Autolycus (submarine detector)

Autolycus was a method for detecting submerged submarines from aircraft. It attempted to detect exhaust fumes from their diesel engines.

Submarines
During WWII, the first submarine snorkels came into service. These allowed diesel submarines to operate near the surface, using their diesel engines, and without being seen by surface search radar. Designs such as the German type XXI U-boat were the first submarines designed to operate primarily submerged, rather than as submersibles that navigated on the surface and could submerge as a means to escape detection or launch an attack. After WWII, this emphasis on submerged operation, battery capacity and higher submerged speed continued. In the US, the GUPPY program rebuilt wartime submarines to emphasise this. New Soviet Union submarines, primarily of the Whiskey class, were based on the four Type XXIs that were assigned to them by the Potsdam Agreement.

In the 1950s, the Fleet Air Arm were patrolling the North Sea and the GIUK gap for patrol and potentially anti-submarine warfare in search of Whiskey, Zulu and Foxtrot submarines. Patrol aircraft operated from RAF Ballykelly in Northern Ireland. A means was needed for the initial detection of submarines in the area. Once detected, other methods such as radar or sonobuoy hydrophones could be used to track and target the contact.

Operation
Autolycus was an ion-mobility spectrometer (IMS). This is an early technique of extreme sensitivity and was a major technique for the initial detection of an unseen submarine.

An IMS measures how fast a given ion moves in a uniform electric field through a given atmosphere. The spectrometer separates ions by shape and charge, so that different species arrive at the detector at different times. Typically this is used to produce a mobility profile characterising the sample. For Autolycus, a boxcar integrator sampled the times for known markers within diesel exhaust.

Withdrawal
Autolycus was withdrawn for a number of reasons. The most immediate one was the withdrawal of the Shackleton carrier aircraft and their replacement with the Nimrod MR1.

The Nimrod was equipped with a Magnetic Anomaly Detector (MAD) tail boom. MAD was considered to be more capable than the Autolycus approach and could also detect submerged submarines operating without diesels or the increasing threat from nuclear submarines. MAD had not been successfully fitted to the Shackleton, possibly because of interference problems from the piston engines and their ignition system.

Although the main submarine threat at the time of Autolycus' withdrawal was still the diesel-engine Golf and Juliett class submarines, increasing numbers of the nuclear-powered Hotel and Echo II were in service in this region. As these did not need to snorkel and did not produce diesel exhaust they were effectively undetectable by either the Shackleton's Autolycus or its ASV Mk 13 radar.

One of the cited limitations for Autolycus was a lack of discrimination between submarine exhaust, trawlers and on-shore sources. In practice this does not appear to have been a major limitation as it was used for initial detection, not identification or tracking.