TheCodeBreakers
Locations" that Layton, when he saw it, considered as "dotting the i's and
crossing the t's" of his own estimates. It placed Akagi and Kaga (Carrier
Division 1), and Koryu and Kasuga in southern Kyushu waters, and
Soryu and Hiryu (Carrier Division 2) and Zuikaku, Shokaku, Hosho, and
Ryujo at the great naval base of Kure. All this was just a more precise
way of saying "home waters."
These estimates were based on the November observations. The call-
sign change of December 1 obliterated the intricate communication
networks that the radio intelligence units had so painstakingly built up
and forced them to begin anew. The Japanese bedeviled them with new
communication-security measures. Dispatches were sent “on the
umbrella"—broadcast to the fleet at large and copied by all ships. This
sort of blanket coverage made identification difficult. Multiple addresses
were used. They sent dummy traffic, which, however, did not confuse the
listeners
annaabi.ee 
