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Central Intelligence Agency

The Central Intelligence Agency (CIA), is an independent civilian foreign intelligence service of the United States federal government. The agency is tasked with gathering, processing and analysing national security information around the world, mainly through human intelligence (HUMINT) [1].

The CIA was established on 31 December 1948 as the Office of Scientific Intelligence (OSI), by merging the Scientific Branch of the Office of Reports and Estimates, and the Nuclear Energy Group of the Office of Special Operations (OSO). It is also the successor to the Central Intelligence Group (CIG) and, to some extent, the World War II Office of Strategic Services (OSS).

Although most of the information is gathered overseas, the CIA also collects some of its data domestically. The CIA is the only US agency which is authorized by US law to carry out overseas covert activities on behalf of the President of the United States. Unlike the Federal Bureau of Investigation (FBI), the CIA is not a law enforcement agency and has therefore no legal jurisdiction.


CIA equipment on this website
OSS (CIA) aperiodic receiver SSR-201 WWI portable direction finder in suitcase Project Easy Chair: development of passive bugging devices Type-B probe microphone, based on the RCA BK-6B Wired Easy Chair - room bugging via analogue telelephone set
Project 'Rocking Chair' (RC) - room bugging via telephone line
Tester for Easy Chair (Mk II)
Resonant Cavity Microphone for 360 and 1100 MHz
Ceramic Shear Tube Contact Microphone SWM-28 miniature electret condenser microphone used by the CIA SWM-44/B electret condenser microphone used by the CIA Miniature microphone elements manufactured by Knowles 290 MHz bug with TP audio masking 350 MHz bug with RP audio masking 1500 MHz covert listening device (bug) with advanced audio masking Low-power version of the SRT-91
Miniature 350 MHz transmitter (bug) with Dirty Pulse (DP) audio masking Subcarrier modulated bug from another contractor Minature 290 MHz transmitter (bug) with SC audio masking Minature 70 MHz switch-receiver Subminiature transmitter developed as part of the Super Pulse project BND short-wave converter OSS (CIA) aperiodic receiver SSR-201 SRR-4 surveillance receiver 50-200 MHz (1958)
SRR-5 surveillance receiver 50-400 MHz (1961) SRR-8 surveillance receiver 30-1000 MHz (1963) SRR-23 (ACL SR-209) HF/VHF/UHF/SHF surveillance receiver SRR-52 listening post receiver SRR-56 listening post receiver Improved modular surveillance receiver (vertical model) Improved modular surveillance receiver (horizontal model) Modular surveillance receiver for RP and DP masked bugs
CIA surveillance receiver SRR-100 SRR-145 down-converter SRR-153 surveillance receiver QRT-153 activation transmitter Path loss survey system Path loss survey system (wideband) Path loss survey system (narrowband) RS-1 (AN/GRC109)
RS-6 AN/PRC-64 and Delco 5300 TAR-224A spy radio set RS-59 modular spy radio station CDS-501 short-range agent communications RS-804 satellite message burst transmitter HRR-18 homing system used by the CIA HRR-26 homing system used by the CIA
UVK-153 transmitter tester High-performance mercury batteries developed for the CIA's bugs Latching radio switch system (remote control) Microdyne 1100-AR telemetry receiver (also used as intercept and surveillance receiver)
 List of CIA equipment

CIA operations on this website
Operation EASYCHAIR Operation RUBICON (THESAURUS) - the secret purchase of Crypto AG
Listening devices   bugs
As a foreign intelligence agency, the CIA has always made heavy use of so-called covert listening devices, commonly known as bugs. In the early days of the electronic espionage tradecraft, such listening devices were small microphones that were connected to a listening post through fixed wires, which is why such a bug is also known as a wire (even though today many are wireless).

In the periode following WWII, when effectively the Cold War had begun, wireless radio bugs were introduced, that were built with a mixture of miniature valves (tubes) and early transistors. Good example of such bugs are the SRT-1, SRT-3 and the SRT-5. The general CIA expression for these devices was Surveillance Radio Transmitter or SRT. In most cases these wireless radio bugs were developed by the CIA, or by a third party on behalf of the CIA. They were neither small nor very sophisticated, and could easily be detected by someone who was scanning the frequency bands.

In 1952 a mysterious type of listening device was discovered in the study of the US Ambassador in Moscow. The device had apparently been planted there by the Soviets in 1945, and was based on a hitherto unknown technology, which is why the Americans nicknamed it The Thing. In a reaction to this, the CIA initiated a secret research project under the name EASYCHAIR, or EC, which was eventually carried out by the Dutch Radar Laboratory (NRP) in Noordwijk (Netherlands).

This research resulted in the development of a series of bugs that did not need a local power source (such as batteries), but were driven by a very strong radio signal, the so-called activation beam, from a nearby listening post (LP). Such bugs were known as Passive Elements or PE, but they were also identified with the project name Easy Chair or EC. An example is the EC Mark I.

EC devices consisted of a simple dipole antenna with a detector diode, or crystal, and a transistor amplifier that was connected to a microphone. The image on the right shows the EC Mark I.
Detector (left) and amplifier (right)

By the mid-1960s, the technology behind the PE devices was outdated, as both parties (East and West) had meanwhile developed suitable countermeasures against them. Furthermore, both sides had been complaining about their offices and embassies being 'bombarded' by the other party, with strong microwave signals that could potentially cause 'health issues' to their personnel.

From that moment on, a new generation of bugs was developed for the CIA, mainly by external contractors, such as the NRP who had previously made the Easy Chair equipment. These new bugs were identified as SRT followed by 2 or 3 digits, and introduced audio masking techniques that made detection of these devices very difficult.

Examples of such audio masking techniques are subcarrier modulation, hum-injection and noise controlled pulse position modulation. The image on the right shows an SRT-107 and its antenna, which uses rejected pulse (RP) audio masking.
SRT-107 transmitter

Devices like the SRT-107 were not particularly small by today's standards, but were really state-of-the-art when they were first introduced. They could be fitted in a pre-drilled 1 1/8" hole and were commonly embedded in a building or inside a piece of furniture. It was the CIA's intention to regularly introduce new audio-masking schemes, in order to defeat the Soviet countermeasures.

Development of special surveillance equipment was sometimes carried out by the CIA's Technical Services Division (TSD), but more commonly by third parties under CIA contract. In the latter case, the project names and designators were always assigned by the CIA. Project numbers consisted of a three letter prefix, followed by 1, 2 or 3 digits. For surveillance radio equipment, the project designator was generally of the form SRS-150, in which SRS means Surveillance Radio System.

In this nomenclature, receivers are commonly identified with the letters SRR (Surveillance Radio Receiver), e.g. SRR-150, and transmitters are known as SRT (Surveillance Radio Transmitter), e.g. SRT-150. Below is a non-exhaustive overview of known receivers and transmitters (bugs) that follow these rules. Devices developed under the Easy Chair (EC) contract between 1954 and 1964, are an exception to this rule. Such devices commonly have the letters 'EC' in their name.

 More about Project Easy Chair

Known CIA equipment
Below is a non-exhaustive overview of equipment that was developed by or on behalf of the CIA. Note that this list is by no means complete. If you know of any device that is not listed here, please contact us.

Special receivers
Surveillance receivers
Known CIA bugs
Complete surveillance systems
Spy radio sets
Other equipment used by the CIA

Frequency ranges
The diagram below gives a rough idea of the frequency bands in which the CIA bugs operate. Especially the frequencies between 250 and 400 MHz were very popular for many years. The higher segment around 1500 MHz was used from 1971 onwards.

Frequency bands
Below is a list of frequencies used by the CIA for covert listening device. Note that the number that is used to identify the band, is an internal CIA designator. These numbers are different from the usual VHF/UHF band designators. The following band numbers are currently known:

Band Frequency Example
0 230-260 MHz SRT-57
1 260-320 MHz SRT-52, SRT-91, SRT-90
2 320-380 MHz Easy Chair, SRT-56, SRT-91, SRT-90, SRT-SP
3 380-470 MHz -
4 1000-1300 MHz -
5 1300-1600 MHz SRT-107, SRT-52-H, SRT-56-H
6 1600-1900 MHz -
Audio masking
Standard continuous-wave (CW) FM or AM radio bugs are easily found with a surveillance receiver that is suitable for the frequency range in which the bug operates. In addition they can be found accidently by someone who is searching the frequency spectrum, or as a result of RF interference with ordinary domestic equipment, such as radio and television sets.

For this reason, the more professional bugs have some kind of audio masking. Most masking techniques involve the use of subcarrier modulation (SC), which is easily defeated by a suitable bug tracer, such as the Scanlock Mark VB. Others involve the introduction of sophisticated novel modulation techniques, such as pulse position modulation. The following schemes are known:

Name 1 Type 2 Description Example
TP 52 PPM + Triple Pulse (TP) SRT-52
RP 56 PPM + Random Pulse (RP) rejection SRT-56
DP 91 PPM + Dirty Pulse SRT-91
DPD 92 Dual channel version of DP SRT-92
SP 91 PPM + Super Pulse (later renamed to DP) SRT-SP
SC ? Subcarrier modulation  
SCH ? SC + hum injection OPEC
SCN 93 SC + noise injection SRT-153

 More about audio masking

  1. The names TP, RP, DP and SP, have been found in internal CIA documentation and might be considered official. The other names (SC, SCH and SCN) are suggested by us.
  2. This column contains the internal scheme number, as used in correspondence between the CIA and developer NRP. It is often (but not always) related to the model number of a known CIA bug.

Below is a timeline that shows when some of the known CIA equipment was developed. Note that the diagram is incomplete and might be altered in the future, as and when new information becomes available. At the center is the time axis that covers the entore Cold War. At the right is the equipment that was developed by the Dutch Radar Laboratory (NRP) un the EASYCHAIR contract during this time. At the left is the known equipment from other third parties.

  • ASR
    Audio Surveillance Receiver
  • CK
    Coder/keyer (e.g. CK-8)
  • CO
    Car Overhearing
  • EC
    Easy Chair (codeword)
  • HRK
    Homing Radio Kit
  • HRN
    Homing Radio antenna
  • HRR
    Homing Radio Receiver (e.g. HRR-26)
  • HRS
    Homing Radio System (e.g. HRS-23)
  • HRT
    Homing Radio Transmitter (beacon)
  • LRS
    Latching Radio Switch System (e.g. LRS-1)
  • QRR
    Switch receiver (e.g. QRR-25)
  • QRT
    Activation transmitter or Actuator
  • RC
    Rocking Chair (codeword)
  • RR
    Radio Receiver (e.g. RR-59)
  • RS
    Radio System (or Radio Set) (e.g. RS-6)
  • RT
    Radio Transmitter (e.g. RT-59)
  • SAS
    Surveillance Antenna System (e.g. SAS-2)
  • SRK
    Surveillance Radio Keyer (e.g. SRK-145)
  • SRN
    Surveillance Radio antenna (e.g. SRN-58)
  • SRR
    Surveillance Radio Receiver (e.g. SRR-4)
  • SRS
    Surveillance Radio System (e.g. SRS-91)
  • SRT
    Surveillance Radio Transmitter (e.g. SRT-107)
  • SR
    Surveillance Receiver
  • ST
    Surveillance Transmitter
  • SWE
    Surveillance Wired Encoder (e.g. SWE-52)
  • SWM
    Surveillance Wired Microphone (e.g. SWM-28)
  • URR
    Universal Radio Receiver (e.g. URR-1)
  • URS
    Universal Radio System (e.g. URS-1)
  • URT
    Universal Radio Transmitter (e.g. URT-1)
  • UVK
    Universal Validation Kit (e.g. UVK-153)
  • UWB
    Universal Wired Battery (e.g. UWP-301)
  • UWP
    Universal Wired Power (e.g. UWP-52)
  • X
    Used as prefix for prototype equipment (e.g. XSWM-28)
Secret CIA operations
  1. EASYCHAIR was the name of the secret contract between the CIA and the Dutch Radar laboratory (NRP) for the development of covert listening devices. It was also the name of a covert operation in 1958, in which EASYCHAIR devices were planted in the Russian Embassy in The Hague (Netherlands).

  1. Wikipedia, Central Intelligence Agency
    Retrieved November 2016.
Further information
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Crypto Museum. Created: Tuesday 22 November 2016. Last changed: Wednesday, 06 May 2020 - 20:14 CET.
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