Originally Posted by
ValMORNA
It was also significant that, prior to a raid, each aircraft conducted a radio check using the a/c call-sign. Thus RAF sigint knew which units and numbers of aircraft would be involved, thereby forewarning Fighter Command.
(Allegedly)
I seem to remember reading somewhere in the dim and distant past that, prior to an op, it was SOP for Bomber Command crews to check out their aircraft, including a ground radio check (on HF!) - all of which was being monitored by the Luftwaffe Signals Intelligence Agency (
Luftnachrichten Abteilung 350).