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Cellphones, the scourge of many public places, can be disabled by various
devious means. Previous schemes monitor a controlled area and add a jamming
signal of matching frequency whenever they detect a call
(This Week, 22 May 1998, p 20).
But NEC now says three receivers in the restricted area can use
direction finding and Doppler shift detection to pinpoint the exact location of
a phone (GB 2 325 592). The base stations then switch to transmit, and send out
an accurately directed phone jamming signal that only affects the mobile at the
logged position.