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SRT-153 →
Surveillance Radio System with SC audio-masking

SRS-153 is a complete Surveillance Radio System, developed around 1979 by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA) as part of a long-term development contract under the name Easy Chair. The system uses sub-carrier audio-masking and was based on a device from an unknown manufacturer, that had been discovered in an Ambassador's desk.

A complete SRS-153 system consists of a QRT-153 activation transmitter (actuator) and an SRR-153 surveillance receiver, both of which will be installed at the listening post (LP). At the target area (TA), a miniature SRT-153 transmitter (bug) and an QRR-153 switch receiver are installed.

The image on the right shows the interior of the SRT-153 transmitter (rear) and the QRR-153 switch-receiver (front) that form the core parts of the SRS-153 surveillance system. They are accurately copied from the original bug that was found in the Ambassador's desk in the 1970s.
SRT-153 transmitter and QRR-153 switch-receiver in font of an SRR-153 surveillance receiver

It is currently unknown why the CIA needed a 'chinese copy' of an existing inferior 1 bug that could easily be discovered by a professional sweep team, but it is clear that they wanted to hide its true origin. Although the NRP managed to duplicate every aspect of the bug, they managed to improve several characteristics, such as temperature range and stability, in a completely invisible manner. The peripheral equipment was fully developed by the NRP from scratch. It is currently unknown how long the SRS-153 system was used, but it was in production until at least 1985.

  1. In this context, 'inferior' means that the CIA had access to far better technology, consisting of bugs with a far better performance and superior audio-masking schemes that were much more difficult to detect.

SRS-153 system components
SRT-153 transmitter (bug)
QRR-153 switch receiver
SRR-153 surveillance receiver
QRT-153 transmitter (actuator)
Transmitter tester
Listening Post (LP) antenna for 300 MHz
Click any of the items in this image for further information
Surveillance receiver Transmitter tester Activation transmitter Transmitter (bug) Switch-receiver

Click any of the items above for further information

Complete setup
The diagram below shows a complete setup of the SRS-153 system. The SRT-153 transmitter is installed at the target area (TA) at the bottom right. It is powered by two strings of five Mallory mercury cells each, under control of the QRR-153 switch-receiver at the top right.

At the listening post (LP), which is generally located across the street from the target area, is the QRT-153 activation transmitter, which can send two carriers (one for the ON command and one for OFF) via a frequency in the 70 MHz band. It has presets for controlling up to four QRR/SRT-153 sets simultaneously. Once activated, the signal from the SRT-153 transmitter can be picked up by the SRR-153 surveillance receiver at the bottom left. The latter can also be replaced by an SRR-90 receiver which has been modified for the reception of subcarrier-modulated transmitters.

In early 1977, the CIA approached the Dutch Radar Laboratory (NRP) with the request to analyse a bug that had been discovered inside a wooden drawer divider in the desk of an ambassador. The bug had already been analysed by an undisclosed third party, but the findings were inconclusive. Although the redacted analysis [3] does not tell us where the bug was found, it reveals this:

The transmitter/Switch-Receiver was concealed in a desk drawer divider resembling those of the Ambassador's desk. It was discovered lying at the bottom of the right-hand bottom desk drawer along with another divider.
The bug's concealment was fabricated to replicate the size, shape, appearance and finish of the existing wooden dividers, so it is likely that one them was 'borrowed' while the concealment was being made. It measures 36.8 x 10 cm and is between 5 and 7 mm thick. The concealment is made of linen-based phenolic that has been milled out to snugly fit the components. The exterior is covered by a thin layer of wood veneer that closely resembles the wood of the original divider.

The low-grade image above was taken from the original report submitted by the CIA [3]. It shows the internal layout of the device after the outer wooden venier layers and the protective phenolic sheets had been removed. The brown colouring was added by us, to show the wooden frame that surrounds the inner linen-based phenolic block. The layout of the bug is further clarified below.

Interior of the wooden desk drawer divider. Copyright Crypto Museum.

The image above shows what is inside the concealment. At the bottom is a row of Mallory RM822 mercury cells, of which the middle one is used to power the receiver. The remaining cells form two parallel strings of five cells each that are used to power the transmitter, under control of the receiver. In order to save power, the receiver is turned ON ever 1.5 seconds for just 23 ms.

It is estimated that, under normal condiditions, the batteries allowed one full year of operation. After that, the bug had to be replaced or was considered 'dead'. A magnetic reed switch controls the power to the switch-receiver. A magnet can be used to switch the entire device OFF to save power once manufacturing is complete. It is removed after installation at the target area.

Not the first time
According to the report of the initial analysis [3], it was not the first time that a bug of this kind had been discovered. It describes the differences in components and manufacturing techniques between this one and earlier versions. Nearly all parts had been sourced in Europe, mainly in Germany and France, and were covered in a sticky white silicone paste. The only American part was the miniature Knowles 2501 microphone that was commonly used in hearing aids of the era.

Although the construction of the switch-receiver is similar to that of the transmitter, there are significant differences in manufacturing and construction techniques, indicating that the two devices may have been manufactured by different suppliers.

The report also describes an earlier version that was much larger and was housed inside a hollowed-out piece of wood. Such transmitters were generally identified as Stick Transmitters, and were often customised to fit under a desk or table, allowing quick and easy installation and removal by an operative. An example of a Stick Transmitter is given in a separate report [4].

Czech 'stick' transmitter, featured in H. Keith Melton's book Ultimate Spy [6].

Similarly concealed bugs were developed during the Cold War by the Czechoslovakian Intelligence services, such as the Štěnice that is described elsewhere on this website. A similar Czech remote controlled transmitter in a wooden stick is shown by H. Keith Melton in his book Ultimate Spy. [6]

Copy and improve it
It was the CIA's intention to do an extended analysis, improve the bug's behaviour and copy it. Copy not only the bug's circuits, but also its enclosures, materials, markings, etc. This might seem a bit strange at first. Although the discovered bug features audio masking, it is by no means secure, as its subcarrier masking scheme is no match for the bug tracers of the era. It makes you wonder why the CIA wanted to use an insecure device that was easy to find, when they had accesse to superior equipment, such as the SRT-91, that consumed far less energy.

Apparenty, the CIA wanted to deploy bugs that, when discovered by a sweep team, would likely put the blame on another country's spooks. It is also possible that the bugs were used as a bait. In such cases, the agency hides several bugs of a different nature in the same target area, in the hope that one of them is found and will satisfy the sweep team. The remaining bugs will then still provide useful intelligence. 1 To quote the unwritten law of the Counter Measures trade (TSCM):

For every bug that is found in a target area,
there are three more that have not been discovered.
The NRP was asked to do an extended analysis and fill in the missing bits of the earlier report submitted by another contractor [3]. This was done by reverse-engineering the samples of the bug that had been provided by the CIA. During the course of 1977, various concepts were tried, resulting in a development proposal in November 1977 [B]. The CIA subsequently gave the green light for development of what was then called SRS-53. 2 The NRP spent most of 1978 on the development of the various parts of the system, whilst simultaneously fine-tuning the analysis.

In February 1979, the NRP was ready and submitted the final version of their extended analysis in a 28 page report, complete with full circuit diagrams and descriptions [5]. In addition, the origin of most components of the original bug has meanwhile been established, and the NRP is ready to take the various devices into production. The performance and stability of the SRT-53 transmitter and the QRR-53 switch-receiver have been improved, and a suitable activation transmitter is ready in April 1979. For the time being, a modified version of the SRR-90 was used for reception.

In 1980, the name of the system was changed from SRS-53 to SRS-153, probably because it was conflicting with other CIA projects. The names of the sub-components were changed accordingly. In September 1981, the dedicated SRR-153 receiver was added to the range, so that the modified SRR-90 was no longer needed, making the SRS-153 a complete and fully self-contained system.

In August 1984, the UVK-153 Transmitter Tester was added to the system. It is suitable for testing all subcarrier-modulated bugs that were used by the CIA at that time, including the SRT-153, the SRT-93 and the SRT-105. The SRS-153 system was in production until at least 1985.

  1. It is also possible that the CIA wanted to use copies of the bug for lower level targets that were less aware of the possibility of being bugged, and that had no means to carry out a proper bug sweep.
  2. The system was later renamed SRS-153. The letters SRS stand for Surveillance Radio System.

SRS-153 - overview of system components
SRT-153 and QRR-153 in front of an SRR-153
SRT-153 transmitter and QRR-153 switch-receiver in font of an SRR-153 surveillance receiver
SRR-153 in front of an QRT-153
QRT-153 activation transmitter placed on top of an SRR-153 surveillance receiver
QRR-153 switch receiver (front) alongside the SRT-153 transmitter (rear)
SRT-153 and QRR-153
SRT-153 compared to the size of a hand
1 / 8
SRS-153 - overview of system components
2 / 8
SRT-153 and QRR-153 in front of an SRR-153
3 / 8
SRT-153 transmitter and QRR-153 switch-receiver in font of an SRR-153 surveillance receiver
4 / 8
SRR-153 in front of an QRT-153
5 / 8
QRT-153 activation transmitter placed on top of an SRR-153 surveillance receiver
6 / 8
QRR-153 switch receiver (front) alongside the SRT-153 transmitter (rear)
7 / 8
SRT-153 and QRR-153
8 / 8
SRT-153 compared to the size of a hand

  1. XSRT/XQRR-53 Operating Notes
    NRP, October 1977. CM302627/A.

  2. Proposal for Prototype SRS/QRS-53
    NRP, November 1977. CM302627/B.

  3. Concise Operating Instructions for QRT-53 Actuator
    NRP, March 1979. 2 pages. CM302627/C.

  4. Manual for QRT-53 Actuator
    NRP, April 1979. CM302627/D.

  5. Environmental Test Report on XQRT-53 Actuator
    NRP, July 1979. CM302627/E.

  6. Environmental Test Report on XSRT-53 Transmitter
    NRP, August 1979. CM302627/F.

  7. Environmental Test Report on XQRR-53 Receiver
    NRP, August 1979. CM302627/G.

  8. Operation and Test Manual for SRT-153 & QRR-153
    NRP, April 1980. CM302627/H.

  9. Operation and Test Manual for SRT-153 & QRR-153
    NRP, May 1980. CM302627/I.

  10. Operation and Test Manual for SRT-153 & QRR-153 (draft)
    NRP, September 1980. CM302627/J.

  11. Concise Operating Instructions for Transmitter Tester
    NRP, 23 April 1981. CM302627/K.
     follow up 26 August 1981

  12. Preliminary Partial Manual for XSRR-153 Receiver
    NRP, 7 May 1981. CM302627/L.

  13. Environmental Test Report on XSRR-153 Basic Receiver
    NRP, July 1981. CM302627/M.

  14. Operation and Test Manual for SRT-153 & QRR-153
    NRP, September 1981. CM302627/N.

  15. Operating and Test Manual for XSRR-153 Receiver
    NRP, September 1981. CM302627/O.

  16. Manual for QRT-153 Actuator
    NRP, September 1981. CM302627/P.

  17. Operating and Test manual for SRR-153 Receiver
    NRP, November 1983. CM302627/Q.

  18. Environmental Test Report on SRR-153 Receiver
    NRP, November 1983. CM302627/R.

  19. Operation and Test Manual for UVK-153 Transmitter Tester
    NRP, August 1984. CM302627/S.

  20. Environmental Test Report on UVK-153 Transmitter Tester
    NRP, January 1985. CM302627/T.
  1. NRP/CIA, Collection of documents related to SRS-153
    Crypto Museum Archive, CM302627 (see above).

  2. Various manual drawings and notes on SRS-53 / SRS-153
    NRP, April 1977 - March 1979. CM302627/x.

  3. Final Report on the Analysis of the

    Unknown author (via CIA). Date unknown but probably early 1977. 35 pages. 1
    Crypto Museum Archive, CM302634.

  4. Analysis of the Stick Transmitter/Receiver
    Unknown author (via CIA). Date unknown but probably mid-1970s. Incomplete. 2
    Crypto Museum Archive, CM302633.

  5. Extended Analysis of Example Transmitter/Switch Receiver
    NRP, February 1979. CM302624.

  6. H. Keith Melton, Ultimate Spy
    ISBN 978-0-2411-8991-7. Page 115.
  1. This document has been redacted by the CIA, hence the appearance of the
    in the title. It marks the position of words that have been removed. This document is otherwise not marked as confidential.
  2. Exact title, date and origin unknown (redacted). Not marked as confidential.

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Crypto Museum. Created: Thursday 18 May 2017. Last changed: Monday, 21 November 2022 - 13:27 CET.
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