 By March 1942, the U.S. was able to decipher up to 15% of the IJN's "Ro" or "Naval Codebook D" code (called "JN-25B" by the U.S.), which was used by the IJN for approximately half of its communications.By the end of April, the U.S. was reading up to 85% of the signals broadcast in the "Ro" code.In March 1942, the U.S. first noticed mention of the "MO" operation in intercepted messages.