[Beowulf] Re: blackbox on Mars?

Jim Lux James.P.Lux at jpl.nasa.gov
Thu Oct 19 20:21:57 PDT 2006

At 03:56 PM 10/19/2006, steve_heaton at iinet.net.au wrote:
>G'day Jim and all
>Pushing further from the topic (given your day job Jim ;) Also 
>freely accepting
>this is pure speculation...
>30% efficiency on the solar cells?! A generous round up I suspect? I thought
>Deep Space 1 only got ~23%? And that was the soopa doopa focus layer, no dust
>out there to speak of flavour?

You betcha.. by the time you can put 4-5000 kg of container on the 
surface of Mars, I suspect that 30% solar cells will be a 
reality.  Actually, Mars Science Lander (MSL) for launch in 2009 is 
probably pretty close to 1000kg (I'd have to go check, but it's 
widely described as being roughly the size and mass of a Mini).

>I also remember hearing some talk of them developing special cells 
>for Mars that
>were better operating at the slightly different 'coloured' sunlight?

Could be, although the color of the sunlight on Mars isn't all that 
much different than earth at least during the middle of the day. 
Probably depends on the amount of dust in the air, maybe a bit more 
UV (no real atmosphere).

>And while lag to Earth in radio is the same for light ('c') what 
>rates might we
>expect if we upgrade to that light carrier technology we've been 
>hearing about?

Optical comm?  It's a ways off for practical use. The robotic space 
exploration folks tend to be pretty conservative in choosing 
technologies and fairly conventional radio appears to be the ticket 
for the immediate future.  What is different from, say, 10 years ago, 
is the use of relay satellites around Mars.  Landed mass costs about 
10-100 times what orbiting mass does, so putting a big antenna in 
orbit with a relay station, and a small low powered transmitter on 
the lander is a good idea.

Big (in terms of wavelength) antennas have to be pointed accurately, 
and that's a lot easier in orbit than on a moving rover.

Optical comm shows a lot of promise as a wideband backbone link 
(because you could go to an optical receiver in orbit around the 
earth, helping solve the "cloudy day" problem), but considering that 
we've been flying radios in deep space for almost 50 years, and only 
a few small experiments for optical (Galileo carried an optical 
experiment), the "maturity" of the technology for RF is a lot more 
than for optical.

There's also the political issue (which all cluster builders will be 
aware of) that you need to have a *need* for the extra bandwidth (as 
in a customer willing to pay for it or willing to accept the 
additional risk) before you can fly it.  At a billion dollars a crack 
for flagship Mars missions, you can't really do the "build it and 
they will come" strategy.  It's worth noting that the large 64m 
antennas at the Deep Space Network were funded back in the early 60s 
to be able to get live TV back from the moon. And we're still using 
the same antennas today: they got enlarged to 70m a while back, and 
the electronics have been steadily improved, but the "dish" and its 
mechanical parts are 40 years old.

We're getting megabits per second back from Mars right now (all those 
gorgeous high res pictures from the Mars Reconnaissance Orbiter), and 
I'm working on next generation radio designs that go up to, say, 100 
Mbps or thereabouts.  To a certain extent there's a Energy per Bit 
issue.. Can't beat Shannon, as they say.  Running faster rates is 
mostly a matter of either bigger antennas or more power.

The other thing that's going on (and actually Beowulf cluster 
related) is that we're getting much better at modeling the antennas, 
or more particularly, the interactions of the antennas with the 
spacecraft.  Vaughn Cable has been developing some automeshing codes 
that take descriptions of the spacecraft from mechanical engineers 
and 3D drafting packages and turns it into suitable segments, which 
can then be run with a variety of Method of Moments codes on one of 
the clusters here on lab.  (The challenge is that the drafting 
packages tend to generate facets and triangles that aren't directly 
suitable for EM modeling.. looks just fine in the rendering, but 
things like duplicate facets or boxes within boxes cause real troubles)

Unless, of course, you were referring to that patented superluminal 
transmitter that also enhances plant growth.

>If this post is too far from topic then somebody slap me... it's 
>just that I'm a
>curious animal ;)

James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875 

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