To find something which is small, quiet, submerged and doesn't wish to be found, Northumberland sent sound waves through the depths as she tested both her hull-mounted and towed-array sonar systems–and also switched to silent running to passively listen for the ultra-quiet submarine.
That tests not just the skills of the sonar operators in the operations room, but the entire ship's company, who have to minimize all noise aboard–even down to removing shoes.
And the frigate pulled out the ace up her sleeve in the form of her Merlin Mk2 helicopter. Mohawk Flight from 814 Naval Air Squadron at Culdrose used her sonobuoys (listening devices dropped in the ocean) and her 'dipping' sonar, lowered beneath the Merlin, to locate and successfully track the Uredd's movements throughout the exercise–and conducted simulated attacks to defend Northumberland.
Descend below the surface of an area of water.
※an ace up one's sleeve:
A powerful and often secret weapon, advantage, etc., that can be used if it is needed.
A system of hydrophones towed behind a submarine or a surface ship on a cable.
A towed array sonar is a vital sensor used by submarines and anti-submarine warfare surface defense.
The tragedy of life is not so much what men suffer, but what they miss.