In article ,
says...

SNIP
In your train analogy above, what is to keep the leading part of
the echo from being QRMed by the trailing part of the transmitted
signal?



The analogy is good in so far as it shows the timing, but I have to
admit it is poor in that I used a solid object, the train, to
represent a wave and their properties are very different. For
example, if two trains hit head on, you are going to have a mess.
That is not the case with waves. If you throw two rocks at the same
time in a pond of still water so that they land some distance apart,
the waves from each impact point move out in concentric rings. When
the rings from one impact point spread out enough to meet the
spreading rings of the second, there is however no "wreck". The
rings of waves of one appear to pass through the rings of the other
with no harm done to either wave.

I don't think that is true. I believe the waves would add, subtract or
be somewhere in between depending on their phase relationship.

What you say is true, but what I said is also true. The waves are not
"wrecked" and after the waves pass each other, they are are the same as
before they overlapped.

I found a reference on the web

(http://www.smgaels.org/physics/amsco.../chapter04.pdf)

that says it much better than I could. On page 109 the author states:


(Beginning of Quote)
"Two or more waves may pass through a medium at the same time. When this
occurs two rules apply. First, the total displacement experienced at any
point where waves meet is equal to the sum of the displacements of the
individual waves at that point. This is known as the principle of
superposition. Second, waves pass through each other, with each wave
unaffected by the passage of the others. After meeting, the individual
waves continue traveling in their original directions and with the same
characteristics as before."
(End of Quote)


SNIP

What you say is interesting, but...wave theory notwithstanding, what we
have are TWO RF signals on -essentially- the same frequency (ignoring
Doppler, libration, etc), the incident and the reflected. And I think we
both agree that the signals "overlap" for .75 seconds (as so aptly
stated in your 'train' analogy).

Actually they overlap for very close to 2 seconds. Thinking back to the
train analogy, the instant the pulse hits the moon (cow catcher on the
engine) and starts back the other way as an echo, we have an overlap.
That means 99.99% of the pulse (train), or almost 2 seconds worth, still
has to hit the surface and reflect before there is no longer an overlap.

Just like the old USSR jamming the VOA
-- two signals on the same frequency. Why wouldn't they interfere for
the .75 seconds in question?

I think they do to the extent of the "superposition" of the waves, and
if you were in a space capsule with a receiver 1000 miles from the
surface of the moon, I think you might indeed hear that interference.


Let me give you one more analogy:

Assume the moon is -totally- absorbent (no RF reflections).

Next assume an earth station sends a 2 second long 7.0000 MHz CW signal
towards the moon.

Finally, assume there is a radio operator on the moon. The -instant- he
hears the earth signal, he turns on -his- 7.0000 MHz CW transmitter.

Again, because of the 2 second earth signal and only a 1.25 second
transit time, the two signals will overlap by 0.75 seconds.

How can they NOT interfere with each other for the .75 second overlap?
This is -exactly- like SWBC jamming (only unintentional) and all the
'wave theory' in the world doesn't mean jamming doesn't work. Sorry, but
I believe two signals on the same frequency during the same period of
time will interfere with each other.


I agree. I thought your opening question, "what is to keep the leading
part of the echo from being QRMed by the trailing part of the
transmitted signal?", was asking why the overlapping signals
(transmitted and echo) didn't QRM the echo at the receivers on earth.
That is why I gave the example of the stones to show that did not
happen. I did not realize you were indicating a hypothetical receiver
where the signals are overlapping. My apologies.