Re: Laser ranging to moon begs questions

On Mon, 29 Oct 2007 18:26:04 +1100, "Peter Webb"
<webbfamily@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

The entire aim of my query was simply to find out how one could assert
3.8cm/yr from one pulse received every once in a while as stated in
the NASA press release. My interest does not extend beyond this and I
should have kept this in mind

Ohhh ***, that's easy.

If you get (say) a single return photon every 10 seconds, you get 360 per
hour or a few thousand per day. After a year, you would have about a million
data points.

So far so good.

Pick the 1% that came back fastest, and you will get resolution
equal to 1% of the pulse width - that's a fraction of a centimetre.

Why didn't you say so in the first place?That's a good idea: pick the
fastest, since it is the only way to simulate the onset of the pulse.
(And, not to *** in, but to double the production to 2% efficiency,
pick also the 1% slowest, marking the end of the pulse with equal
accuracy).

The rate of photo return isn't an issue; its the absolute number of photons
you get back.
A high rate of photon return makes for good signal/noise ratio, but if
it is a case of single returns it doesn't apply, but does mandate
tight noise-avoidance gating.
Even with very occasional return photons, run it long enough
(eg for weeks) and you will get a huge number of data points to analyse.

Take a couple of weeks or months (after which they probably connected
chimes to announce the next entry). The gear is I believe dedicated,
but you couldn't do that on the Hubble, where everyone wants a piece
and a 6z takes a 2 week time exposure (I read once).
So, now you know.

I am happy with this turn. Now I can get back to the dark matter
problem. Oh, I have it solved, but now the only problem is how to make
it palatable.

John Polasek
.