Re: A couple of quick replies

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<DISCLAIMER>
I'm _not_ familiar with full thrust, but as this discussion originated
as
a StarGrunt question...
</DISCLAIMER>

On Sun, 25 Mar 2001, Richard and Emily Bell wrote:

> 
> 
> Derk Groeneveld wrote:
> 
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> >
> > On Sun, 25 Mar 2001, Richard and Emily Bell wrote:
> > > Warships will not use active sensors in the same fashion as the
civilian
> > > vessels.	Warships will use their active sensors to take a
snapshot of a
> > > bogie first detected by passive sensors.	The snapshots are taken
with
> > > microwave "flashcubes"  derived from explosively pumped, high
powered
> > > microwave weaponry, which are destroyed by the energies that they
focus
<snip>
> > On the other hand, such extremely high powered pulses are just about
> > impossible not to detect. 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). Problem with super-broad band flashes is that
you
> > need super-broadband receivers, which means receiving a HECK of a
lot fo
> > noise. And since you're doing only very short pulses, you have no
means of
> > discerning between noise and a coherent signal, no processing that
can
> > help to any extent, except inbetween successive PULSES. Which is
orders of
> > magnitude less effective than what you can get from a coherent
signal
> > processing.
> 
> The problem with a "low powered" signal is that the reflected power
that
> can be detected at the sender falls off at the fourth power of the
> range, but the power detected at the target only falls off with the
> square of the range.	In space, if you can detect a return from a ship
> at one thousand kilometers (10^6 meters) with your "low powered"
signal,
> then that signal can be detected at one billion kilometers (10^12
> meters).  You get a lot more, with less danger of giving too much
away,
> by examining the emanations of the target. 

Yes, no doubt that passive sensors have a significant advantage. But IF
you're going to go active, I'd MUCH rather use long, low power
transmissions, than short high power transmissions. Due to the
modulation
on the signal, you can get in increased signal to noise ratio, and hence
the same performance as a high power transmission, with a MUCH lower
probability of intercept.

> Target speed is hard to deduce if there is only one sensor, but two
> sensors on one large ship, or two sensors on seperate vessels with
exact
> knowledge of relative positioning.  

With the distances involved in space warfare, there is no way you can
use
the parallax from two sensors on the same ship, until the target is
REALLY
close. seperate ships work, of course, but as you said, your position
info
needs to be frighteningly accurate. Narrow-beam ranging inbetween the
ships or somesuch, I guess. Very tight-beam communication between the
ships would also be a prerequisite. Asuuming you wouldn't want to
broadcast your position with your comms traffic, that is ;)

> The flashcubes are for quickly
> generating extra information, or for lone small ships to generate a
> range, from which they can extrapolate a bearing and speed.  The other
> use for flashcubes are objects that only barely radiate, usually by
not
> applying drive energies and by having their PDS units switched off. 

I can imagine attackers coasting in from long distances well outside
your
sensor range? I don't see why they'd have to do their acceleration burn
while in your sensor range. Of course, once they'd decelerate to
engagement velocities, they'd light up their engines. Doesn't strike me
as
an altogether outlandish concept.

> > Instead of going for very 'loud', short pulses, you could use long,
> > modulated, much lower power signals. harder to detect, and give you
a big
> > performance boost against noise.
> 
> Loud and short reveals the least information about yourself as you
interrogate the
> contact.  Low power, active sensors in space combat is very relative. 
You either cannot
> detect a target at a useful range, or you are announcing your presence
to the heavens.

Are you familiar with low power frequemncy modulated continuous wave, or
pulse compression techniques? Both work with low power signals, which,
lacking the very same source of modulation, are hard to detect. They are
techniques currently in use and continuously being expanded upon. They
work, and I can't think of a single reason why they wouldn't work in
space.
 
> > I'm curious where this idea came from, and what makes you state it
as if
> > it were chapter and verse? Is it FT canon?
> 
> A 1988 (may have been 89 or 90) issue of IEEE:SPECTRUM described high
powered microwave
> weaponry aimed at applying EMP-like surges on enemy equipment.  One of
the methods of
> powering such a device is an explosively compressed coil, as these
have the potential to
> convert 25% of the chemical explosive's energy into electrical output
(explosively
> compressed coils are why you could see belt fed, autofiring energy
weapons spewing out
> spent casings outside of badly written anime).  That the resultant
explosion will
> destroy the microwave device is not an issue as the pulse has left the
travelling wave
> tube before the explosives disassemble the device.  You simply make
the TWT barely
> capable of handling the power once.  I merely posited another use for
these things, as
> there are applications for terawatt radars (peak) with low pulse
repitition frequencies.

I guess I wasn't very clear. i have no problem with the concept of the
transmitter. I do, however, havbe a problem with these very high peak
signals offering an _advantage_ in being harder to detect, than chirped
(frequency modulated) longer duration, low power transmission. 

> I describe them as "chapter and verse" simply out of artistic license
(being unable to
> write even to the length of a short story leaves me with few outlets
for my desire to
> write science fiction).

Okay, no problem :) I never  played full thrust, so as far as I knew,
this could have been straight from the rules.

> > > The PDS's employ a more conventional active sensor system as the
range
> > > is much smaller and a fire control solution is needed sooner than
would
> > > be available from target motion analysis.
> >
> > Makes sense.

The more I think of it, the more I am convinced that with the ranges
that
space combat suggests, you're going to need such high update rates for
general anti-shipping weapons as well, unless you're talking homing
missiles etc. A fraction of a milli radian off-target and it's going to
be missing by miles? What sort of ranges does FT combat happen at?

Cheers,

   Derk

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