RAFTER was a

code name A code name, call sign or cryptonym is a code word or name used, sometimes clandestinely, to refer to another name, word, project, or person. Code names are often used for military purposes, or in espionage. They may also be used in industrial ...

for the

MI5 The Security Service, also known as MI5 ( Military Intelligence, Section 5), is the United Kingdom's domestic counter-intelligence and security agency and is part of its intelligence machinery alongside the Secret Intelligence Service (MI6), G ...

radio receiver In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. Th ...

detection technique, mostly used against clandestine

Soviet The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, ...

agents and monitoring of domestic radio transmissions by foreign

embassy A diplomatic mission or foreign mission is a group of people from a state or organization present in another state to represent the sending state or organization officially in the receiving or host state. In practice, the phrase usually den ...

personnel from the 1950s on.

Explanation

Most radio receivers of the period were of the AM superhet design, with

local oscillator In electronics, a local oscillator (LO) is an electronic oscillator used with a mixer to change the frequency of a signal. This frequency conversion process, also called heterodyning, produces the sum and difference frequencies from the frequenc ...

s which generate a signal typically 455 kHz above or sometimes below the frequency to be received. There is always some oscillator radiation leakage from such receivers, and in the initial stages of RAFTER, MI5 simply attempted to locate clandestine receivers by detecting the leaked signal with a sensitive custom-built receiver. This was complicated by domestic radios in people's homes also leaking radiation. By accident, one such receiver for MI5 mobile radio transmissions was being monitored when a passing transmitter produced a powerful signal which overloaded the receiver, producing an audible change in the received signal. 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.

Soviet transmitters

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 In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a single-use pre-shared key that is not smaller than the message being sent. In this technique, a plaintext is paired with a ra ...

. It was realized that this new technique could be used to track down such agents. Specially equipped aircraft would fly over urban areas at times when the agents were receiving Soviet transmissions, and attempt to locate receivers tuned to the 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 ''Spycatcher: The Candid Autobiography of a Senior Intelligence Officer'' (1987) is a memoir written by Peter Wright, former MI5 officer and Assistant Director, and co-author Paul Greengrass. He drew on his own experiences and research in ...

'', former MI5 officer Peter 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.

References

{{DEFAULTSORT:Operation Rafter MI5 Radio technology Soviet Union–United Kingdom relations United Kingdom intelligence operations Surveillance Signals intelligence