[Milsurplus] "mystery" modem on 8780/7160 kHz

[John Vendely]

Greetings,

Can anyone identify the modems in use on 8780 and 7160 kHz USB, and who the users might be?  THey appear to be some kind of parallel multitone modem, occupying a nominal voice bandwidth, distinguished by an unusually long synch sequence of just under 3 seconds at the beginning of the transmission.  Signals have been heard at various times in the late afternoon and evening at moderate to strong levels here on the Florida space coast, up the east coast, and throughout the midwest.  Transmissions seem to be most common around the top of the hour, and around the half hour.  Alas, at present I have no directional antenna facility on these frequencies.  Apparently operating in frequency diversity, transmissions take place simultaneously on both frequencies.  As I first heard them on 7160 kHz, I assumed they were digital voice operations by ham operators. However, no call signs or other activity are ever heard, and upon discovering the second channel at 8780 kHz, it became obvious that this wasn't the explanation.

73,

John
K9WT