Hal Murray wrote:
Concerning my query about what's good enough to count as a contact...
We've done Moonbounce with 3mW (Hobart - Dwingeloo) in JT65 - but a
26m and a 25m dish is stretching 'amateur' a bit again.
Googling for JT65 finds a nice paper:
http://www.arrl.org/FandES/field/regulations/techchar/18JT65.pdf
The JT65 Communications Protocol
Joe Taylor, K1JT
It's a fun read. 17 pages.
The basic idea seems to be that amateurs (hams) have to exchange station IDs.
That's more than a few bits, but not a huge number.
JT65 is a compact protocol for doing that in a (very) weak signal
environment. Their packet format is 72 bits expanded to 378 by forward error
correcting. On top of that, they use half of the time for a synchronizing
signal so the receiver can find the transmitter's time and frequency. Each
72 bit packet takes 1 minute to send.
Their modulation scheme is 1 of 65 tones. 6 bits per baud. The extra tone
is the synchronizing signal.
6 bits per symbol. 1 baud is 1 symbol per second.
Happy to see someone using baud, just unhappy about seeing it being used
incorrectly.
An amusing error was found in one of our early datasheets. For some
reason they wanted to tell the signal rate, so they said 1,0625 GBaud/s.
I found that very amusing to have symbol acceleration... it gets faster
every second!!! Just keep a fixed bit/symbol ratio and you have a hell
of a product. Later in life it will transport all of universe into a
black hole.
Cheers,
Magnus
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