The AN/SQS-504 Diver[1] was an early naval medium-frequency active variable depth sonar[2] (VDS) developed in Canada.
The AN/SQS-504 emerged from a post-Second World War program to improve the detection of submarines between the thermal layers in the Gulf of Saint Lawrence.[3] A transducer derived from a wartime British Type 144 sonar was placed in a towed body[4] and deployed from the stern by the AN/SQA-501 handling gear.[5] The AN/SQS-504 was succeeded by the AN/SQS-505, which used a then-modern transducer and receiver.[6]
It was deployed in the 1960s by the Royal Canadian Navy (RCN), the British Royal Navy (RN), and the Royal Australian Navy. The United States Navy (USN) did not adopt the AN/SQS-504, but the design of the towed body handling gear influenced later USN VDS.[7]
The success of the AN/SQS-504 has led to claims that VDS was invented[8] or primarily developed in Canada.[9]