Distress signal
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A distress signal, also known as a distress call, is an internationally recognized means for obtaining help. Distress signals are communicated by transmitting radio signals, displaying a visually observable item or illumination, or making a sound audible from a distance.
HMS Romney aground off the island Texel in 1804. In Richard Corbould's print, Romney's blue ensign at the stern is shown inverted, as a sign of distress
HMS Pique in a gale, flying the white ensign upside down
Signal for "yes, I need help"
Digital selective calling
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Digital selective calling (DSC) is a standard for transmitting predefined digital messages via the medium-frequency (MF), high-frequency (HF) and very-high-frequency (VHF) maritime radio systems. It is a core part of the Global Maritime Distress Safety System (GMDSS).
A Sailor VHF DSC unit
A DSC touch screen control panel showing a (simulated) distress message. This unit made by ICS controls DSC messaging for VHF, MF and HF transceivers