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Originally Posted by Puster Bill
Just by way of explanation for the uninitiated, when messages are passed in Morse they are often preceded by a "preamble". Often, this contains things like a message serial number, a date and/or time group(s), the number of groups in the message, and sometimes other administrivia.
Sometimes that information is actually encoded in the message, though. The danger in doing that is that you give the enemy a stereotyped beginning which can help him break your codes/ciphers. Sending the information in the clear helps prevent that.
Here is a notional example of a preamble for a message:
NR 123 GR 8 0221 0904 BT
THISI SSUPP OSEDT OBEAN ENCRY PTEDM ESSAG EXXX
AR
NR 123 is the serial number, ie., this is the 123rd message from this entity
GR 8 is the number of groups, 8.
0221 is the date the message was sent, February 21st
0904 is the time the message was sent, 09:04
BT is a 'prosign' that means "Break Text", and it separates the preamble from the actual message.
After the 8 groups are sent, the operator sends "AR" which is a Morse prosign that means "End of Message".
I have yet to find evidence that the U-bootwaffe radio operators used preambles, or indeed if they even used callsigns to call up the other side. To the best of my knowledge (which is admittedly scant and circumstantial), it seems as if Ubootwaffe communications were sent "blind" on fixed frequencies with no calling involved. In other words, if BdU wanted to contact KL Schmidt on U-127, they would transmit just the encrypted part of the message, depending on the radio operator on U-127 to be alert and copy it (and all the other boats at sea would copy it also, unless they were submerged to deep to receive it). The radio operator on all the boats would decrypt the message, but for most it wouldn't apply.
Likewise, the operator on the boat would send a message "blind" to BdU, but because BdU had extensive receiver facilities and never submerged obviously, they were more likely to receive messages. BdU would acknowledge the message by repeating it with a serial number attached.
The frequencies involved would have been picked according to what worked best at that time of day for "DX" communications, higher frequencies for the day and lower ones at night. BdU would also transmit using their VLF transmitter, which could be received underwater and at great distances. The HF frequencies would be relative: boats in the North Sea would use generally lower frequencies than boats in areas farther away, because otherwise the station they intended to contact would be within the shadow of the "skip zone" and wouldn't be heard. Different operational areas would have different sets of frequencies.
Probably the best way to simulate the sound of a message being received would be to send a couple sets of V's (often used by operators to "warm up" and make sure the transmitter is operating properly), followed by some 4 letter code groups, and ended with an "AR" and perhaps an "SK", so it would look something like this:
VVV VVV
INDI CATR IFYO CANR EADT HISY OURT WODA MNED CLOS EXXX INDI CATR
SK
VVV VVV is the operator sending some V's to warm up, check his transmitter, and to alert the receiving operators that something is going to be sent.
INDI and CATR are the "Indicator" groups, sent at the beginning of the message and repeated at the end, with the actual message in between.
The SK or VA at the end is the prosign that signals the end of the transmission. I prefer the "SK" usage because that's what I'm used to, but in practice it makes no difference whether you copy it as SK or VA, the two characters are "run together" to make one big prosign, so it's a chose your preference kind of thing.
If you wanted to be really funny, you could make an 'Easter Egg' in the form of a message encrypted in Naval Enigma, formatted like an actual message, and record it using fast hand-keyed Morse. Might be an interesting exercise.
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