A submarine's whole purpose is to disappear. It hides beneath hundreds of metres of dark, noisy water, moving slowly and quietly. Finding one from an aircraft flying overhead sounds almost impossible. Yet maritime patrol crews have been doing it since the Second World War, with two very different tools: buoys that listen, and a boom that feels the Earth's magnetic field.
On Monday FlightGlobal reported that South Korea's navy has started the process of replacing its seven P-3CK Orions, which are due to retire in 2030. It is a good moment to look at how these aircraft actually find a submarine.
Informazioni rapide
- Sonobuoy: Small expendable sonar buoy dropped from aircraft; typically about 13 cm wide and 91 cm long
- Passive buoys: Listen for sounds from ships and submarines
- Active buoys: Send out sonar pings and listen for echoes
- Data link: Buoys radio what they hear back to the aircraft by UHF/VHF
- First operational use: July 1942, RAF Coastal Command, code name "High Tea"
- MAD: Detects tiny distortions in Earth's magnetic field caused by a steel hull
- MAD range: Typically a few hundred metres
- P-8A Poseidon: Most versions carry no MAD; India's P-8I does
Sonobuoys: Ears in the Water
A sonobuoy is a slim tube, about 13 centimetres across and 91 centimetres long, launched from the aircraft in a canister. When it hits the water, a float with a radio transmitter stays on the surface while hydrophones sink to a set depth. Whatever the hydrophones pick up is radioed back to the operators on the aircraft.
There are two main families. Passive buoys emit nothing and simply listen for propellers, pumps and machinery. Active buoys send out a sonar ping and listen for the echo, giving range and bearing, but they also tell the submarine it has been found. Some can be commanded by radio from the aircraft to ping or scuttle themselves.
One buoy on its own hears little. The real skill is laying patterns, lines and grids of buoys, so that their signals can be combined and the submarine's position worked out. That is why patrol aircraft carry racks of them and why a hunt can use dozens.
A Wartime Idea
The idea grew out of sonar work in the First World War and US radio-buoy experiments in the 1930s. Sonobuoys first went into operational use in July 1942 with RAF Coastal Command, under the code name High Tea, flown by No. 210 Squadron's Sunderland flying boats. After the war they became standard on the P-2 Neptune, the S-2 Tracker, the S-3 Viking and the P-3 Orion.
MAD: Feeling the Hull
A submarine is a huge mass of steel, and steel bends the Earth's magnetic field around it. A magnetic anomaly detector, or MAD, is a very sensitive magnetometer that picks up that tiny distortion. It is entirely passive, and it does not care about weather or sea state, which is why it can work in rough seas where sonar struggles.
The catch is range. The anomaly fades very quickly with distance, so MAD typically only works within a few hundred metres. That is why MAD-equipped aircraft fly low and slow over a suspected contact, and why the sensor sits at the end of a long tail boom like the one on the P-3 Orion: it has to be as far as possible from the aircraft's own metal and electrics.
A restored Cold War film on anti-submarine warfare, showing how patrol aircraft tracked submarines.
The P-8 Changed the Rules
Its successor, the Boeing P-8 Poseidon, shows how the job has changed. Built on a Boeing 737-800 airframe and in US Navy service since 2013, most P-8s do not carry MAD at all: the Navy deleted the requirement to save weight. India's P-8I is the exception and carries a MAD.
Instead the P-8 leans on sonobuoys, radar and processing power, and it is being fitted with a kit that turns a Mk 54 torpedo into a glide weapon that can be released from as high as 30,000 feet. The tools have changed, but the principle has not. A submarine can hide from eyes. It is much harder to hide from patient listeners and the physics of steel.
Sources: Wikipedia (Sonobuoy; Magnetic anomaly detector; Boeing P-8 Poseidon), FlightGlobal, U.S. Navy
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