Although incapable of detecting submarines directly, measurement sonobuoys , suffixed  as  BT, are critical to the ASW detection process. Prior to the deployment of detection sonobuoys, it  was advisable to obtain a vertical temperature profile of the local ocean environment. This information could then be directed to  the ASW operator as to what depths it might be useful to deploy detection sonobuoys.

Officially known as a SSQ-36 BT buoy, it was a standard 'A' size sonobuoy that measured the water temperature from the surface to a depth of about 1,000 feet from which a sound velocity profile (SVP) could be determined. The SVP indicated the changes in the speed of sound with depth which in turn indicated the refraction of sound causing layers or shadow zones that could affect the ability of a sonobuoy to detect the sounds generated by  a submarine. The SVP determined the median detection range (MDR) of a submarine and hence the spacing between sonobuoys in a submarine search field.
Comment: This sonobuoy provides an Electronic Function Select (EFS) for use prior to loading and launching. This allows the operator to select the desired RF channel.

Maximum operational depth:
305 meters (1,000 feet) for the SSQ-36
800 meters (2,625 feet) for the SSQ-36B

Weight: 7.7 kg (17 lbs)
Transmitter Power Output:  .0.25 W
RF Transmitter Operating Frequency:  136.000 to 173.500 MHz in 99 selectable channels. .
Sensor Temperature Range: 2.2C to 35°C
Operating Life: 12 minutes

The readings obtained were printed on a AN/ARR-52  data recorder.

AN/SSQ-36 BT bathythermograph.(Photo via Sonobuoy Tech Systems)

RO501 AN/ARR-52 bathythermograph data recorder for use with SSQ-36. It was located in the JEZEBEL bay.

Extract from manual EO 05 120A-1

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Credits and References:

1)  Ernie Cable  Shearwater Aviation Museum.
2) SSQ36 image https://www.yumpu.com/en/document/view/25976996/an-ssq-36b-sonobuoy
3) Manual reference: EO 05 120A-1

 
Sept 20/23