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Ship's radio room clock. Twice each hour, all stations using 2182 and 500 kHz were required to maintain a strictly enforced three-minute silence and listening period, starting at h+00, h+30 and h+15, h+45. This allowed any station with distress, urgent or safety traffic the best chance of being heard at that time, even if they were at some distance from other stations operating on reduced battery power, or perhaps reduced antenna efficiency, as for example in a dismasted vessel. As a visual aide-memoire a typical clock in a ship's radio room would have these silence periods marked by shading the sectors between h+00 to h+03 and h+30 to h+33 in green. Similar sectors were marked in red for what used to be the corresponding silence and listening period on 500 kHz. These silence periods are no longer required as the introduction of GMDSS has produced alternative automatic watchkeeping systems.
Om een lang verhaal kort te maken... Elk kwartier moest er een radiostilte zijn van 3 minuten om eventuele noodoproepen te horen.