Re: [FT]Flashes was: A couple of quick replies

Derk Groeneveld wrote:

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> On 26 Mar 2001 KH.Ranitzsch@t-online.de wrote:
>
> >
> > >Absender: derk@cistron.nl
> > > On the other hand, such extremely high powered pulses are just
about
> > > impossible not to detect.
> >
> > If I understood the proposal correctly, these would be 'flare
> > grenades', fired some distance from the ship. This makes it
difficult to detect
> > the launching ship unless it, too, is illuminated by the flash. You
would
> > need directional flashes.
>
> Mmm. Okay, I didn't read it as such. You'd need to have very accurate
info
> on the timing of the fired pulse, as well as the position, to get your
> ranging right. And of course it would be sort of obvious what you're
> doing, you're just not giving your exact position away, but still your
> general whereabouts.
>
> Also, it's going to light up your own ship in an even stronger way, to
the
> enemy? Or is it a directional flare grenade? I'm gonna reread the
original
> post :)

They are directional.  Using the technology needed to implode a nuclear
weapon, you
can fire several individual transmitters simultaneously, as a phased
array.	The
power of each transmitter is limited by the dielectric strength of free
space, but a
large number of transmitters can be ganged together in a single
flashcube.  Judicious
design of the power outputs of individual transmitters can be used to
eliminate the
side lobe that points to the ship.

>
>
> > > Also, there's a lot you cannot measure this way,
> > > which you could measure with a longer, low powered signal, like
> > > target speed (doppler shift).
> >
> > You don't need really long signals to measure doppler shift, just
> > something that has a well-defined spectrum.
>
> Well, that's the problem with an extremely short pulse, isn't it? I'm
> guessing this would appraoch the characteristics of a Dirac pulse, and
> would therefore be rather broadband. Would be very hard to measure the
> doppler, on that, as the doppler shift is orders of magnitude beneath
the
> bandwidth of the transmitted signal.

The problem with sending out a pulse train is that the passive sensors
are akin to
the SETI project, except that it is examining a much smaller volume of
space, and the
signal processor is on par with (or much better than) a Beowulf cluster
of thousands
of UltraSparc's running SETI@home as top.  While each individual pulse
of an LPI
radar is within the noise distribution, the sum total of all of the
pulses will show
up as an anomolous, and very large, amount of random noise that all
comes from the
same source.  LPI radars work because it takes an order of magnitude (or
two) more
computer power to detect than to employ, and noone puts that much
computer power into
an ESM system.	However, as the cost, volume and energy requirements of
processing
power comes down, LPI radars become harder to implement as each
individual pulse must
be closer to the noise floor and all of the pulses must be spread over a
larger
spectrum.  At some point you encounter the problem that if your
pulsetrain is
indistinct from noise at the target, it is also indistinct from noise at
the
receiver.
[My description of LPI is nothing more than an educated guess]

The flashcube accepts this inevitibility and sends one short, powerful
burst, and the