[Laser] Laser Digest, Vol 61, Issue 3

[Dave]

> Message: 1
> Date: Thu, 27 Aug 2009 21:48:51 -0700 (PDT)
> From: Tim Toast <toasty256 at yahoo.com>
> Subject: Re: [Laser] Laser retroreflectors on Sats
> To: laser at mailman.qth.net
> Message-ID: <325903.40199.qm at web37903.mail.mud.yahoo.com>
> Content-Type: text/plain; charset=us-ascii
> 
> I'm intrigued by these iridium sats and their bright reflections 
 > of sunlight. I know there are places online where you can get very
 > accurate predictions of the sun glints. Are there any programs that
 > give predictions for reflecting light from two ground based locations
 > instead of just the sunlight ones??

I don't know of a site, although you could plug the orbital elements
into quite a few of the orbital simulation programs and determine when
any particular satellite had a ground footprint that covered the two
observers.  Then, it would be just a matter of determining whether the
satellite had a reflecting surface that would allow a reflection from
one user's direction to the other user's direction.  Of course,
complicating that is that the geometric shapes and orientations of most
satellites aren't published (or even necessarily known).

Also, for the Iridium satellites, note that the Sun is not a point
source.  Also, note that the Iridium satellites are in a stabilized
orientation.  Plus, note that the Sun is quite bright.  :-)

> Someone off the list asked me the other day (almost at the same 
 > time the topic was brought up) if we had tried bouncing beams off
 > LEO sats and also the iridiums. Some quick math was done and looked
 > almost favorable - i know there's always a catch though. The holy
 > grail seems to me to be the geo-sats which don't move much and are
 > nice and shiny sometimes, "midway" between the moon and leo.

Run the numbers, and I think you'll find the answer is midway between
totally impossible, and almost totally impossible.  :*)

Consider that a typical (or, maybe slightly better than typical) laser
pointer has a beam (half-)divergence of about 1.5 milliradians.  That
doesn't sound like much, and it isn't.  Except, when you figure that
geo-sats orbit at slightly above 23000 miles up, that means you get a
beam radius of 34.5 miles at the geo-sat altitude.  That corresponds to
an area of about 3,739 square miles (if I'm doing the numbers
correctly).  So, your 5 milliWatt laster pointer is going to have a
power density out at the geo-sat altitude of about
.333 femtoWatts/square-inch.  How many photons is that?

Now, consider that those few photos that manage to strike the geo-sat
are going to be reflected or scattered.  If they're scattered, how many
of them will make it back to your receiver (Hint: Not very many!).  If
they're specularly reflected, even given a reasonable reflection
efficiency, the power level back on Earth at your receiver is going to
be pretty weak.

You could, of course, significantly increase your chances by increasing
the power (although firing multi-GW quantities of laser light into the
sky is discouraged), or by increasing the integration time.  But, the
easiest way might be to find a satellite with a laser retroreflector on
it (Aren't there some LEOs up there with those?).

Dave