beowulf in space
James.P.Lux at jpl.nasa.gov
Mon Apr 14 18:34:33 EDT 2003
At 02:46 PM 4/14/2003 -0400, Mark Hahn wrote:
> > Has anybody considered the theoretical aspects of placing beowulfs on a
> > cluster of satellites?
>if this interests you, I highly recommend reading Vernor Vinge's
>recent books (A Deepness in the Sky, for instance). Robert Forward
>has some topical ones, too. they are science fiction, though...
> > I understand that communication will be slower AND
> > unreliable,
>well, to the extent that such a cluster would be spread out,
>I can understand the "slower" part. though c in vacuum is higher
>than c in fiber or TP.
>I don't see the "unreliable" part. are you presuming some kind of
>traditional RF modulation? using free-space optics seems like the
>more obvious way to network satellites, and I don't see why that would
RF for short (<1000km) links can be very reliable (certainly better than
Ethernet, once you've factored in collisions, etc.). Don't take 802.11
kinds of links as an example.
> > and it would restrict the set of problems that could be solved. I'm
> > for papers/tech reps etc on the subject.
>I doubt they exist, simply because there's no practical reason,
>given the huge cost and unclear advantage.
Certainly, flying a Beowulf to provide computing services to a terrestrial
user makes no sense, but flying a Beowulf to provide insitu computing
crunch for, e.g., data reduction on a deep space mission, makes a lot of sense.
While a broadband high rate "pipe" from GEO orbit isn't too tough (all it
takes is money to buy or rent a transponder and suitable ground station
equipment), the same from a LEO orbit is much more of a challenge. Take
something like the Shuttle Radar Topographic Mission (SRTM) as an example.
The 2 radars produce 180 and 90 Mbit/sec raw data rate for C and X band,
respectively. There isn't any convenient way to get that kind of data pipe
for something orbiting the earth every 90 minutes or so. So, they recorded
the data on a whole pile of tapes, which they brought back, and which will
take some years to ground process the 10 Tbyte of data. And that's for a
mere 10-11 days of data.
Clearly, some sort of onboard processing would be useful. SRTM was designed
to measure the topography of all land surfaces on a 10 meter (or so) grid.
Figuring the Earth's surface at 564E6 square kilometers, figuring 40% land
area, and 1E4 measurements/square km, you're looking at about 2E12
measurements reduced from around 1E13 bytes of data.
Topography is actually one of the easier measurements.. the ground
elevation doesn't change much on a day to day basis (usually).
Now consider a couple much more difficult problems:
1) quasi real time imaging of some parameter that varies quickly (wind,
2) moving target detection.. Say you wanted to track all airplanes in
flight with an accuracy of, say, 100 meters.
For a constellation of spacecraft, one could do things like atmospheric
sounding or tomography, the latter of which requires some serious
processing crunch to reduce the raw data to usable output. Imagine that you
want to tomographically process atmospheric sounding through the atmosphere
of Jupiter, but you need to send the data back through a datalink with a
bandwidth of, maybe, 1 Mbit/second, 8 hours a day. It's also got to
tolerate the somewhat(!) harsh radiation environment of Jupiter.
One can argue that for deep space missions, costing hundreds of millions of
dollars, that you're not going to be using commodity PCs mail-ordered from
WalMart stacked up on baker's carts. However, one might very well use the
Beowulf concept of lots of fairly simple, fairly slow processors,
interconnected by a high latency, moderate bandwidth fabric of sorts.
>I can imagine some really great advertisements for colo though ;)
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James Lux, P.E.
Spacecraft Telecommunications Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
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