This could be an ID modulation to serve the receiver's logic. In
other words, correction is based upon both signal strength AND
modulation.


I think it's even simpler than that... It could be based on the signal
strength alone.

As long as the received signal is treated in an analog manner, it
would be easy to tell the difference between a strong and a moderately
strong signal. I'd bet they don't do anything as sophisticated as a
digital decode to identify the transmitter.

Be on the lookout for 'vlf transmitter, part II.......coming to this
newsgroup soon. Basically, I ran the transmitter into a spectrum
analyzer and saved the output spectrum of the transmitter. It's very
interesting and leads to another whole set of questions.

Regards.

A