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RE: Fw: Mars? Why not P3x in moon orbit...?

>    Hi Tony,
>      I am aware of the Moon's liberation and realise that the 
> antenna needs
> to be
> steerable but it only has to move around on a monthly cycle and so low
> powered (low geared) actuators could do that and the "aiming 
> system could
> simply be a camera such as the SCOPE camera which could be 
> mounted on the
> side of the antenna and provide earth images "for all" as well as the
> feedback to the control system to keep the antenna aimed at Earth.

That's true, the speed of the antenna tracking would be very low, which
would certainly simplify things.  One would also need substantial battery
power at the site, if full time operation is desired, to last through the 2
week long lunar night.  Daytime power would be ample, as long as you can
carry enough solar cells to lay out after landing.

Thermal control would be critical as well, since the transponder would have
(up to) 2 weeks of unbroken sunshine, followed by 2 weeks of extreme cold
(whereas ordinary satellites are only partly illuminated most of the time,
with 0 - 50% of their orbit in eclipse - with a period measured in hours,
not weeks).

>     My first thought was, "how much gain is practicle?" 
> Considering the cost
> and difficulty of landing an "OSCAR" on the Moon, it would 
> seem to me to be
> best value to have the highest practical gain antenna steered 
> towards Earth.

That would make sense, all other things being equal.
>     Earth has a diameter of about 12732 km which means that 
> from the Moon at
> an average distance of about 385000 km the Earth subtends an 
> angle of about
> 1.9 degrees. If we were to consider the half power beam with 
> of the lunar
> antenna should be about 1.5 degrees (stations who want to 
> comunicate close
> to Moon rise or Moon set need a good view of the horizon 
> which most of us
> don't have anyway and so a reasonable compromise might be to need more
> preformance in these situations - atmospheric losses would be 
> worse in those
> situations anyway) then we could benifit from about 42.7 db 
> (my first guess
> was a bit high).
> For 2400 MHz this would be a 5.4 m dish.
> For 5800 MHz this would be a 2.2 m dish.
> For 10400 MHz this would be a 1.25 m dish.

Interesting...  :)

>     An interesting way of using a 5.4 m dish would be to have 
> a mult band
> feed so we could experiment with the higher gains of the 
> higher frequency
> bands when the Moon was high over head. Might be a good way 
> of downloading
> images etc.?

That could be very interesting.  And in our winter, we would get excellent
shots near the centre of the 10 GHz beam on the high gain dish.  Of course,
the tricky bit is getting it there and aligned, though NASA (albeit with
much larger budgets) have managed that feat on multiple occasions. :)
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