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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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