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Path loss survey system

URS-3 is a complete test system for investigating the path attenuation or path loss in a UHF 1380 MHz radio system, developed in 1975 by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA). It was used for analysing the link budget 1 of a communications system, consisting of a covert listening device (bug), a surveillance receiver and two antennas.

The complete URS-3 system was supplied in two Samsonite briefcases. In its basic configuration, it consists of an URT-3 transmitter with Pulse Position Modulation (PPM), an URR-3 receiver, suitable antennas and a range of accessories.

When doing a site survey, the measurements could be recorded on paper, using the supplied pen recorder, and/or on audio tape, using an optional UHER tape recorder. This allowed the survey to be reviewed later. In some cases, an additional Replay Unit was supplied, allowing the results to be viewed without access to a receiver.
URR-3 receiver

The URS-3 did not replace the URS-1 path loss survey set, but was used complementary to it, as it supports the 1380 MHz band, whilst the URS-1 was used for measurements in the 316 MHz band. Both systems were used extensively in the following 20 years (at least). The URS-1 was eventually succeeded by the URS-4 in 1980, which added support for VHF-H band (170 MHz). Note that the URS-3 is a wideband system (25 MHz) whereas the URS-1 and URS-4 are both narrowband systems (30 kHz). Together, the URS-3 and URS-4 covered all CIA frequency bands.

  1. In a telecommunications system, the link budget is the sum of all gains and losses from transmitter, through the medium to the receiver.  Wikipedia

URS-3 path loss survey system URR-3 receiver
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URS-3 path loss survey system
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URR-3 receiver

Complete system

The diagram below shows the basic setup of the URS-3 system. At the left is the transmitter that is fed by a 9V battery which is installed at the bottom end. When ON, the transmitter produces a constant beep that should be picked up by the receiver. An optional external push-button can be connected to a socket to make the transmitter send a MARK tone, allowing easy identification at the listening post (LP). The transmitter is normally connected to an SRN-58 'plexiglass' antenna.

At the right is the URR-3 receiver that should be connected to the SRN-55 reference antenna. The receiver produces a visible output on an indicator at its front panel. Furthermore, sockets are available for the connection of a pen recorder and an audio tape recorder, for later reference.

The illustration below shows some of the factors that attribute to path loss, starting with the type and position of the bug and its antenna at the target area. The transmitter's signal is attenuated by the distance to the receiver, the material in which the antenna is embedded, the walls of the building, any furniture, and by anything else that is in the signal path to the Listening Post (LP).

Possible causes of path loss

Other factors may attribute to the gain of the signal, such as the transmitter's output power, the gain (if any) of its antenna, the gain of the receiving antenna and any pre-amplifiers. In order to predict the propagation of the signal with some degree of reliability, it may be useful to calculate the sum of all gains and losses and compare it to the link budget of the entire system. A detailed path loss survey was usually carried out by the CIA before planting a bug at a given target.

Modified Samsonite briefcase Universal radio transmitter URT-3 (low-band) Universal radio receiver URR-3 Replay unit URR-3 Data recorder/writer UHER tape recorder Listening Post (LP) antenna for receiver Listening Post (LP) antenna for receiver
Samsonite briefcase
All parts and accessories of the URS-3 survey system were supplied in two standard Samsonite executive style briefcases, such as the one shown in the image on the right. The manual and the antennas are contained in the set.

All accessories are stowed in the bottom part, that consist of two layers. The transmitter and the receiver are directly accessible. Adapters and cables are stored in the bottom section.
Samsonite briefcase with URS-1

Transmitter   URT-3
The URS-3 came with a dedicated calibrated transmitter, the URT-3, that transmitted on a fixed spot frequency in the 1380 MHz band using Pulse Position Modulation (PPM). It was powered by a standard 9V battery and had a button for sending an identification tone.

Unfortunately, the URT-3 is not in our collection, so we are unable to provide an image of it at this time.

No image available

Receiver   URR-3
The URS-3 came with a fixed-frequency receiver, the URR-3, that was set to the same 1380 MHz spot frequency as the URT-3 transmitter. The receiver has outputs for headphones, audio tape recorder and pen recorder. Recorded surveys can be played back for processing at a later moment.

As we don not have a full URR-3 receiver in our collection, we are instead showing an image of the Replay Unit, which is nearly identical.

 More information

URR-3 receiver

Replay unit   URR-3
The optional URR-3 Replay Unit could be used in addition to the URR-3 Receiver. It is housed in the same enclosure, and has all parts fitted, except for the RF front-end and the IF-stage.

The Replay Unit could be used at the head office, for replaying the data from a recorded survey. If necessary, it could later be converted into a complete URR-3 by installing an upgrade kit.

 More information

URR-3 receiver

Data recorder   RE-501
When performing a site survey, the results of the path loss measurements can be printed onto paper, using the Goertz RE-501 MINOGOR pen recorder shown in the image on the right.

The device is battery powered and prints the data, relative to time, onto a 20 cm wide thermal paper strip for later processing.
Goertz Minigor RE-501 data recorder

Tape recorder   UHER
In addition to printing the measurement data onto paper, it was also possible to record it onto a magnetic (audio) tape, using the UHER recorder shown in the image on the right.

UHER tape recorders were very popular during the 1960s and 1970s, as they were among the first affordable portable tape recorders with professional performance.

 More information

UHER 4000 Report-S

Receive antenna   SRN-55
A suitable 1600 MHz directional antenna should be used with the URR-3 receiver, such as the SRN-55 shown in the image on the right.

Initially, this antenna was not part of the URS-3 survey set, but from 1977 onwards, it was issued as standard with every URR-3 receiver.

 More information
SRN-55 directional antenna

Transmit antenna   SRN-58
Each URT-3 transmitter was supplied with an sleeve antenna embedded in a plexiglass stick, that was suitable for the 1500 MHz band.

The same antenna was supplied as standard with every covert listening device that operated in this band, such as the SRT-107.

 More information


RE-501 data recorder with dust cover closed Goertz Minigor RE-501 data recorder RE-501 data recorder with open dust cover Goertz Minigor RE-501 data recorder Control panel Thermal paper roll
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RE-501 data recorder with dust cover closed
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Goertz Minigor RE-501 data recorder
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RE-501 data recorder with open dust cover
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Goertz Minigor RE-501 data recorder
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Control panel
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Thermal paper roll

The first designs for the URS-3 date back to November 1972, when, at the request of the CIA, development was started of a path loss survey set that would be suitable for the newly assigned L-band (1000-1600 MHz). The existing URS-1 set was only suitable for the 315 MHz band [A].

After several experiments and design changes, the first XURS-3 prototype was delivered to the CIA in August 1974. The evaluation led to further changes and modificiations to the design. The final design was ready for release in December 1975. The set was officially released during the course of 1976. In 1980 is was complemented by the URS-4, which shared the same user interface, but offered narrowband support for the VHF (170 MHz) and UHF (416 MHz) bands.

Equipment list
  • Receiver URR-3
  • Replay unit URR-3
  • Transmitter URT-3
  • Pen recorder Goertz Minigor RE-501
  • Antenna SRN-58 (with OSM/M plug)
  • Calibration attenuator 60 dB
  • Remote mark switch and cable
  • OSM-BNC adapter cable
  • 2 BNC-Banana adapter cables
  • 2 DIN-DIN extension cables
  • 6 DIN spare plugs
  • Amphenol Sumbinax/M spare plug
  • 2 Carrying straps
  • 10 spare fuses 100 mA
  • Extender board for URR-3
  • Screwdriver
  • Operation and test manual
  • Set of official acceptance test data sheets
Technical specifications
Receiver   URR-3
  • Frequency
    1000 - 1600 MHz
  • IF frequency
    60 MHz
  • Bandwidth
    25 MHz
  • Subcarrier
    20 kHz
  • Path loss indication
    quasi logarithmic
  • SC bandwidth
    10 kHz
  • Indicating ranges
    30-100 and 60-130 dB
  • Demodulation
    Compatible with URT-3
  • Audio out
    1 mW into 600 Ω
  • Temperature
    0°C to +60°C
  • Power supply
    Internal, 4 x 9V block battery
Transmitter   URT-3
  • Frequency
    Tunable 1000 - 1600 MHz (spot frequency)
  • Pulling figure
    < 10 MHz
  • Pulse modulation prf
    20 kHz average
  • Pulse width
    0.2 µs
  • Duty cycle
    1 : 250
  • Peak power output
    300 - 3000 mW
  • Power adj. range
    5 dB
  • Modulation
    Pulse Position Modulation (PPM)
  • Audio response
    300-5000 Hz at -3dB
  • Indentification
    1000 Hz
  • Audio compression
    2 x linear dB-wise
  • Audio input
    500 µV at 4000 Ω
  • Temperature
    0°C - +60°C
  • Power supply
    Internal 1 or 2 x 9V block battery
  1. Engineering Report for XURS-3
    NRP, August 1974. CM302516/A.

  2. Collection of notes, correspondence and circuit diagrams related to URS-3
    NRP, December 1975 - November 1976. CM302516/B.

  3. Manual for URS-3
    NRP, November 1976. CM302516/C.

  4. Manual for URS-3
    NRP, December 1976. CM302516/D.

  5. Proposal for Production of URS-3 Systems
    NRP, March 1977. CM302516/E.

  6. XY-YT Miniaturschreiber Minigor, Type RE 501, Bedienungsanleitung
    Goetz, Metrawatt AG, Nürnberg (Germany). Date unknown. CM302559/A.
  1. NRP/CIA, Collection of documents related to URS-3
    Crypto Museum Archive, CM302516 (see above).
Further information
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Crypto Museum. Created: Thursday 10 August 2017. Last changed: Monday, 14 August 2017 - 12:47 CET.
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