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Re: Lunar Architecture Moon Base



Bruce etal:

At 09:45 AM 12/6/2006 -0600, Bruce Bostwick wrote:
>
>There are definitely a lot more Part 15 radiators on 2.4 GHz than  
>there were 10 years ago.  How much of an issue they pose in terms of  
>raising the noise floor of your receiver depends on where you are and  
>how well your antenna rejects off-axis signals.
---snipped---
 
>I can't see any other way terrestrial 2.4 GHz signals would make a
difference  
>one way or the other except in terms of raising the noise floor of  
>your receiver's LNA.

It will make a big difference if the signal level is below the noise.  

Space loss from the Moon is:
L = 32.4 + 20LogF + 20LogD, F=MHz and D=km
D = 356,400km at perigee, = 406,700km a apogee
1296:  L=205.7 to 206.8 dB
2400:  L=211.0 to 212.2 dB
3456:  L=214.2 to 215.4 dB
5760:  L=218.6 to 219.8 dB
10368: L=223.8 to 224.9 dB
actual difference in space loss from perigee to apogee is 1.15 dB (some
round-off error in list above).

So what does this mean?  Next step is calculating SNR:  We'll do it for
F=2400 MHz
S/N = Ps - Pn
Pn = 10Logk + 10LogTe + 10LogB, Te = system noise temp, K and B =
bandwidth, Hz
Pn = -198.6 + 10Log150 + 10Log2500 
Pn = -142.9 dBm (the receiver noise floor)

Ps = Pt + Gt + Gr - L, Pt power transmitted in dBm, Gt = Tx ant gain, dBi,
Gr = Rx ant gain, dBi
Lets say the tx dish is 8-foot and receive dish is 4-foot, and the Moon Tx
output is 10w (+40dBm)
Gt = 34.2 dBi, Gr = 28.2 dBi
Ps = 40 + 34.2 + 28.2 - 212.2 dB
Ps = -109.8 dBm

S/N = -109.8 - (-142.9) = 33.1 dB
A nice strong signal on SSB

If Te is compromised to 500K, and we use 2-foot dishes (G=22.2 dBi)
then we get an S/N = 18 dB.  I guess that is still OK but a lot less strong.

try the numbers for 5670 MHz.
BTW all this math is from pp 13-2 and 13-3 of the Satellite Experimenters
Handbook, 2nd Ed. by Davidoff.

>
>As far as engineering the transponder itself .. the two biggest  
>problems of putting a transponder on the moon (1, getting it there  
>and deploying it properly, and 2, keeping it powered up through the  
>lunar night to survive the cold soak) get a lot simpler if there's a  
>spacecraft already going there that you can hitch a ride on (at  
>however many $M a pound!), astronauts (i.e. trained eyes and hands)  
>there to set it up, and an external source of power to keep it  
>running without sunlight.  It starts to get maybe sort of practical  
>if you squint real hard, once those two problems are solved.  Not  
>*really* practical in the sense of being affordable on an AMSAT-type  
>budget, but getting an order of magnitude or so closer at  
>least .. :) .. enough so that if NASA can be sold on it as an  
>emergency communications backup (with the caveat that this means it  
>could be taken over in the event of a station emergency), it might  
>just become feasible.

This thread was based on NASA's announcing plans for a manned Moon Base, so
there is your ride, and personnel to deploy the station.  The site was
described as always in the sunlight (never sees dark) so solar power is
good.  Never dark means it is not so cold!  NASA probably has the same
concerns as we do.

>The libration problem does get tricky, because the transponder's  
>antenna either has to have a wide enough main lobe that Earth stays  
>in it most or all of the time (the exact percentage being a very  
>critical engineering tradeoff), or there's some tracking mechanism to  
>steer the antenna for a tighter beam, which gets into the additional  
>failure modes of moving parts as well as the control system to steer  
>the antenna (and ways in which that can get out of sync with the  
>Earth's motion causing intermittent LOS and requiring Earthside  
>control ops commands to get it lined back up!), also a critical  
>engineering tradeoff.  Which of those two solutions is better is for  
>better minds than mine to decide, but those are the choices antenna- 
>wise ..

With a manned base I would offer that a tracking antenna is practical.
Perhaps it could be made piggy-back to NASA's comm system?  Of course there
is always electronically steered arrays.  Perhaps a quad of dishes pointed
to cover the 10-degree area of lunar sky the earth moves thru?  Then only a
4-way coax switch to track.  The 8-foot dish on the Moon has a 3.65 degree
bw at 2400 MHz.

In any case, what I am trying to say is this is just a matter of looking at
the engineering parameters and coming up with the solution.
The ride/power/maintenance issues are the significant ones...and they may
be solved by 2020 when Moon Base One is established.

I actually suspect that by then that 10-GHz will be reasonable as an
downlink frequency with uplink on 5-GHz.
Play with the numbers.

 
73's
Ed - KL7UW 
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