My understanding of basic Alpha-Numeric Message decoding (not necessarily EAM decoding):
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Each crew has a code book that they check out right before they go to fly.
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They write down the message they receive a message via HF radio as well as the zulu day/hour/minute the message was sent
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Open the code book to the page corresponding with the correct zulu day, hour & minute and read the table of code words
that correspond to each letter or letter-pairs to yield a plain-text command (eg: return-to-base, weather at destination is below
mins…divert to alternate, contact HQ for updated guidance, etc.)
Note on time-of-day: I believe most "minutes" are actually groups of minutes (01-02 minutes after the hour or, 01-06 minutes after
the hour, etc).
The alpha-numeric code that is broadcast openly on the airwaves will decode to completely different clear-text messages for each
progression of the minutes field
To accurately decode the message you need all 3 elements: 1) decode book, 2) the alphanumeric message, &
3) DTG (zulu date/time group) the message was broadcast.
Bottom line: without the absolute most current decode book, the alphanumeric letters are completely meaningless to me, you,
the enemy, and the bomber crew that accidentally took off with yesterday's or tomorrow's code book (yes, It happens)