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Re: [amsat-bb] Laser Satellite Comms



Franklin Antonio wrote:
> [snip]
> Aren't you missing a factor of 2 here?  The spot spins around the disco
> twice as fast as the disco ball spins.  5/(21600*2) = 115 microseconds.
> 
> In any case, I agree with your conclusion.

Yes -- there is another factor of two, making the glint be twice as fast.
And of course there were other approximations (like e = pi = 3) in my
comments too ;<}

A comment was made about the lunar reflector. In point of fact, MANY
satellites do carry retro-reflectors, including the new SUNSAT. In
all these cases (save one), the reflectors are corner cubes, which
reflect directly back to the transmitter over a fairly wide angle.
These reflectors have their beams slightly "spoiled" (reflected beam
slightly larger than the diffreaction limit) so that the abberation
of light due to hitting a moving target allows the transmitter to
be seen back at the co-located receiver (the amount of the "spoiling"
is ~v/c = (7/3e5 km/sec)= a couple of arc seconds. 

In the case of the lunar reflectors, the "spot" seen back on earth 
is ~10 km in diameter, making it very hard to work DX with lunar 
laser EME! Lunar echoes are TOUGH!!! to get. Since the reflectors
were deposited on the moon in the late'60s and early '70s, the 
observatories around the world have managed to grab less than a
million laser EME photons, but have gotten some pretty fantastic
scientific results from same. Up in Pennsylvania, K3PGP has been
trying amateur lunar laser ranging and (as I recall) has claimed 
some modest success.

Of course, the purpose for these reflectors is science -- by getting 
the ranges to a few large observatories scattered around the earth
allow for the computation of very precise orbital parameters. This
in turn has provided invaluable data for the determination of the
earth's gravity field, calibration of the radar altimeters used to
measure variations in sea level, etcetera.

The one "mirror ball" that would work better for communications was
the Japanese Ajisai, launched with the first Fuji-Oscar satellite some
years ago. Ajisai was big -- ~2 meters in diameter, and the mirrors were
curved with a radius ~20 meters. This increased the "glint" time 
significantly and the solar reflections off Ajisai were quite spectacular.
But for precise laser ranging on Ajisai, the scientific community relied
on a few corner reflectors between the mirrors on the ball's surface.

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