RE: starship-design: Massively Distributed Computing for SETI

["L. Parker" <lparker@cacaphony.net>]

> This may be true but the beam's does not need to updated that
> much, I guess
> once a minute and that is mostly with the feq of the beam.

As Zenon's post points out, this is all dependent upon the beam aperture and
distance to target. I posted the relevant equations several years ago, they
should be in the archives.

Without actually sitting down and figuring it out, I would say that Zenon's
guess at ten minutes would be the outside limit for retargeting in 3
dimensions. The frequency shift depends upon two things, the acceleration
rate and the nature of the sail and how forgiving it is. Probably not less
than every ten minutes though.

A typical non-mobile phased array in use today has around 100,000 emitters.
Each of which requires a separate solution. There is a limited ability to
"group" them to reduce the computation load to manageable levels, which
results in some tradeoffs in other areas, but since we aren't talking about
using this as a sensor, the tradeoffs aren't relevant.

Zenon's best case was a 100 km antenna, which is probably the one we should
go with given the power densities we need. Since even relatively small
airborne phased array antennas of approximately one meter square contain
between 2000 and 3000 emitters, we can use the smaller number to estimate
the number of emitters and therefore the computing power necessary.

A 100 km antenna contains 10,000,000,000 m^2 * 2,000 emitters or
20,000,000,000,000 emitters each of which requires several Fourier
transforms to update its targeting. The computers controlling the one meter
square airborne radars are state of the art digital signal processors and we
would need the equivalent of 20 billion DSPs to perform these computations
for the emitter processing alone.

Now factor in the overall targeting solution using the three body solution
problem I began with and you begin to see the size of the problem. We
haven't even touched on several other of Zenon's objections such as
downrange beam degradation, etc.

Lee