Originally Posted By: unimogbert
Internet search suggests that the British used heliographs into the 1940s.

Yes - the British used heliographs in World War 2, and most of the British heliographs that come up on Ebay these days were built during World War 2.

The first six seconds of this video show British operational use of a heliograph at Sollum Bay in 1942:
[video:youtube]https://www.youtube.com/watch?v=ydHuSUZffnY[/video]

There are a fair number of photos of British heliograph training in WW2, and I've read military dispatches describing their use for ship to shore communication during the D-day and Italian landings. (One made the point that, unlike signal lamps or radio, they didn't consume batteries).

I have documentation of the Australians using heliographs in the 1950s, and I have two (second hand) accounts of the Australian military teaching their personnel to use heliographs in the early 1960s.
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A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)