From: turner@bigbang.astro.indiana.edu (George Wm Turner)
Subject: Re: Early Warning of missiles and meteors
Reply-To: turner@bigbang.astro.indiana.edu (George Wm Turner)
Organization: Indiana University
Date: Sat, 15 Aug 1992 18:41:11 GMT


>In article <6202@ucru2.ucr.edu>, judson@watnxt2.ucr.edu (Michael Judson) writes:
>
>A friend and I were having a discussion as to whether or not the early
>warning system used in detecting missiles can actually detect meteors
>that pass through the Earth's atmosphere.

i know that they can track anything larger than a softball in close
earth orbit (70's technology, today maybe they can track metallic tennis balls.)
being able to detect something with radar is dependant
upon what the object is made of.  i would assume a iron meteor would
be pretty shiney but even a stoney should have a pretty good radar
cross section (compared to giant nerf ball.)   the size of object
one can detect with a radar is limited by the wavelength of the
radar; you cannot see anything smaller than the wavelength you 
use for the radar.  the earths atmosphere is opaque to some wavelengths.
the major radio band opens up just under 1 cm  and goes to almost
10 meters.  there are a couple of narrow transparent bands around .9 mm
10 mm & 40 mm that could also be used.  so there are several options
to look for a 1 meter object.

finding an object with a radar is also dependant upon search rate, 
i.e. how fast can you scan, how far can you see with your radar,
and how fast is the object moving.  today's over the horizon radars are 
phased array radars and their scan rates provide sub-second horizon to horizon
scanning (no more revolving radar antenna).  this leads to the fact that there
are various kinds of radars, some pointing over the horizon, some searching 
line-of-sight, some looking from the ground up and some looking from the 
top down (i avoid saying space to ground because ground based 
over the horizon radars bounce their beams off the ionosphere back 
toward the ground).  You have phased array radars which move their beams 
electronically and revolving radars which take time to spin.  with
a slow enough scan rate anything can slip through

a design constraint in radars is how far can you see with the device.
a radar works by sending out a pulse of radio waves then waiting for
the return of an echo.  the amount of time you wait for the return
echo determines how far you can see with the radar.  the distance being
that time times the speed of light.  these systems are built to look for
object relatively close to the earth.  they probably do not look much
farther than geo-stationary orbit (22K miles).  while on the descission of 
distance, of course you need enough power to get detectable return signal.  
the intensity drops off as the inverse square of the distance 
(double the distance and you have 1/4 the intensity)

as for how long they had to see the object, that would depend on how fast
the object was moving and what was the entry trajectory.  if it came 
in slow from the north it would be more likely to be seen than if
it came in fast from the south.  our biggest fear was the soviets launching
on us and we have tended to look for the threat from the north
rather than from the south.  (watch this change if brazil ever
gets into medium range missles)

these radar systems were developed during the cold war to search for 
objects on ellipitical or ballistic orbits.  they weren't really set
up to search for an object on inbound hyperbolic orbit.  this means
that they probably didn't see it coming but did they see it pass through
their systems as it entered.   it also depends upon wether they were
looking.  i would think so.  soviet subs patrol off the east & west coasts.  
the cold war threat from these subs was that they could launch missles 
that would have short flight time to their targets. for a ballistic 
orbit this is on the order of 5 minutes to hit washington & 15 minutes 
to hit the mississippi.  for sub-ballistic orbits these times drop
to about 2-3 minutes & 7 minutes respectively.

in looking for sub launched ballistic missles and discarding sonar detection 
(launching a ballistic from a sub is a very noisy process), 
you would look for the hot pulme of the missle`s exhast or detect 
with radar.  orbiting satellites are used to search for the hot plumes but 
before that, you only had the radar system to search off the coast.  surely the 
govt has had them.  i think that was what the cobra dane radar was for
but i don't really remember.  there were the cobra dane & cobra ???? radars.
these two radars were build in the mid 70s.  i don't remember where they
were or where they pointed but i'm thinking the pointed out off the coast
versus pointing norhtward.

>This was brought up because of the tidal waves that occurred in Florida
>which meteorologists theorized may have been a meteor.  My friend asked
>why didn't early warning pcik it up, and I argued that early warning
>could not pick it up.  If it could, then why did the meteorologist have
>to theorize about the meteor and instead not have solid proof from SAC?
>--
>I was contemplating Socrates' immortal words: "I drank what?"
>judson@watserv.ucr.edu

they could detect it; if their systems were looking.  the military
is rather sensitive about how sensitive their equipment is.  they
wouldn't brag about seeing a 5 inch nick in the theorized meteor
nor would they admit that a 1 meter chunk of iron plopped down 
in the good ole U.S. of A. & they missed it.  (they were probably
looking for plane loads of marijuana anyway.)

nit-picks : SAC no longer exists.  it too has gone the way of cold war.
it has been replaced by some command that combined sac, tac & maybe
mac.  i don't remember the command's new name.  the command that did
the early warning stuff used to be called ADC {Air Defence Command}
and they may have been absorbed into the Space Command.  ADC searched the
skys and did the airborne intercepts of threats; they protected the 
home front.  SAC's job was to provide the offensive nuclear threat.

   george wm turner        turner@bigbang.astro.indiana.edu

				if you want i.u.'s opinion, ask them, not me.
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