The Royal Navy call sign equivelent to the Army's CROMWELL Alert was "Blackbird". This was the signal that the German invasion fleet was sighted, identified and inbound. All Royal Navy warships within reason were instructed to return to the nearest RN base ASAP, re-fuel, re-arm and make ready to do battle against them in the channel. This meant that RN destroyers would abandon their convoys and make a mad dash back to base to re-arm and re-equip.