Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!ncar!tank!uxc!uxc.cso.uiuc.edu!m.cs.uiuc.edu!kenny
From: kenny@m.cs.uiuc.edu
Newsgroups: sci.space.shuttle
Subject: Re: Info wanted on Atlantis "secret" mi
Message-ID: <22000011@m.cs.uiuc.edu>
Date: 8 Dec 88 21:15:00 GMT
References: <684@pyuxd.UUCP>
Lines: 35
Nf-ID: #R:pyuxd.UUCP:684:m.cs.uiuc.edu:22000011:000:1834
Nf-From: m.cs.uiuc.edu!kenny    Dec  8 15:15:00 1988


/* Written 11:06 pm  Dec  6, 1988 by karn@ka9q.bellcore.com in m.cs.uiuc.edu:sci.space.shuttle */
[Sun-synchronous orbit] is a major advantage since many pictures can
be taken of the same spot over time with a relatively constant
illumination angle.  The 57 degree inclination orbit of Atlantis's
payload has some nasty characteristics as far as earth illumination
goes; as the orbit plane precesses, there will be long intervals
during which most of the ground underneath the satellite is in
darkness, which would make a visible light satellite rather useless.
/* End of text from m.cs.uiuc.edu:sci.space.shuttle */

Say again?  Any satellite orbit, over the long run, spends half its
time with its ground track in the night hemisphere.  Sun-synchronous
orbit passes over any given spot at the same local time each day
(modulo 12 hours, not 24), but the time is latitude-dependent.  It
gets consistent viewing angles, but has the same problems with
darkness.  (Let me qualify this; some sun-synchronous satellites
follow the sunrise-sunset line, but the low illumination angle isn't
terrific for photoreconnaissance, as the shadows get really long and
contrasty.)

About the only exception is the Earth-Sun L2 point, which always sees
the sunlit hemisphere. You'd need kind of a big scope, though, to do
recon work from there. 8-)

A non-sun-synchronous orbit is definitely second best because of the
constant illumination angle you get with a syn-synchronous orbit, but
is would be far from useless.  Considering, too, that the STS cannot
achieve an orbital inclination of greater than 57 degrees because of
range safety considerations (look at the STS-27 ground track sometime,
and you'll see) the NRO might be willing to accept second best.

The idea that the device is a synthetic-aperture radar still makes a
lot of sense.

A-T