Flash spotting

Flash spotting was a military method of detecting the position of enemy guns at long range where the gun could not be observed directly, and was developed during World War I. The flashes could be observed at night as reflections from the sky. The purpose was then to call up friendly guns to destroy the enemy guns position. Theoretically this could be achieved by several observers spotting the flash from a particular gun and then plotting the intersection of the bearings.

This was extremely difficult with multiple guns firing since several observers could not be sure they were all looking at the same particular gun. The British solved this using a flashboard located at HQ fitted with a combination of buzzers and signal lights connected to the observers by telephone wires, which operated in such a way that after a sequence of observations, all observers could be sure they were looking at the same gun flashing and its position determined by triangulation.

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Wikipedia