Re: [amsat-bb] Moon-based amateur radio transponder

[Bruce Bostwick <lihan161051@xxxxxxxxxxxxx>]

> 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.

The airbag landing was mainly a means of reducing the complexity of the 
landing automation.  The payload package deploys a parachute and a 
small solid rocket pack that fires at a preset altitude indicated by 
the landing radar, bringing the payload to a complete stop something 
like 20 feet above the surface.  The payload then separates from the 
parachute and free-falls the remaining distance, and the airbags 
cushion the landing.  The tetrahedron shape of the package ensures that 
it either lands upright or turns upright when it opens, although the 
latter case does cause a small shock when the center of gravity shifts 
and the payload rolls over to the upright position.

What it doesn't do is reduce the delta-V required for a landing.  It 
simply reduces the precision of control required to land safely -- the 
early Viking probes required an extremely complex and precise landing 
control system to do a LM-style throttled landing on the Martian 
surface, and the cost of duplicating that was prohibitive.  All the 
current crop of landers have to do is come to a stop right over the 
surface and then free fall, much easier to accomplish.

> 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?

You'd definitely want the command/telemetry link to come up as soon as 
the lander is on the surface, or as soon after that as is practical.  
Once that's up, you can use that to monitor and/or intervene if the 
high gain antenna doesn't deploy right or whatever happens.  Something 
WILL go wrong, count on it, and you'll want command and telemetry up to 
be able to do whatever you can to deal with it.

> 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.

What you'll be able to count on is being within some fairly large 
number of degrees of level at landing, and that's about it.  You wont 
have any control over orientation relative to lunar north (and as if 
that wasn't enough of a hassle, there's no magnetic field to speak of, 
so a magnetometer won't tell you much) so you'll have to have some 
means of determining your orientation in space to point the high gain 
antenna.  And there are tradeoffs involved .. see below.

> 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.

Assuming the payload is intact and has full power and all the equipment 
still works, you have two choices as far as how to get the high gain 
antenna pointed right.  The earth will appear to move back and forth in 
the sky due to libration -- the moon is tidally locked to the earth but 
due to the eccentricity of its orbit it oscillates back and forth by 
some distance over the course of a lunar orbit.  If you have a tight 
beam focuse, you'll lose gain during part of the month because the 
earth won't be in the center of the main lobe part of the time.  If you 
loosen up the antenna pattern a bit to allow the earth to be in the 
main lobe during the entire libration, you'll pay for that in overall 
gain.  Moving the antenna to track the earth as it moves is probably 
out of the question because it will take too much power, and because 
sooner or later those tracking mechanisms will eventually jam.  Better 
to set in one position and leave it -- you can probably only count on 
enough life in the antenna positioners to get it in the optimum overall 
position and then lock it in place.

You might be able to compromise with an elliptically-shaped dish that 
has a narrow elliptical main lobe that covers just the earth's path in 
the sky IF you can accurately determine your orientation and have 
enough travel in the antenna positioning gear to get the antenna into 
the right position.  Those are BIG ifs, btw, and will involve other 
tradeoffs in terms of what else you can carry along.

Practically, what will probably work best is to optimize as much as you 
can for the middle of the libration, maybe help out some with an 
elliptical outline dish that concentrates more of the signal in that 
more or less linear path, and just have people put up with a little 
loss of gain twice a month .. it might actually work better for some 
folks since it will make the transponder a bit harder to work during 
the fades and a lot of the appliance operators will go elsewhere.  But 
that's all to be taken into account in the decision making process.

> 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.

Easy is relative on the lunar surface.  Getting there is the biggest 
obstacle, but to be practical this transponder has to work for a long 
time, longer than anything solar powered on the lunar surface has ever 
worked.  If it goes up and lands, and doesn't work, or works for a 
couple of weeks and then dies after one lunar night, you've wasted an 
ENORMOUS amount of money.  The up side of that is that if you can pull 
it off, and get something to work up there for the long term, you'll 
open the door for a lot of other developments ..

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