"Bill" wrote in message
...
On Dec 1, 2:37 am, "Al Gillis" wrote:
I'm guessing the OP intended to type selcal. If so, that stands for
Selective Calling, a scheme where two audio tones sent before a
conversation
starts "unlock" the receiver's squelch so the conversation can be heard.
Other radios on the frequency which receive those tones (the SelCal
"code")
don't unlock and the conversation is unheard. I've heard SelCal used
when
aircraft control stations are calling trans-Pacific commercial
aircraft.http://en.wikipedia.org/wiki/Selective_callingprovides a pretty
good
explanation.

Al

Al,
That must be it then. Thanks for your input. There must be a tone
that my radio didn't pick up and instead picked only the ones at the
end of the conversation. My only question is how a user would know
the frequency is in use? Could there be some kind of indication
(maybe visual?) so he won't cause interference? Thanks again.
-Bill

I'm not sure how the tone frequencies are assigned but I'd guess it's done
much like assigning "squawk" codes for a RADAR transponder. In that case
some FAA system assigns the code and marks the aircraft (in the computer)
with that code. Then the Ground Control person radios the code to the pilot
along with taxi instructions they provide. And it's likely that in the case
of airlines the pilot already has the four-digit code based on some sort of
flight instructions he receives from his company computer system.

That's my guess - anyone who knows more is please encouraged to post some
enlightening facts for us!