When the men of the 7th RSM were listening to the radio on the frequencies that the Japanese used, they were listening for some very peculiar signals. The Japanese written language consists of a series of tiny pictures called ideograms, one for each word, while by contrast English consists of groups of individual letters put together to form words. To transmit English over the radio is simple: a Morse Code group (of long and short beeps) for each letter or number is sent; the recipient just has to read through the run-on string of letters and then insert spaces between the words where appropriate. (The famous signal "SOS" sent by sinking ships is three short beeps, three long, and three short.) Any Boy Scout learns the Morse Code alphabet -- but by contrast, a Japanese radioman had to perform the extra step of converting the ideograms to another alphabet of individual characters (called "Kata Kana") -- which is admittedly kind of like how English works, though there are more Kata Kana characters than English - and then send the Morse code groups for those characters.
"Kata Kana, sometimes called 'hen tracks,' is a simple pictorial means of phoneticizing the Japanese spoken language. In 1923, the Chief of Naval Operations, perhaps unaware of the nature of the Japanese telegraphic code, requested that Asiatic and Pacific fleet radio operators listen in their spare time for enciphered foreign radio messages.... A few other operators on the Asiatic station somehow learned to write Kata Kana and copy Japanese messages with a pencil, though none ever gained the stature of Harry Kidder [the first American to learn how].... In 1924, to obtain more legible copy and eliminate writer's cramp, Washington purchased a few specially designed Japanese typewriters."
Harry Kidder was the first American to learn the array of long and short beeps, or dots and dashes, which comprise the Kata Kana radio alphabet, and which the men of Grandpa Hurley's unit were listening for. The dots and dashes were written down as the American radio operator heard them, and then grouped into Kata Kana characters. However, these couldn't be instantly converted into ideograms because they had been encoded: instead of the characters themselves being transmitted, usually it was a group of five numbers that were sent. The numbers had been added to randomly-chosen digits from a book that was carried by every Japanese radio operator. Based on the first four digits, the sender and receiver could tell which page to use to find the digits which would allow them to encode and decode the message. This meant that the Americans had to figure out who was sending the message and to whom in order to guess at which code was being used, and then they also had to get a copy of the book full of random numbers for that particular code. Some extraordinarily complex math and plain old statistics could be used to guess at the digits in the book, but finding a copy was a lot simpler - and happened more often that you would think.
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