> Absender: derk@cistron.nl
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.
> Also, there's a lot you cannot measure this way,
You don't need really long signals to measure doppler shift, just
something that has a well-defined spectrum.
Greetings
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> On 26 Mar 2001 KH.Ranitzsch@t-online.de wrote:
> >Absender: derk@cistron.nl
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:)
> > Also, there's a lot you cannot measure this way,
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.
Cheers,
> Derk Groeneveld wrote:
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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,
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
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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
You would
> > > need directional flashes.
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,
I'm
> > guessing this would appraoch the characteristics of a Dirac pulse,
> The problem with sending out a pulse train is that the passive sensors
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
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,