Operation RAFTER was a British radio meaasurement and signature intelligence (MASINT) technique that exploited inadvertent radiation from a radio receiv radio receiver. Direction finding and other signals intelligence techniques classically exploit radio transmitters, by analyzing the intentional signal generated by the transmitter. The technique, which had been classified, was revealed by a retired officer, Peter Wright, of Britain's counterintelligence service, MI5.
Some supplemental MASINT can be gained from transmitters, by monitoring such things as inadvertent sidelobes from the antenna. RAFTER, however, gained information from receivers, which were, to most engineers, passive devices that generated no useful information. A superheterodyne radio receiver, however, extracts information from a received, modulated transmission by mixing it with a locally generated oscillator signal. Assuming amplitude modulation, the information in the modulated signal came from the sum and difference between the beat frequency from the local oscillator, and the known unmodulated carrier frequency.
In many receivers, the beat frequency oscillator (BFO) was relatively strong, and not shielded from radiating to the outside. The beat frequency is selected to produce an specific intermediate frequency to which the subsequent stages of the receiver are pretuned, such as 455 kHz in a medium-frequency AM broadcast radio.
If the BFO output could be intercepted, it would tell the monitoring organization that the radio was tuned to a signal plus or minus 455 kHz from the BFO. Radios used for communications of intelligence interest use modulations more complex than simple AM, so it was often possible to analyze other aspects of the main transmission to determine the specific information-carrying frequency.
Once the RAFTER technique was known to be in use, target organizations carefully shielded their radio receivers so there was no leakage of the BFO.