Re: A couple of quick replies

Derk Groeneveld wrote:

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> On Sun, 25 Mar 2001, Richard and Emily Bell wrote:
>
> > > > 2) Advanced Sensors - My take is opposite - advanced sensors are
passive.
> > > > The better the quality, the less need for active sensors, hence
the less
> > > > detectable.
> > >
> > > Basic sensors are by definition passive, unless you include
'Hello??
> > > Anyone there?". Anything above basic will probably be to some
degree
> > > active. But yeah, I can think of some very advanced passive ones
as well.
> > > So maybe it would be: Basic - passive. Enhanced - active	&
passive.
> > > Advanced: Mainly passive, some active etc.
>
> > Actually quite the opposite.  Truly basic sensors (more basic than
the basic level
> > sensors described in MT), such as those mounted on freighters are
entirely active
> > because they only have to avoid objects, not shoot at them, and
power is cheap
> > while sensitivity is expensive.  FT warships (including the Komarov
and Von
> > Teghettof) are actually too small and maneuverable for the double
transmission
> > delays of active over passive sensors to provide an adequite fire
control
> > solution.
>
> I may have used the wrong term, buty I was referring to the d4 you get
for
> Mk1 eyeball...
>
> And we were discussing stargrunt rather than FT ;)
>
> > 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
> > on the target (vessels carry a large number of these small and
> > expendable devices).  The vessel's passive sensors (and those of
other
> > vessels in the formation) use the reflection of the single pulse to
> > determine range, size and shape of the target.  Because these images
are
> > used to aid in the passive tracking, they need not be taken very
often.
> > As the pulse is very short and the target does not know when the
pulse
> > is arriving, there is very little that the target can do besides
being
> > unreflective.  A method under investigation to defeat stealth
coatings
> > are broad-spectrum flash cubes powered by small nuclear devices.
>
> 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.  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.  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.

>
>
> 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.

>
>
> 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 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).

>
>
> > 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.
>
> Cheers,
>
>   Derk
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