generally the wolfpacks would be coordinated through BDU... one boat would report contact, BDU would then order that boat to begin broadcasting a locator beacon which other boats could home in on, when BDU was satisfied with the number of boats that had reported in that they had arrived at the convoy the green light would be given for the attack.
more or less thats the way it worked as i understand it.
ships and airplanes could then ... and still do today often navigate off of AM frequency bands. so closing in on a submarine transmitting on a specified freq would have been as easy as following an indicator needle on a direction finder.
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