[Laser] Re: another lunar experiment

[Garnier Yves]

>I remember you calculated one time that a 10 watt signal in a
>2 degree beam width could be detected easily from lunar
>distance with FFTDSP.
   
  Be carreful Tim :
  This calculation is from a point to point loss ONE WAY.
  With the lunar distance plus the return to the earth after bouncing the losses are gigantic because the returned beam is wery wide and angular diffused ! (b.e. Tycho area)
  73 Yves F1AVY

Tim Toast <toasty256 at yahoo.com> a écrit :
  You are probably right Yves. If i understand correctly, in
the Frequency Domain, it is all about average power. So it
is like comparing 3 watts with 1000 watts in the end. I am
definitely leaning toward the heliograph mirror idea now. I
remember you calculated one time that a 10 watt signal in a
2 degree beam width could be detected easily from lunar
distance with FFTDSP. A 1 meter heliograph would have 100
times the energy of that one way link, so maybe it is the
only (low cost) way to even come close to doing passive
EME.

The xenon strobe has a really long pulse length when
compared to the ultra narrow pulses used with the Apollo
retro-reflector experiments and i thought maybe that could
make up the difference somehow. If i could get pulse rates
close to 500 or 1000 Hz, the pulse train would become more
like a modulated CW signal. But there is probably no way to
get that kind of performance out of the small tubes and
simple circuits used in modified cameras or disco lights. 
And I think the clouds, rain and tornado's are trying to
tell me something too 8)




--------Yves F1AVY wrote----------

Tim
The most important is the energy you transmit and 
proportionally you receive.
If you use the FREQUENCY DOMAIN to detect your signal by
FFTDSP, a one kW 
average power (square signal modulated at 50% duty cycle)
will be incomparably 
better your 3 kW peak 3 watts average power to hit the
moon.
Many reasons for that:
The number of photons you will receive is in the energy
ratio by time unit.
To be sensitive the FREQUENCY DOMAIN FFTDSP must accumulate
in high 
resolution during long time. 
T duration short pulses must be amplified in a 1/2T
bandwidth that means 
enormous extra noise.
A short time low rate pulsed signal gives very numerous
harmonic frequencies.
With the FFTDSP FREQUENCY DOMAIN the harmonic FFTDSP
spectral lines will be 
very weak because the energy is divided in each of them. 
To detect low rate short pulses, you must use a TIME 
DOMAIN receiver with 
ultra high accuracy synchronic time window, adapted 
filters, correlation, 
convolution... many complex electronic processes.
For example an ultra short pulse can be detected via an
APD in Geiger mode 
which is over biased just at the expected arrival pulse
time and during the 
same duration the pulse width.
It needs very accurate and difficult time controls....
73 Yves F1AVY




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