[Spooks] Unmodulated carriers on 6993 kHz

[Martin VE3OAT]

Does anyone know anything about the multiple unmodulated carriers on 
6993 kHz, apparently reliably reported to be from transmitters in 
Florida, USA?

My original source of information about them was Glenn Hauser and his 
World of Radio broadcasts.

At my location 2000 km north of Florida, they are heard each 
evening/night, and one or two are easily seen on a spectrum analyzer 
during local afternoons (though still too weak to hear by ear).

I have logged these on 6992.990, 6993.000, 6993.010, and 6993.020 kHz. 
  The one on 6993.000 is always the strongest, but usually only QSA2 
here.  When strong enough to actually hear by ear (evening/night), 
they are easily picked out by the audio 10 hertz beat-note between them.

Similar carriers are also heard on 4443.000, 4443.010, and 4443.020 
kHz, "heard" here (that is, more easily seen on spectrum analyzer) 
only in the evenings and nights.

My guess is that they are used in some sort of ionospheric experiment.

Wondering if anyone here knows more.  Thanks for any info.

... Martin