CRT-1B Expendable Radio SonoBuoy (ERSB)

© Brooke Clarke 2014 - 2018

CRT-1B Sonobuoy
Fig 1
Historical Significance
CRT-1 Description
ARR-3 Sonobuoy Receiver
ASQ-1 MAD equipment


A couple (GSQ-154, GSQ-160) of the outdoor Intrusion Detectors from the Vietnam era use cylindrical modules that seemed to come from sonobuoys so I tried to find the sonobuoy that was the source, but so far have not found it.  My Sonobuoy web page has information on a few buoys as well as Vietnam era equipment that was re-purposed for outdoor intrusion detection.  In the process of making that web page I learned that the CRT-1 was the first U.S. sonobuoy.

Sonobuoy transmits an FM modulated signal:
CRT-1A: 67.7 to 71.7 MHz
CRT-1B: 62.9 to 66.9 MHz
Used with the AN/ARR-3 twelve channel receiver. Used in conjunction with the AN/ASQ-1 MAD equipment.

In 1942 Magnetic Anomaly Detection (Wiki MAD) was invented (see Sonobuoy MAD patents) but it could not tell the difference between a sub near the surface and a much larger ship on the bottom.  The sonobuoy was the answer to that problem.  The CRT-1 was first put in service March 1942.  The CRT-1 could only detect a sub if the propeller was cavitating.  Under the best of conditions the range was about three and a half miles. (Wiki)Ref 1.

I believe this was used as the basis of the Project Mogul "flying disks" used to listen for atomic explosions and/or missile launch noises in the stratospheric sound channel.  For more on this see my Roswell Connection paragraph on the Sonobuoy web page.

I have another web page for Torpedoes, depth charges, mines and hedgehogs, Submarines, VT Fuze, Sonobuoys, SOSUS,

The CRT-2 was a jammer.  In the 1950s I had the clock mechanism from what I've just learned was the CRT-2 jammer.
The CRT-3 is known as the Gibson Girl lifeboat radio because of its shape.

Historical Significance


This was the first operational sonobuoy (ever or just the U.S.? tell me).

Roswell Connection

When investigating the connection between the Underwater Locator Beacons used on aircraft black boxes, like for the MH370 disappearance, I discovered the underwater sound channel and it's parallel upper atmosphere sound channel that led to the MOGUL balloon flights.  More on this on my sonobuoy web page.

But now it has come full circle in that the CRT-1 was the payload used to listen for the Russian atomic bomb testing.  This shows up in the book Roswell Report: Fact vs. Fiction in the New Mexico Desert, 1995, R.L. Weaver USAF.  Sonobuoy mentions:
(note the use the spelling Sonabuoy instead of the more modern sonobuoy)

38, 39
26, 27
Report of Air Force Research Regarding the "Roswell Incident"
195, 196, 211, 231, 232
15, 16, 31, 51, 52
Transcript from 8 Jun 94 Interview w/ Proffessor Charles Moore
pg31 "Pi Ball" should be PIBAL
272, 273, 275, 277
Synopsis of Balloon Research Findings
The Roswell Incident (pg 268)
"regular sonobuoy"
"Watson Laboratory scientists utilized an AN/CRT-1 Sonabuoy" (sic)"
NYU Tech Report NO. 93.03
Constant Level Balloon Project
R-2A/ARR-3 sonobuoy receiver has AFC
707, 716, 717, 718
2, 11, 12, 13
Journal Transcripts A.P. Crary
"sonobuoy worked into GR3"
put McCurdy low frequency amplifier in circuit before GR8 and have plenty of signal
9 June 1947Sid West's ranch:  recovered sonobuoy, microphone, and lower part of dribbler.
Aug 6, 7, 8 1947 - Flight 25B depended on sonobuoy signal

pdf pg 275
"1. Descriptions of the debris provided by Brazel, Cavitt, Crary's diary, and the photos of the material displayed in General Ramey's office. These materials were consistent with the components of a MOGUL service flight, with neoprene balloons, parchment parachutes, plastic ballast tubes, comer reflectors, a sonabuoy, and a black electronics box that housed the pressure cutoff switch (Atch 3). "

pdf pg 277
From research, it appears that the wreckage displayed on July 8 consisted of unclassified components of a MOGUL balloon assembly. Possibly withheld, if it was indeed recovered, was the AN/CRT -1 Sonabuoy, which could have compromised Project MOGUL. Although the Sonabuoy was not itself classified, its association with a balloon would have exposed a specific military purpose, an obvious violation of project classification guidelines (Atch 9). A device described in "crashed disc" publications as "a giant thermos jug" was allegedly transported from Fort Worth AAF to Wright Field.38 This description is consistent with the appearance of an AN/CRT-1 Sonabuoy such as was used on flight no. 4 (Atch 4). At some point General Ramey decided to forward the material to Wright Field, home of AMC, the appropriate agency to identify one of its own research devices or a device of unknown origin. If the debris was determined to be from an unknown source, the AMC, T -2, Intelligence or Analysis Division, would conduct scientific and/ or intelligence analysis in an attempt to discover its origin. But since the balloons, reflectors, and Sonabuoy were from an AMC research project, the debris was forwarded to the appropriate division or subdivision, in this case the Electronics Subdivision of the Engineering Division. There, it was identified by Colonel Duffy, under whose purview Project MOGUL operated. Colonel Duffy, a former project officer of MOGUL with specific directions to "continue to monitor upper air programs," was the appropriate headquarters officer to make an identification, which he apparently did. According to Captain (now Colonel) Trakowski, the officer who succeeded Colonel Duffy as project officer on MOGUL, after returning from the Alamogordo II field trip, Colonel Duffy contacted him by phone at Watson Laboratories and informed him that the "stuff you've been launching at Alamogordo," had been sent to him for identification. He described the debris to Captain Trakowski, and Trakowski agreed that it was part of his project (MOGUL).39"

Notice in the description below that the sonobuoy is a derivative of the radiosonde.  It's interesting that the first sonobuoy was also deployed as the payload for this project  But there were at least two modifications:

CRT-1 Description

The idea was influenced by the work of Dr. Thomas H. Johnson of the Bartol Research Foundation of the Franklin Institute who used telemetry to get data from high altitude balloons (Bartol: Hodoscope 1933).  By the early 1930s the expendable meteorological Radiosonde (Wiki) carried a simple radio transmitter using a vacuum tube and dry batteries.  The chronometric type was in use where multiple channels were being transmitted.  This technology was mentioned in a (18 May 1941) letter from Professor P.M.S. Blackett (Wiki) to C.G. Darwin (Wiki) and the follow on letter from Darwin to the U.S. National Defense Research Committee (Wiki: NDRC) on 24 May 1941.  At this time the Magnetic Detection System (MDS, now called MAD) only had a range of about 200 feet so was of no use in tracking a sub from a plane. See Sonobuoy Ref 7 pg 31.

The expendable sonic buoy (Wiki) and the Magnetic Airborne Detector (as they were called then) (Wiki) came out of a project of the NDRC (Wiki) Section C-4.  See Ref 2 page 181 - 184.

2397844 Signaling apparatus, Dewhurst Wallace W (Haddon Heights, NJ), Rca Corp, Filed: Oct 1, 1942, Pub:Apr 2, 1946, 367/3, 138/89, 114/198, 455/99, D10/107, 441/11, 73/322.5 -  This may be THE sonobuoy patent.  Ref 2 says RCA developed the first convey buoy which was to be dropped from the stern of a ship and would detect any sub that came up from behind the convey, which was the MO of "Wolf Packs", but these were soon replaced by the air dropped type. 
Note 19 patents cite this one as prior art.

Note that a sonobuoy is not a weapon, that's to say it can not damage an enemy submarine.  Once the sonobuoy detects a sub then some munition needs to be used.  Although there were some attempts made to use bombs these failed since dropping a bomb from a moving aircraft and hitting a moving submarine or ship is almost impossible with a ballistic bomb.  The depth charges (Ash Cans) used early in W.W.II had a very slow decent rate and so were not very effective.  The later streamlined depth charges worked better, especially when fitted with contact or functional influence fuzes.  But the most effective munition was the torpedo and later extremely effective the FIDO Mk 24 depth charge (Wiki) (actually a sound homing torpedo).

YouTube Title: Navy Depth Charge Demonstration (1946) - really about CRT-1 Sonobuoy.  THe CRT-1 probably could only detect a sub at high speed (cavitating Wiki). Note the use of "Ash can" depth charges that had a poor record because a can with its centerline axis horizontal descends slowly.  It was replaced with the Mk 9 depth charge that was shaped like a bomb and included tail fins, much faster decent giving subs much less time to get away.

First sonobuoy, dropped from a ship in a convoy

These were proof of concept units first used September 1941 and weighed 60 pounds.  The key motivation in 1941 for the sonobuoy was that other detection methods like Visual, RADAR or ASDIC (active SONAR) resulted in the submarine escaping 95% of the time.  See Sonobuoy Ref 7 pg 23.

2397844 Signaling
                  apparatus, Dewhurst Wallace W, Rca Corp, Apr 2, 1946
2397844 Signaling
                  apparatus, Dewhurst Wallace W, Rca Corp, Apr 2, 1946

1696109 Electric stopping device for stretching machines, Dewhurst William (Methuen, Mass), not assigned, Dec 18, 1928, 26/74, 192/126 -    
Tuning indicator, Dewhurst Wallace W (Haddon Heights, NJ), Jones Nevell R, Gen Electric, Sep 27, 1932, 116/262, 74/10.7, 361/300, 74/568.0FS, 74/568.00R - string drives pointer type  
2023517 Electrical resistance device, Creager Frederick L, Dewhurst Wallace W (Haddon Heights, NJ) Rca Corp, Dec 10, 1935, 338/319, 439/549, 338/332, 248/201, 439/560, 29/854, 338/327, 338/322, 29/838 - leads form solder points for terminal board (but requires slot in board).  
Binding post, Wallace W Dewhurst (Haddon Heights, NJ, Eskuchen Frank, Rca Corp, Apr 14, 1936, 439/816, 220/DIG.400 - hollow post maybe for use on terminal board.  
Adjustably tuned coupling unit, Bennett Paul R, Dewhurst Wallace W (Haddon Heights, NJ), Rca Corp, Apr 4, 1950, 334/65, 336/136, 334/74 - slug type coils   
2555475 Radio receiver tuning mechanism, Dewhurst Wallace W (Oaklyn, NJ), Rca Corp, Jun 5, 1951, 455/178.1, 334/77 - tunes two slug type coils in such a manner that as one coil goes up in inductance the other coil goes down, like needed for tuning the input and oscillator circuits. 

3130384 Artificial target, Atchley Raymond D, Downs George W, Secretary of the Navy, Filed: Mar 27, 1946 (18 year delay) Pub: Apr 21, 1964, 367/1, 236/68.00B, 114/25, 434/6, 116/27 -  cites the 2397844 Signaling apparatus patent as prior art.  This device can be launched from a torpedo tube and simulates a submarine as a decoy for destroyers or in can be used to train destroyer crews in sub hunting.

Second, air dropped sonobuoy

First operational sonobuoy as of August 1942 and by the war's end about 60,000 had been made. (Sonobuoy Ref 7, pg 39)

2629083 Expendable radiosonic buoy, Barkson Joseph A, Mason Russell I, Mcnary James C, Secretary of the Navy, Filed: Sep 21, 1944, Pub: Feb 17, 1953, 367/3, 343/709, 455/99, 441/33, 441/23, 343/705, 343/901 -  
2629083 Expendable
                  radiosonic buoy, Barkson Joseph A, Mason Russell I,
                  Mcnary James C, Secretary of the Navy, Filed: Sep 21,
                  1944, Pub: Feb 17, 1953
2629083 Expendable
                  radiosonic buoy, Barkson Joseph A, Mason Russell I,
                  Mcnary James C, Secretary of the Navy, Filed: Sep 21,
                  1944, Pub: Feb 17, 1953


From Ref 4.

Audio (Hz)
300 to 8,000
100 to 10,000


When the aircraft was at a few hundred feet altitude (so that the MAD detector will work) the radio range for hearing a CRT-1 was about 10 miles.  The hydrophone had a range of 200 to 3,000 yards.  (Sonobuoy Ref-7, pg 39). 

Pattern used with Non-Directional Sonobuoy

Notice that the operational depth of a submarine was on the order of 1 to 3 times its length.  The MAD detector required the airplane to fly at or below 500 feet above the surface (200' for the early versions).  Once the MAD detected a sonobuoys would be dropped to both mark the area and also to locate the sub by dropping a pattern of buoys.



Just under 40" tall with the antenna collapsed and eleven pounds without batteries or the shipping tube.  Tube is just over 4-1/4" diameter.  The size envelope was determined by the British 500 pound bomb (Wik) since a bomber would be used to drop the sonobuoye.

The telescoping antenna was frozen as received.  I've applied a few drops of Kroil and maybe tomorrow it will fully extend.

Surrounding the antenna at the top is a black plastic cylinder about 6" high and 2-5/8" diameter inside of which is a coiled up synthetic rope which I guess was attached to a parachute prior to deployment from an airplane.  It's marked MX-211/CRT-1A.  Note the "A" was not changed when the sonobuoy was upgraded.

The hydrophone appears to be made from some type of wire wrapped around a hollow ferrous cylinder 5" tall by 3" outside diameter.  It's connected to the main body by what looks like common household AC power line cord.  Normally it would be held inside the bottom of the tube but it came out when the sonobuoy was removed from the shipping tube and since the wire is now very stiff it would break into bits if forced back into the tube.

Vacuum Tubes

The three 1L4 tubes for audio amplification and FM modulation and the 3Q4 RF oscillator and 3A4 final RF amp tube all have their filaments wired so they can be powered from a 1.5 Volt Zinc Carbon battery.  The RF oscillator and amplifier run directly from the B battery (135 Volts), but the 1L4 tubes have a voltage divider so they run at a lower voltage.


The cord between the body and hydrophone was to reduce the noise caused by the sea lapping against the body.  Down 24 feet the hydrophone was more sensitive.  Note there was no amplifier down with the hydrophone.  For the CRT-1 to detect a submarine it needed to be traveling at cavitating speed.  Note that as the depth gets deeper the speed of cavitation goes up, so a sub can go faster without cavitating as it goes deeper. Ref 5

Detection Range
Sea State
3 1/2 Miles
7 knots
90 Yards
3 knots


Fig 1 Overall
CRT-1B Sonobuoy

CRT-1B Sonobuoy shown outside the shipping tube.
Antenna all the way down.

The hydrophone has come out of the bottom and probably will not go back in because the wire is brittle.

Wire length said to be twenty four feet.
Fig 2 Top
CRT-1B Sonobuoy

The telescoping antenna has been pulled part way out.
Said to be about one meter long when extended.  A 1/4 wavelength antenna 1 meter long means a wavelength is 4 meters or about 75 MHz. (Mc back then).

At the left you can see a small black rubber plug.  Function unknown.

Rad Tag Reads:

Replace pin immediately
to conserve battery.
Note: tag connected to one of the parachute shrouds and that shroud connected to short (1/2" long) pin that will be pulled when the parachute deploys.  In order to reinstall the pin a spring loaded plate needs to be moved to uncover the hole in the top.

Label reads:

NXsa-65296,  13415 :CABC
Fig 3 Main Label
CRT-1B Sonobuoy

Color <red> (related to transmission frequency)
NXsa-65283,  2805-CEX
Fig 4 Hydrophone (Wiki)
CRT-1B Sonobuoy

5" tall x 3" dia. Nickel cylinder wrapped with wire.  There's a permanent magnet inside the hydrophone (a nearby compass is strongly influenced).

NXsa 65283
Ser. 5717

  942897 Apparatus for receiving submarine sounds, Thomas Alexander Garrett, William Lucas, Dec 14, 1909, B06B1/08
               Magnetized Nickel rod develops voltage in coil & is more stable than carbon mike.
1811127 Balanced magnetostrictive oscillator, Harrison Jamison R, Wired Radio Inc, Jun 23, 1931,
331/157, 367/168, 331/59, 331/168, 318/118
               aimed at using magnetostrictive element to replace a quartz crystal, but a simple 2-tube oscillator circuit.

1882397 Magnetostrictive vibrator, Washington Pierce George, Oct 11, 1932, 333/201, 318/118, 361/206 -
               mechanical arrangements of magnets and material
Apparatus for transmission and reception, Leonard Steinberger Raymond, Dec 15, 1936,
               367/153, 318/118, 367/174, 181/164, 181/168
2166359 Magnetostrictive device, Lakatos Emory, Bell Labs, Jul 18, 1939 - basic info on Magnetostriction w/plots
               333/201, 420/581, 252/62.55, 148/312, 367/168, 420/459, 310/26, 148/315, 335/215, 318/118, 367/176
2328496 Magnetostrictive microphone, Mar 22, 1939, Yves Rocard, 367/168, 333/148, 335/215, 310/26, 381/190
2414699 Magnetostrictive signal translating apparatus, Rca Corp, Dec 30, 1944, 367/168, 310/26, 318/118
2415407 Submarine signaling apparatus, Hugo Benioff, Submarine Signal Co, Dec 9, 1942, Feb 11, 1947, 367/168
thin cylindrical shell 49 of magnetostrictive material..."
Art of tuning magnetostrictive elements, Rca Corp, Nov 29, 1944, 310/26, 318/118, 367/168
2435487 Electromechanical vibrator, Robert Adler, Zenith Radio Corp, Feb 1, 1943, Feb 3, 1948,
331/157, 335/215, 331/138, 331/107.00R, 335/224, 310/25, 310/113, 310/26, 335/229, 318/128, 335/221
                Although this device can provide mechanical vibrations up to 2.8 MHz using non ferrous materials, most
                of the implementations use magnetostrictive material (Fig 3 and up).  diagrams showing magnetic poles

Electroacoustical transducer, Turner Jr Edwin E, Submarine Signal Co, Nov 18, 1942, Mar 9, 1948,
                367/168, 381/182, 381/190 - uses permanent magnets for bias
2468270 Magnetostrictive transducer, Rca Corp, Dec 30, 1944, 367/168
2472388 Magnetostrictive oscillator, Thuras Albert L, Sec of Navy, Filed: Jan 15, 1944, Pub: Jun 7, 1949, 367/168, 310/26, 369/146 - ultrasonic
2521136 Hydrophone, Albert Thuras (New London CT), USA, Sep 5, 1950, 310/26, 381/190, 367/168, 381/163 - CRT-1
2566984 Magnetostrictive device, George Firth Francis, Sep 4, 1951, 310/26, 165/84, 367/156, 366/127, 310/16
               although designed to generate ultrasonic with some power it would make a good microphone
2631271 Tubular hydrophone, A.L. Thuras (New London, CT),  Sec of Navy, Mar 10, 1953, 367/168 -  CRT-1 
2641751 Hydrophone casing, Bernier Jr Hector F, Mason Russell I, Ripken John F, Filed: May 11, 1944, Pub: Jun 9, 1953, 367/173 -
2749532 Hydrophone, Harris Wilbur T, Harris Transducer Corp, Jun 5, 1956, 367/168, 310/26 - Nickel sheet metal cylinnder similar to CRT-1
Microphone, Filed: Sep 11, 1943, Granted: May 12, 1959,  367/168, 310/26, 114/25, 367/157, 381/190 - magnetostrictive
Dual magnetostrictive hydrophone, Reginald A Hackley, Mar 29, 1946, Apr 11, 1961 (SECRET for 15 years),
367/156, 333/201, 181/165, 116/DIG.180, 381/427, 381/162, 310/26              
3509523 Helical-wound magnetostrictive line hydrophone, Parker David E, Prentice Winslow W, Us Navy, Apr 28, 1970, 367/168 
" ...can be economically fabricated in a continuous manner to provide any desired length."
3059217 Transducer-hull for underwater use, Boswell Vance F, Oct 16, 1962, Clevite Corp, 367/173, 310/26, 114/20.1 - made of Nickel sheet metal very similar to the CRT-1              
Magnetostriction (Wiki) -

The Maritime web page says this is how it works.

"...a cylindrical magnetostriction unit that is wound on a nickel shell. Its construction permits the storing of the cable inside the hollow shell and effects a reduction in length of about 4 inches compared with earlier models."

CRT-2 Sonobuoy
                  hydrophone 2631271 Tubular hydrophone, A.L. Thuras
                  (New London, CT),
2631271 Tubular hydrophone, A.L. Thuras (New London, CT),  Sec of Navy, Mar 10, 1953, 367/168 -  CRT-1 
Fig 5 Tags at top of Antenna
CRT-1B Sonobuoy
Electronics Follow up and evaluation Tag (not filled in)

T.O. 49.49
2 each BA-51 (2 * 67.5V = 135 V B+)
4 each BA-30 (parallel connection 1.5V filament)

Ready for Issue Tag
Part No. AN/CRT-1B
Nomenclature: Transmitter
Stock No. R-16-T-9171
BU or Serial No. 2805 CEX
Fig 6
CRT-1B Sonobuoy
(not filled in)


(paper under staple) Nav??p 2650
Changes Incorporated
Change No. B50 40-49
Shop No. NASSD
Shop Insp. HK

Stamps 42(anchor)11, 42(anchor)30
Fig 7 Schematic of Transmitter
CRT-1 Sonobuoy
                  Transmitter schematic

The circuit shows an input jack and no DC bias on the transducer, so it may be working without any electrical bias, but there is a permanent magnet to provide the bias.
Fig 7 Electronics assembly
RCA jack in lower left is hydrophone input
CRT-1B Sonobuoy
Fig 8 Electronics assembly
135 Volt B Battery above A battery
1.5 Volt A Battery at bottom (maybe 4 "D" cells?
Fig 9 Close-up Electronics assembly
T 1B/CRT-1
Clear plastic cylinder removed
Chassis plate is shock mounted
Tube on left is marked 3A4
RF Oscillator & Final Amp
Fig 10 Close-up Electronics assembly
Clear plastic cylinder removed
Tube on right is marked 1L4

ARR-3 Receiver

Used in aircraft.  Powered by 24 VDC.  The 12 Volt filament tubes are paired so filaments are powered by 24 VDC.  A "rotary transformer" supplies the B+.

ARR-3 Photos and information supplied by Michael, VK4ZKT

Fig ARR-3 #1  Front with 1629 Magic-Eye tube.
First made by Freed Radio, New York City.
ARR-3 Sonobuoy
Fig ARR-3 #2 Back Inside
ARR-3 Sonobuoy
Fig ARR-3 #3 Bottom
FET converter PCB allows reception of FM broadcast
ARR-3 Sonobuoy
Description (pdf) with schematic
CRT-1 & ARR-3
                  Tech Information & schematic
2017 June 30  from eBay UK seller  jet-art-aviation
JetArt web page
ARR-3 Sonobuoy

ASQ-1 MAD equipment


1336497 receiver mounting for submarine signals - through hull microphone Apr 13 1920, 381/177; 381/412


Ref 1. Aircraft versus Submarine in two world wars by Dr. Alfred Price, 2004 (Price wrote Instruments of Darkness about ECM)
Ref 2. Scientists Against Time, James Phinney Baxter, 3rd Ed, 1946 - 1952 (Wiki)

Ref 3 YouTube: Navy Depth Charge Demonstration (1946) 24/10/1946. FILM ID:2347.03 - Really about Sonobuoys and probably the CRT-1.  "Various tactical situations require different procedures and exact patterns.  Only by learning these and adhering to them can the full effectiveness of sonobuoys be realized".
Ref 4. The Evolution of the Sonobuoy from World War II to the Cold War, Holler, Jan 2014.
Ref 5. Aircraft Versus Submarine in Two World Wars (1 & 2), Dr. Alfred Price, 2004 - has some new details on the CRT-1 including capabilities.


Outdoor Intrusion Detectors
Torpedoes, depth charges, mines and hedgehogs


Richard Muller Physics Lecture 11 - Waves 1 - W.W.II underwater channel and the Roswell connection - origin of the term "Flying Saucer".

PRC68, Alphanumeric Index of web pages, Products for SaleBrooke's Military Information 
page created 17 Dec 2014