Pay attention at the back, ancient history/old technology, but a True Greenwich Mean Time Dit.
To minimise vunerable time spent at the roof, W/T transmissions were then Broadcast on VLF in high speed Encrypted Morse at scheduled times, immediately preceeded by the GMT time signal Pips.
On receipt, this signal traffic was recorded on reel to reel tape recorders then played back at slow speed whilst deep again when the sparkers then de-crypted the result on their ancient, enigma-style KL-7 Machines. (Forked Stick Mark Two)
So - We came up to peri depth at W/T Schedule time, had quick all round look, but no Signal traffic for us so we went back deep to continue fast passage routine.
Newby Nav pops into W/T shack, knowing there was no traffic but needed his chrono checked at such times.
Sez the newby Nav - Did you get the Time Pips OK?
Sez PoTel (Innocently) - Oh yes sir, we have recorded it especially for you.
Exit puzzled Newby Nav and all collapsed in a crumpled heap.....
Baby polar bear swims to mum on the freezing ice floe. "Mum am I a genuine Polar Bear?" "Of course you are!" mum replied. Baby Polar Bear says "I mean mum, has there ever been any hanky panky in the family, could I be a thowback to a grizzly bear?" No son I've always been faithful to your dad why dont you ask him.
Baby bear swim over to next ice floe, hauls himself out of the water. "Hello son whats the matter."Dad am I a proper polar bear?" "Of course you are," dad replies. I mean dad you haven't been unfaithful with a grizzly or anything, have you dad?"
"Son!", replied dad. "Look at yourself lovely white fur coat, beautiful black eyes, standing here on this ice floe, Of course you're a Polar Bear, why do you ask?"
Baby Polar Bear looks up and says to dad..."I'm fcuking freezing!"