[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: Repeater on the Moon

> How do satellites survive the heat cycles?  Spinning satellites is one 
> way.
>  AO-40 has heat pipes.  Why not shield the electronics with solar 
> panels
> and use heat from the panels to heat the package (or just the battery
> pack).  Bury the electronics below the surface for moderating the 
> extremes.

Sats don't have to survive heat/cold cycles nearly as long as lunar 
surface packages.  They can generally use passive thermal mass to keep 
from getting cold soaked too badly, because a LEO sat is going to be in 
eclipse for maybe an hour or two at the most.  A two week cold soak is 
much harder to ride out.  Spinning is more for thermal compensation -- 
Apollo used to use a "barbecue roll" motion called PTC to distribute 
the solar heating evenly and prevent large temperature differentials -- 
and neither spin nor heat pipes help much when the entire package is in 
cold soak for extended periods.

> I would say put it to sleep at night with a photo-cell.  Keep the
> cpu/control receiver on a separate back-up battery.

The best suggestion I've heard so far has been to use an RTG or some 
other low-power long-life source to keep the package powered up 
continuously.  If it's powered up, it stays warm much better.

It's unlikely this package will need much of a control/telemetry unit.  
What I'm envisioning is a very simple and rugged translator and TWT 
amplifier with a cavity duplexer and high gain dish, or a pair of high 
gain dishes .. see below ..

> Don't point them.  Select adequate beamwidth to illuminate the whole 
> earth
> (2 degrees) plus the angle of Moon motion.  Ground stations should be
> designed for 6 dB over minimum to accomodate the "squint-angle" drop 
> off.

It's likely the positioning of the high gain antennas will need to be 
adjusted -- I don't see an astronaut in an EVA suit being able to line 
up the dish on earth too accurately, it's not easy to handle/position 
stuff in that environment.  I realize that any moving parts are likely 
to fail shortly after deployment -- not counting on them lasting more 
than a couple of weeks or so -- but this would allow some final 
adjustment after the package is set up and there's no one there to 
tweak it.  Once the dish is in an optimal position, you're right, it 
doesn't need to be moved after that.  I'm also not counting on any 
control/telemetry hardware lasting long, so it's probably better to 
design the translator/amp to operate independently of the control unit 
so *when* the control unit fails from radiation or whatever the 
transponder will still be working with its high gain antenna pointed 
correctly.  Think of it mainly as a way to get a few extra weeks of 
setup time that won't require any intervention from the guys whose 
labor costs a few billion bucks an hour .. the more we can do on our 
own, the less it will burden the EVA.

> Think manned mission to accomplish set up (pointing antennas and 
> burying
> electronics package).  Sell it as an emergency back-up to manned 
> mission
> communications.  This may have to wait till the establishment of a 
> manned
> lunar base.  Didn't someone say the Chinese were planning a manned 
> mission.

I don't know if it will sell as an emergency manned mission backup 
unless it has short-range omni capability, and even then, you'll need 
at least a 30 KHz passband to pass FM voice for suit-suit and 
suit-lander comm frequencies, plus the suit and lander radios will need 
to be designed to work the right split.  That's feasible but may be a 
tough sell.  I don't know if anyone uses SSB voice on EVA these days, 
so it'll probably have to be wide enough to pass an FM voice channel 
for that application.

I agree that it will be much easier to do this if a permanent manned 
lunar base is established, because if nothing else, it will be possible 
to negotiate use of their power facilities or ride along as an add-on 
to their communications facility, which solves a few major problems 
right off the bat.  That does raise some other concerns, most notably 
reassuring whatever space agency we're dealing with that we won't be 
endangering their mission in ANY way .. and the history of SAREX will 
tell you that isn't an easy thing to convince them of .. but it does 
change the situation quite a bit, better in some ways, worse in others 

> Only unmanned mission I can think of that might be considered is 
> joining
> the a far-side lunar astronomical observatory (which is being 
> discussed by
> the scientific community).  They will need a radio data link 
> positioned on
> the limb of the Moon in view of Earth to relay data/control commands.
> Perhaps we can help with that part of the project and add a ham package
> "piggy-back" to it.  Maybe the German-AMSAT group could take on such 
> after
> proving themselves with their P5 project.

Being on the limb of the moon would have some drawbacks, mostly 
distortion of the antenna pattern by ground effects and to some extent 
raising the noise floor of the LNA by having lunar surface in the 
boresight.  Same reason offset dishes don't work quite as well in 
"birdbath" mode as they do in the conventional position -- the feed is 
seeing something other than cold sky, and the overall noise temp goes 
up.  Ideally, we'd want to be someplace in the middle third of the 
moon's diameter as seen from earth, because otherwise the high gain 
would be pointed too low to the horizon.  If they're running a cable 
far enough to get from the limb to the center of the lunar farside, 
they'll probably run it far enough to get this far from the limb 
because these effects will degrade their performance as well.  If it's 
close enough to the center of the near side, it's an option, but we're 
not going to be able to do the kind of engineering that it would take 
to operate from the lunar limb without serious penalties ..

"Go ahead and do it, you can apologize later." -- RADM Grace Hopper, 
"The sunset is an illusion, but the beauty is real." -- Richard Bach

Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org