wrote in message
oups.com...
For the feds, the band can be fixed or any mobile except aero. For the
rest, it's fixed.

Thanks for checking. I checked WUN logs and found
nothing.

I tend to think it is commercial but it could be military
(or diplomatic). The FDM aspect is definitely a
diversity thing, to overcome atmospheric fading.
All channels carry the same data. If fading temporarily
wipes out one channel the data still gets through.
There are numerous single channel 850 shift 75 Baud
fsk signals out there. I'm guessing they are military.
HF has notorious fading so I imagine they use
a diversity scheme too but with the parallel channels
spaced out over MHz, not just in one voice width
band. Being the military they have all sorts
of resources so they can use such widely spaced
frequencies. This FDM FSK thing requires less
coordination in setting it up yet still offers
some diversity and immunity from fading.
But the more modern way of doing that is with
OFDM, so this signal is kind of unusual.
It is modern primative.

--
rb


wrote:
I did a FCC search, but found nothing. The NTIA allocation just says
fixed. I'll check the redbook.


Ron Baker, Pluralitas! wrote:
It is transmitting right now.

I regularly pick up an unusual data signal
at 15937.5 kHz (USB). It is 7 fsk signals stacked
in frequency. They are 170 Hz shift and there
is 170 Hz between them. The Baud rate is 75.
All seven channels send the same bit stream
but offset in time by as much as 6.66 seconds.
The bits show a flat autocorrelation.
Anybody know what this is/ who is sending
it?

--
rb