Check-mate system

It was used by the Royal Navy when on patrol looking for German auxiliary cruisers and others ships that had been disguised by Axis forces.

German auxiliary cruisers prowled the Atlantic and Indian Oceans searching for Allied merchant ships.

Axis merchant ships overseas when the war began were used to refuel these auxiliary cruisers, pocket battleships, and long-range U-boats.

From that date the Allies estimated the daily position of every independently routed ocean-going merchant ship in the world.

[2] If identification was uncertain, the Allied patrol could make a plain language radio query to shore and expect a reply within a few minutes.