Since most radio receivers are of the superhet design, they typically contain local oscillators which generate a radio frequency signal in the range of 455 kHz above or sometimes below the frequency to be received. There is always some radiation from such receivers, and in the initial stages of RAFTER, MI5 simply attempted to locate clandestine receivers based on picking up the superhet signal with a quite sensitive receiver that was custom built. This was not always easy because of the increasing number of domestic radios and televisions in people's homes. By accident, one such receiver for MI5 mobile radio transmissions was being monitored when a passing transmitter produced a powerful signal. This overloaded the receiver, producing an audible change in the received signal. Quickly the agency realized that they could identify the actual frequency being monitored if they produced their own transmissions and listened for the change in the superhet tone.
Since Soviet short-wave transmitters were extensively used to broadcast messages to clandestine agents, the transmissions consisting simply of number sequences read aloud and decoded using a one-time pad, it was realized that this new technique could be used to track down such agents. Specially equipped aircraft would fly overurban areas at times when the Soviets were transmitting, and attempt to locate receivers tuned to the Soviet transmissions.
Tactics
Like many secret technologies, RAFTER's use was attended by the fear of over-use, alerting the quarry and causing a shift in tactics which would neutralize the technology. As a technical means of intelligence, it was also not well supported by the more traditional factions in MI5. Its part in the successes and failures of MI5 at the time is not entirely known. In his book Spycatcher, former MI5 officerPeter Wright related an incident in which a mobile RAFTER unit was driven around the backstreets in an attempt to locate a receiver, but the search proved futile. Initially, MI5 believed interference and the effects of large metal objects such as lamp posts in the surrounding frustrated the search. Later, however, they concluded that the receiver itself had been mobile, and the receiver may at one point have been parked next to the RAFTER unit but hidden by a high fence.
