. 2017-11-20



Background  Notes



  See also :  Sonobuoys-1 : AN/CRT-1 aboard Cananadian Lancaster Back to :  ASW Weapons  


he early work on SOund Navigation And Ranging (SONAR), which was named after RAdio Navigation And Ranging (RADAR), was done utilizing audio frequencies were an operator would listen on headphones (for those systems that used stereo) or a loudspeaker.  The first sonobuoys also used audio with a human operator listening to the sound.  This is called passive SONAR (code name Jezebel).  There are no pings and whoever is doing the listening is not giving away their presence or position.  When it works it's the preferred method and is by far the most common.  In the 1950s the SOund SUrveillance System (SOSUS) which makes use of LOw Frequency Analysis and Recording (LOFAR) rather than the use of human audible sound was put into service.  This worked on snorkeling diesel electric subs and on nuclear powered subs like the Soviet Hotel , Echo & November class subs.

Active SONAR comes in two flavors, the most commonly known is the active ping like in any movie involving submarines.  A ping is sent out and the time measured until it returns.  The early pings were audible to humans, later ultrasonic pings were used and later still the frequency of the ping changed it into a chirp.  If the propagation speed is known the distance to the target can be calculated and with the later types the radial speed of the target can be determined.  The less well known active SONAR method (code name Julie) involves setting off an explosion of a couple of pounds of TNT using either the Mk-15 (Mod-12) or Mk-61 Signal Underwater Sound (SUS).  The explosion generates a spike in the underwater pressure which is similar to a ping at all possible frequencies, it's the most useful type of ping, but can only be used occasionally because it requires a small bomb for each pulse.

Modern  diesel electric subs are very quiet when running on battery power underwater.  The explosive type active SONAR is good at deteting these subs.


The word sono-buoy is based on sound and a floating object.  Sonobuoys  have been around since about May 1941 when P. M. S. Blackett, head of the British Admiralty committee for antisubmarine measures, proposed the idea.  In June 1942, the AN/CRT-1 became the first operational sonobuoy, and on July 25, 1942, the first successful launch of a sonobuoy from an aircraft was made from a U.S. Army B-18 bomber.

They are nomenclatured SSQ-nn.  They are part of anti-submarine warefare.  "All sonobuoys currently in inventory are normally launched from standard A-size tubes via pneumatics, free fall, or a Cartridge Actuated Device (CAD).  Shipboard personnel may also launch them by hand or Over the Side (OTS). All are powered by either salt water activated magnesium or silver chloride, lithium chemistry, or thermal batteries and are designed to scuttle at some point after usable or selected life expires".


Modern sonobuoys have an outside diameter of 4-7/8" (fit 4-15/16" launch tube commonly called 5 inch) and are 1 yard long (36").  The max weight is 39 pounds.  This is a convient size for one man to handle on a P-3 Orion aircraft.  The larger sizes are not easy to handle.  There are smaller sizes based on getting some interger number of them inside the A size outline.  3 each is called "F" and 2 each is called "G".  The other sizes are pretty much not used in volume.


A hydrophone is the common sensor for sonobuoys and is typically deployed at 20 meters (65 feet: shallow) or 120 meters ( 328 feet: deep).  A sonobuoy might have 50 depth settings that can be set prior to ejecting it from an aircraft.  This is important because of what's called the thermocline  which is where the temperature of the water changes rapidly.  This changes the speed of sound .  This causes the sound to change direction just as light will be bent by a change in refractive index .  And just like light there's conditions where the bending acts like a mirror and all the sound (or light) is reflected off the layer instead of just changing angles.  So if the hydrophone is on the wrong side of the thermocline it may not hear a sub that's on the other side. Bathythermograph This is a device that measures the water temperature as a function of depth.  For example the SSQ-36 might first be dropped and the temperature profile recorded.  Then the ideal depth for the hydrophone determined and programmed into the sonobuoy.  Then the sonobuoys would be dropped.

The speed of sound in water depends on temperature so it slows down as the depth moves from the warm surface to deeper depths, but at some point the pressure caused by deepth will cause it to speed up again.


The service life can also be programmed prior to launch for 1, 3 or 8 hours.


A VHF vertical whip antenna is used.  One feature of this type of antenna is that there's a null directly above the buoy so when an aircraft directly overflys the buoy there's a characteristic signal drop out.  This allows confirming the buoy location.  The 1 Watt transmitter is FM modulated and covers an audio bandwidth of 10 Hz to 20 kHz (about the same as a Hi-Fi system or entertainment FM radio).


The aircraft can transmit to the sonobuoy to change the commands while it's in the water which is much better than the old way of making the command decisions prior to launch.


These buoys use directional hydrophones covering 5 or 10 Hz to 2400 Hz combined with a magnetic bearing sensor and transmit this information.  They can be used to passively listen, to listen for reflected pings or the shock wave from a small explosion.  Whale researchers use them.  Because the lower frequency limit is below Hi-Fi audio, consumer grade tape recorders could not be used so instrumentation type recorders were used.  With the advent of Digital Audio Tape (DAT) recorders the DIFAR signal could be recorded on a DAT tapes

TABLE OF SONOBUOYS - Naval Consolidated Sonobuoys @FAS -

  Function Start End

5 vacuum tubes single channel FM transmitter between 67 & 72 Mhz.  -  Link : A683MP
CRT-1A: 67.7 to 71.7 MHz
CRT-1B: 62.9 to 66.9 MHz




6 channels


separate web page


upgraded CRT-4

not UK  



Start of: Sound Surveillance System (SOSUS)




Julie explosive


Julie RO B-size



1956 1964.11


1960 1964.11

The BT sonobuoy is an expendable thermal gradient measurement sonobuoy that operates on one of three or one of 99 Radio Frequency (RF) channels. It consists of a thermistor temperature probe that descends through the bottom of the sonobuoy canister producing a continuous reading of temperature versus depth. The thermistor temperature probe will descend to 1000, 2000, or 2625 feet, depending upon the sonobuoy selected.

Fig 36-01 Tubes - Ship: 7" x 45" - Launch: 5-3/8" x 39-1/4" - Housing: 4-3/4" x 35-7/8" - Sonobuoy_01.jpg


30day omni-directional LOFAR - (replaced SSQ-28) - 10 to 6,000 Hz - 31 chan

1964 1961.06

single hydrophone


(replaced Julie explosive system) - active ping omni directional range only - replaced by SSQ-50


replaceddby SSQ-41B




31 RF Channels - 10 Hz - 2.4 kHz - 90 feet fixed depth


90 or 1000' depth - 1 or 8 hours



Since the sonobuoy  has a cylindrical ( 4-4/7" O.D.) shape it makes sense to have the electronics in the form of cylindrical modules that can be stacked end to end. These modules are about 2-3/4" O.D. and have a circular connector around the outer edge.


Photo from Wiki Sonobuoy page

From Ships and Aircraft of the U.S. Fleet (2005):
The P-3C Orion.tail-mounted ASQ-81 Magnetic Anomaly Detector (MAD) and 48 external (fuselage) sonobuoy chutes and four in-flight reloadable (internal) chutes; a total of 84 buoys normally are carried.


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 was invented 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.


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. 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.

Credits and References: