The ship is sending out an SOS signal.

这条船正发出呼救信号。

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The captain found it necessary to send an SOS signal.

船长察觉到必须发出求救信号。

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When they are in danger at sea, they send out an "SOS" signal.

当他们在海上遇到危险时，就发出“SOS”的信号。

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It is believed the first ship to have sent out an SOS signal was the American steamer Arapahoe in 1909.

BBC: Titanic

Coastguards in the UK acted after they picked up the SOS signal.

BBC: NEWS | UK | England | Bristol/Somerset | Stricken rowers rescued by ship

Sherman thought he was sending an SOS signal to the world by activating an "epirb" -- short for emergency position indicating radiobeacon -- which is a satellite-connecting beacon that resembles a floatable walkie-talkie.

CNN: Shipwreck survivor recounts drama on high seas