sea markern. a device which can be dropped from an aircraft to produce a distinctive patch on water below it.
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sea marker1933
1933 Gloss. Aeronaut. Terms (B.S.I.) 64Sea marker, a device dropped from an aircraft on to water, providing a distinguishable patch for determining the drift-angle.
1944 ‘N. Shute’Pastoral iii. 43 He began a chat with the Equipment Officer about sea-markers that did not mark.