RT-1185/GRA-114 Sound Observer Receiver Transmitter

Brooke Clarke 2003 - 2014

GRA-114 Front Panel
Tx Crystal Sockets
RF Module has two sockets for Tx crystal units (SO1 and SO2)
RF Module
Inside of RF module


The AN/TNS-10 system was a wire line system for determining the location of explosions, like from artillery firing or impacting.  A number of special microphones, typically installed in shallow holes in the ground, are amplified and low pass filtered. A square array of 9 microphones fed an electrostatic pen recorder.  Knowing the microphone positions and the speed of sound the bearing (and maybe distance) to the loud noise could be calculated.

Since the chart recorder has a high speed paper feed it's important to have good control of starting and stopping the system to conserve the paper.  The system specifications have all kinds of requirements to be sure paper is not wasted.  The output charts would need to be interpreted by a human.

The GRA-114 replaces all the microphone wiring with radios, saving a lot of wire.

My guess is that the TNS-10 and GRA-114 have been replaced with the TPQ-36 mortar location system and the TPQ-37 artillery location system.  These TPQ-xx systems use radar to track the ballistic projectile and compute the source location before the first round impacts.  These radars were made by Hughes in either El Segundo or Torrance California.

The KY-671 "Decoder-Indicator" that works with the PRC-25 may be related to the GRA-114 sound locating system.

Accoustic location of gun fire based on time of arrival is now being used in major cities.  For more accoustic gun shot location see my Sensors web page.

Artillery Sound Ranging - using British tube equipment.

The RT-1183/GRA-114 Transmitter Receiver, Radio Data Link, Sound Ranging is a similar but larger unit that probably uses the same Tx and Tx crystal units.


From the training course:
"The Radio Data Link, Sound Ranging AN/GRA-114, hereafter referred to as AN/GRA-114, is a digitized special purpose communications system capable of transmitting voice when desired and analog representations of signals derived from weapons firings for the purpose of locating these weapons. A complete network consists of two sound observer (SO) stations which initiate system operation, eight microphone position (MP) stations (two spares) for detecting and transmitting signal information, and one command post (CP) station which receives, decodes and initiates the action required."
The GRA-114 is a Radio Data Link, Sound Ranging. Parts of the GRA-114 are:


The label reads:
Receiver-Transmitter RT-1185/GRA-114
PN: SM-D-706404 Ser No 01010A NSN 5895-01-064-9795
DSGN ACT: 80063   Contr: DAAB07-83-G-E001-2000  Mfr: 54125  US  
Note that the GRM-114 is an general purpose communications test set made by IFR.  This is a GRA-114
CAGE code 54125 belongs to:
FERRANTI INTL SIMULATION AND TAINING /US/ INC 1995 some to GEC Marconi, but even they went to Mexico.
Status: H - Obsolete Record
Address: 801 S 18TH ST
ZIP: 17512
State: PA
CAO-ADP: S3912A - SC1010
Voice Phone Number: 717-285-7151 (only old legal and corp office, nothing technical)
Date CAGE Code Established: 8/19/1974
Last Updated: 7/25/1995

Power Source

There is only provision for battery power using the +15 Volt tap on a BA-4386, BA-5598, or my 257475BA Battery Adapter.  The battery box is the standard CY-2562/PRC-25 used on the PRC-25, PRC-77 and many other equipments.  The boards run directly from the +15 Volt battery just with dropping resistor and cap filters on the boards.  The CMOS ICs are rated for 15 volts.


The top row (Tx functions?) has: three 10 position CODE switches (have diodes mounted for 8,4,2,1 binary weighting., IDENT of SO1 or SO2, a START DATA push button and the POWER switch has three positions: OFF, MED and HI.

The bottom row (Rx functions?) has type-N antenna connector, VOLUME control, SQUELCH ON/OFF switch, 5 pin HANDSET connector and a pair of wire terminals marked REMOTE.

Component Parts

Front Panel & Chassis SM-D-706596

No active components other than the diodes on the CODE switches.

Tx Module SM-D-706622

There are two sockets that hold the transmitter crystal units.  The appearance of these 2 sockets matches the socket on the TS-3630/GRA-114.

DB-9 connector has only 6 wires:
1 = Blue
2 = nc
3 = White/Red -> Power switch (Hi only) <-> Fuse <-> Battery +15
4 = White/Purple - tbd
5 = White -> POWER switch (MED and HI) <-> Fuse <-> Battery +15
6 = nc
7 = Black = Gnd
8 = White/Black - tbd
9 = nc

 Crystal Unit Frequencies
From Crystal Test Set TM 11-5895-1097-40
The top number is the marked frequency on either a Tx or Rx module, there is a Tx module and an Rx module with the same frequency marking.
The lower numbers are the actual Tx/Rx frequency of the module.

Tx Crystal Units
The Tx crystal units probably look like small metal box 0.9" x 0.6" x 0.4" high.  2 pins on the bottom about 0.16" apart.
These are just overtone crystals. 
The actual Tx frequency times 2 plus 19.25 MHz is the marked frequency.

Rx Crystal Units
There are 3 SMB connectors.  The input connector takes in the marked frequency and an LO and RF signal are output such that the difference is 18.1 MHz. 
P3 (J4) in the input for the marked channel frequency.
P1 (J5) is fed +6.2 volts DC and is one of the outputs.
P2 (J6)  is fed +12 volts DC and is one of the outputs.
P1 and P2 are close toghther and P3 (J4) is seperated by about twice the P1-P2 distance.

The actual Rx frequency plus 18.1 MHz is the marked frequency.

Marked Freq
acutal Tx/Rx freq
Note that the Rx and Tx frequencies are more than about 1 MHz apart when they are in the same band or the Tx frequency is about 2 times the Rx frequency if they are in different bands.  This way a single 1/4 ground plane antenna will work well for both Rx and Tx.



The actual operating frequencies seem to be within US TV channels 2 and 3.  They are also within the tuning range of  VHF low band Squad Radios.

Function Module SM-D-706623

Marked SM-D-706623 Ckt OD ASSY, SO FCTN  MP-19730-D.  Seems to be all analog circuitry with transistors and op amps.

Encode Module SM-D-706624

Has 9 ICs either 14 or 16 pin, most likely CMOS type. A metal box marked "Y1" with two leads, probably a crystal, but with no markings on the box. L2 is a pot core inductor probably in the mH range that's near 5 transistors.

Y1 is connected to a Motorola 14 pin IC marked JM38510/05202BCA (also made by Harris, National, RCA and TI).  Some data at the National Museum of American History, Series 10 Integrated Circuit Engineering Collection  GOVERNMENT TASC box # 17.  The TI part was equivalent to the CD4001 CMOS Quad 2-Input NOR Gate.

T/R Switch Module SM-D-706635

Has 3 coax connectors: to Tx module, to Type-N antenna connector on the front panel and to a connector near the back of the Rx module.  The two diodes are biased through J3 which goes to the Tx module.

Receiver Crystal Module

A missing module that would have three SMB Jacks: two attach to the receiver module and one to attach to the connector routed to the T/R Switch.  Most likely 3/4" x 3/4" x 5 " long.  Probably supplies the RF and LO signals to the Rx module.  Although the transmitter can transmit on one of two frequencies it's not clear what the SO1 - SO2 switch does in receive mode if anything.

Another possible function of this module is to act as a duplexer allowing voice coms on one frequency and data signals on a separate frequency.

Rx Module SM-D-706651

One transistor crystal oscillator in center of board.  All analog circuitry, transistors and a CA3028 amplifier.  DB-9(m) connector has pins 4, 8 and 9 as grounds.  Total of 4 wires on DB-9 connector:
1 = White - tbd
2 = Orange - tbd
3 = Red  -> POWER switch (MED and HI) <-> Fuse <-> Battery +15
4 = Black = Gnd
5, 6, 7, 8, 9 = nc


TM 11-5895-1095-12


TM 11-5895-1097-12 Covers the crystal unit test set.
the test set uses the SG-1112 to supply the received frequency test signal.
TM-11-4940-479-23P is for the SG-1112 which is the HP8640 combined counter & signal generator.


Code generation test set.   SO1 is a START command and opens the line, SO2 is an End Of Word command and closes the line.
The description of how this test set works should be very similar to how the RT-1185 works.
Part of the test set is a 19.25 MHz output that's amplified and fed to the RT under test.
TM 11-5895-1096-40 * -40P


Lawrence Bragg's role in the development of sound-ranging in World War I - William Lawrence Bragg (Wiki)

Technology for Artillery Location, 1914-197, By Alister J Mitchell - from the start of indirect fire using the flash-to-bang timing to the GRA-114.

Wiki: Sound Ranging, Gun laying, Prism paralleloscope, Mortar, Artillery

2500643 Condenser transducer independent of ambient atmospheric conditions, Munson Wilden A, Wente Edward C, Bell Telephone Labs, Dec 7, 1946, 381/174, 381/337, 381/191

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page created 5 March 2003.