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Re: Moon-based amateur radio transponder



Does it need to get to the lunar surface, how about lunar orbit?

Rudy

----- Original Message ----- 
From: "Michael Kjorling" <michael@kjorling.com>
To: "AMSAT-BB" <amsat-bb@AMSAT.Org>
Sent: Saturday, December 27, 2003 9:22 PM
Subject: [amsat-bb] Moon-based amateur radio transponder


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> This subject was brought up some time ago by PE1RAH, and generated some
> interesting discussions on historical aspects of the problem of getting to
> the moon, but as he pointed out few new ideas arose. Well...
>
> A disclaimer might be appropriate to begin with: I really don't know what
I
> am talking about here. I am *not* a rocket scientist nor an engineer by
> profession, and many of the calculations necessary to embark on a project
> like this are way over my head. Nevertheless, and despite not having much
> experience as an amateur "under my belt", I would like to think that while
> tossing random thoughts back and forth, I actually came up with something
at
> least moderately useful to *someone*.
>
> We know that the most recent way of getting to the moon is by using an
> ion-powered engine. That's a good idea, as long as you can get the
spacecraft
> into earth orbit, but for our purposes it'll only get us half way. Lunar
> orbit would be the best we could hope for with such an engine alone
because
> of its lack of output thrust to safely take our equipment down. So,
assuming
> that we can get into earth orbit and that such an engine will get our
> equipment to the moon, what next?
>
> I think that's where the call is for pretty conventional technology:
ordinary
> chemical rockets. The problem is that we'll have to reduce the velocity
> *considerably*. I don't know what it takes to get to the moon, but it's
> probably safe to assume that it is far into the kilometer per second
range.
> We'd need to get that down to at most the two-figure meter-per-second
range
> for a managable landing on the lunar surface. Without special means to
> protect the equipment, I would say that the one-figure meter-per-second
range
> is probably more like it. Of present technology, I am not aware of
anything
> but chemical rockets that can do that, especially during a descent onto a
> celestial body, even though fairly small.
>
> But, one of the recent Mars probes (was it Surveyor?) used airbags to
soften
> the landing. Why not do something similar here? Since there is no
atmosphere,
> it would not take much gas to create a pretty good positive pressure. That
> would mean that we can reduce our delta-V during the landing at the
expense
> of a fairly small quantity of a lightweight gas (helium, perhaps?).
Another
> advantage could be a possible saving on attitude thrusters for the
descent -
> if the "down" side of the lander is significantly flatter than the others,
it
> would seem to me that if the lander rolls around on the surface, it should
> settle down on that side rather than the others. This does add complexity
and
> quite a few variables to the equation, though - I will leave it to others
to
> determine which way is better.
>
> Once safely on the surface, and assuming a favorable location (not in a
> shadowed portion of a crater or something like that), the station would
need
> to do several things before it can become operational: deploy solar
panels,
> power up non-landing systems, and deploy the various antennas, preferably
> directing them towards earth. At that point it's pretty much up to the
> designers to decide what it should do: turn on a telemetry beacon right
away,
> or just activate the earth-moon command uplink and let the operators on
the
> ground transmit a command to turn on a telemetry beacon, the transponder
> passband(s), etc?
>
> The biggest problem as I see it, assuming that we can get something onto
the
> lunar surface in the first place, would be the antennas. How to get
adequate
> gain, and how to deploy these antennas in an automated fashion. After all,
> this isn't free space... we have more physical obstacles than just our own
> equipment to consider.
>
> As for necessary transmitter power output and antennas, if we assume a
> receiver sensitivity of -127 dBm at 435 MHz and a receiver antenna gain of
20
> dBi on the lunar surface, the earth station would need to put out about 50
W
> +13 dBi for a 10 dB S/N ratio on the moon. With 10 W output at 2400 MHz
and a
> similarly sensitive receiver, the total antenna gain (earth plus moon
> stations) would need to be 55 dBi for 10 dB S/N at the lunar receiver
> (theoretically 45.1 dBi to be at the receiver's noise floor) - for
example,
> 30 dBi on the moon and 25 dBi on earth. Of course, these numbers would
also
> apply for a downlink. Be sure to figure in any applicable converter gain.
>
> Anyway, the really tricky part would be to get something to the lunar
surface
> in the first place. I think that the rest would be fairly easy to work
out,
> especially in comparisation.
>
> - -- 
> Michael Kjörling - michael@kjorling.com - SM0YBY QTH JO89XI  ^..^
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>

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