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

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Mmm. I'm not sure we're still QUITE on topic for a GZG list. Unless told
otherwise, I'll reply to any further messages on this topic, in private
email.

On Mon, 26 Mar 2001, Richard and Emily Bell wrote:

> > > 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. 

Is this a single 'thing', or are you firing a cluster of them? For if
it's
a single thing, you simply don't have the antenna dimensions to make any
sort of narrow beam. Keep in mind that for current radar systems, to
make
beams of a few degrees in width, we need (depending on wavelength)
antennas in the 1-10 metres of size. For the sort fo ranges you
encounter
in space, you'd want a MUCH larger antenna gain, therefore a larger
antenna. Phased array or no, you'll run into this problem anyway. If
anyone could give me sone suitable ranges, I could give a ballpark
indication?

Eliminating the ship by tuning the side lobes is no problem.

Also, for each such flare, you'd need not a few, but MANY individual
transmitters. Just for comparison, modern day phased arrays use
thousands
of transmitting elements (or in the case of the active phased array
antenna being developed here, thousands of actual transmitters)

Also, you'd need positioning equipment on the flare to point it
_exactly_
at the target, and to determine the _exact_ position of the mothership.
Also, you need very exact timing info from the flashcube to the
mothership.

Where would you have gotten the exact info on where to point the
flashcube
anyway? You wouldn't have gotten that kind of exact information from
your
triangulation, I'd imagine. having ONLY a pencil-beam active sensor is a
bit of a disadvantage.
 
Strikes me as if this is rapidly becoming a very
expensive system - for each individual measurement!
 
Mm. And something I'm just thinking of. I'm not at all sure whether
phase
modulation actually WORKS with really short (wide bandwidth) pulses,
since
the actual difference in phase, as seen from the direction of the
target,
would be different at the different frequencies. I'm not sure, though,
I'd
have to give that angle some further thought.

> > > > > > > > > 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. 

Just as your EW processors become much more advanced, and capable of
lifting a signal from the background noise, so do your radar receiving
processors. I would suggest that the advantage is still with the LPI
radar, since it can concentrate all it's effort on a small part of the
received signals, both space-wise, frequency-wise, and modulation-wise,
since it KNOWS what the transmitted signal looked like. Even if
generated
completely random, _you_ know the random numbers that went into the
generation, and the other side doesn't.

I'm not saying LPI is impossible to detect. It's not _in_detectable
today.
But it _is_ hard to detect, and expensive to detect, today.

> [My description of LPI is nothing more than an educated guess]

> The flashcube accepts this inevitibility and sends one short, powerful
> burst, and the passive sensors reconstruct the reflecting object,
based
> on the return signal. 

This still does not give you any doppler measurement, or any other means
of lifting your received target echo out of the noise floor. Never mind
a
target near any sort of clutter, be it planet, asteroids or what's not.
Also, you're smearing this transmitted energy over a wide part of the EM
spectrum (due to the extremely short pulse). This results in further
losses in signal-to-noise, since you either need wideband receivers
(much
noisier) or accept that you're only using a fraction of the transmitted
energy for actualdetection.

Anyway, interesting discussion ;)

Cheers,

 Derk
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