From: fcrary@ucsu.Colorado.EDU (Frank Crary)
Subject: Re: Space based combat--the next frontier
Organization: University of Colorado, Boulder
Date: Mon, 25 Jan 1993 17:30:43 GMT


From fcrary@ucsu.Colorado.EDU (Frank Crary)

In article <C1D842.95p@law7.DaytonOH.NCR.COM> Brian Lane <NETOPRBL@ncsuvm.cc.ncsu.edu> writes:
>Let's say that two sides have somehing the size of a naval destroyer out in
>deep space somewhere. how close would they have to be before radar could
>pick the other ship up, how about passive sensors (what kind of sensors
>would they be?)?

Radar detection would depend on how much power one wished to put into
the signal: Range scales as the fourth root of power. A low power system,
similar to current aircraft systems, might range out to ~200 km. A 
higher power system might extend this to ~1000 km before power consumption
became unreasonable. However, you should also consider ladar: A radar-like
system using laser instead of radio energy. Such a system would extend 
range, but at the cost of a narrower region scanned (or a longer time to
scan over the entire sky...) The issue is also confused by the possible
developments of stealth technology over the 50 years you are thinking of.

As for passive sensors, the real question is one of observation time and
sensors: A modern astronomer, with a good infrared telescope, could
detect such a ship out to an unreasonable range (millions of kilometers),
but the search would take a few weeks to cover the entire sky. What
another warship (which, presumably, wouldn't have delicate, large
telescopes on board...) could do in a reasonable time, is a different 
question. The background is also improtant: If the ship wasn't in
deep space, but had a planet in the background (e.g. is it were
in a lower orbit than the searching ship), passive detection above
the planet's emissions wouldn't be realistic. I think IR would be
the best choice for passive detection, since a ship of the
size you suggest would have to use (and therefore, eventually,
emitt as heat/IR radiation) somewhere between tens of kilowatts and
a few megawatts. Worse, the most effective heat rejection systems
concentrate the heat: Instead of a 1000 m^2 area radiating at 250 deg
K (a cool, dim source), you might have a 10 m^2 radiator panel at
800 deg K ( a hot, bright source.) On the other hand, an unmanned
craft using little power would be very hard to detect passively.

>How close would they have to be to get an adequate fire
>control solution? (I assume the energy weapon they'r using (laser or some
>other energy weapon) would have a longer range than the fire control.)

Well, a pointing accuracy of 1 arcsecond is probably the best that could
be hoped for. For a 100m sized target, that would imply ranges of
about 20,000 km (i.e. well beyond active detection ranges).
Lasers, however, have another limit: Difraction. Beyond a range of
(roughly) the laser optics' diameter squared divided by the wavelenght,
the beam will start to spread and power will decrease as the inverse
square of range. (I.e. a 10 micron IR laser using 5cm diamerer wide
optics will start to spread after (0.05m)^2/(10e-6) = 2.5 km. Higher
frequencies do much better: A 100 nm UV laser from the same sized
optice would travel 250 km before spreading. Also, the laser is still
effective as a weapon after the beam begins to spread, but it's intensity
(and therefore the damage it would do) starts to drop off rapidly with
range.

                                                         Frank Crary
                                                         CU Boulder