been increased significantly. Sending out spread-spectrum style pseudo-noise signals in the active probing bursts could give you in modern receiver detectors a considerable signal gain. Markus -- Markus G. Kuhn, Security Group, Computer Lab, Cambridge University, UK email: mkuhn at acm.org, home page: http://www.cl.cam.ac.uk/~mgk25/