[Laser] Re: Lunar downlink

[Tim Toast]

This all brings back memories of a couple years ago
when there was some talk on here about a lunar optical
repeater setup. 
Since the main objective of the lunar range
experiments is to measure the distance, they have to
use short pulses to be able to resolve the distance
accurately. In one example i saw here it was stated
they used 200 mj pulses of 300 picosecond duration.
That equates to a peak power of 6.6 x 10E8 watts (660
million watts). But the peak energy is still only 200
mj (0.2 watts per second). If the repetition rate was
10 hz, then that's an average energy of 2 watts. I
assume the same number of photons is sent whether it
is in the form of 10 200 mj pulses or in a one second
CW transmission from a 2 watt laser. 

If your main objective is to just hear the signal and
not to do precise distance measurements, doesn't that
mean you could use a 2 watt CW source (modulated at
some low frequency) and receive it with DSP programs?
This is assuming the same optics and your detector can
produce a signal with only a few photons per period. 

It seems a bit odd saying 2 watts CW power from a
laser could make a return signal as strong as the one
the high powered laser system obtained, but the amount
of energy is the same. Incidentally, a few photons per
second is the approximate definition of the brightness
of a star of the 6th magnitude, which is about the
faintest that can be seen without optical aid -
eyeball (assuming dark and clear skies). 

Now, having said all that, imagine the improvement in
signal strength if the distant beacon or repeater is
only at 1/10th the distance of the moon, in
geosynchronous orbit. An active or passive repeater
located there would have 1/10th the path loss of the
moon based one while still having almost (2/3 i think)
a whole hemisphere view of the earth. The earth
appears about 18 degrees wide from that distance. A
passive retro-reflector on a geosynchronous satellite
would only need to be 1/10th the size of a lunar based
one to have the same angular size, and path loss. 

I was thinking about the relative sizes of
retro-reflectors at several different distances for
comparison:
A 1000 meter diameter area on the moon would be the
same angular size as a 100 meter area in geostationary
orbit or a 1 meter area on a LEO satellite. 
moon distance = 360,000 km
geo-sat = 36,000 km
LEO (space station) = 360 km (when overhead)

Scaling this on down to earth, the 1 meter reflector
on the space station at 360km would appear the same
size as a 10cm reflector at 36 km or a 1 cm reflector
at 3.6km. 

I thought maybe this meant you could simulate the
performance of a theoretical 1 km diameter
retroreflector on the moon by using a 1 cm retro
reflector at a distance of 3.5km and using a laser
power level of around .002 milliwatts (1/100,000th of
that 2 watt CW laser)

Hopefully this will put some things in perspective
with hopefully not too many math errors :)
 
an ISS lunar transit:
http://www.spaceweather.com/eclipses/08nov03d/Morana1.jpg

Tim Toast
http://www.aladal.net/toast/exp.html

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