[Boatanchors] Strange Sound On Air- Data Burst?

[David Stinson]

For years I've occasionally heard this sound on the bands:

http://home.netcom.com/~arc5/pulses.wav

It is not swept; it's specific to a frequency.
Often, there is only one set of two pulses.  
Sometimes there are three sets, other times several more.

I've been told these are data bursts sent from one RDF site
to another, which replies.  These bursts are used to adjust 
the site calibration vs. real-time propagation, giving a more 
accurate RDF "fix."   I think this was the person's theory,
rather than any certain knowledge.

Does anyone have an idea?

73 DE Dave AB5S