Subject: Doubts about FAS "Lacrosse" homepage...
Date: 27 Jan 1997 11:16:36 GMT
From: aufsj@imap2.asu.edu
Organization: Arizona State University
Newsgroups: sci.space.policy
I was recently looking about on the net and I found the
Federation of American Scientists site. They have a page that purports
to show a blueprint (one would suppose a conjectural one) of the
"Lacrosse" radar imagery satellite. There were no disclaimers, so I
would think that the data represents the best guesses of the people at
FAS. I have *very* grave doubts about what is portrayed, however.
http://www.fas.org/SPP/military/program/imint/lacrosse.htm
In summary, I think that the spacecraft pictured is a SIGINT
spinner, and that some of the other data (like the orbit) don't make any
sense at all for Lacrosse. Well, on to the thinking (well, I call it
that...).
First things first. The drawing shows possible modes of data
collection. I recognized it as a copy of documents of the Canadian
RadarSat (naturally, a radar imaging system). After some looking, I
found the artists conception at:
http://radarsat.space.gc.ca/Graphics/RADARSAT/rsat-op.e.gif
Not only do the modes correspond, but the rendering is for all intents
and purposes identical. So the diplayed data is from the RADARSAT
program, and is highly unlikely to be from Lacrosse. Apparently there is
no hard data on Lacrosse modes.
Satellite: This thing is a radar imaging satellite? The primary
feature appears to be a large high-gain antenna, which is identified as
the radar antenna. The problem, however, is that the state of the art in
radar imagery birds (even back when Lacrosse was designed) is the use of
an array. As a general rule, big dishes don't work out for operational
imagery.
The Magellan space probe used such a dish, true, but that was
primarily because the antenna also needed to double for communications to
earth, a long ways from Venus, and hence the need for high gain. In
addition, IIRC Magellan mapped by varying its orbit so it overflew the
planet, rather than by pointing the antenna in a 'near realtime' manner.
It took Magellan months to map a planet, an operational imagery bird
needs to be able to scan quickly.
In other words, in order to scan (image) an area as the
spacecraft flies over it would probably have to find some way to move
the radar 'beam' across the area to be scanned. With an array (the
RadarSat for example) this is done electronically. If you have a big
directional dish, however, you run into major problems. Moving the dish
is a big problem, that takes motors and/or servomechanisms and
whatever---all complicated things that spacecraft designers avoid like
the plague ( they have enough problems just getting damned antennas to
deploy, let alone trying to steer them on a regular basis ). In
addition, moving around that big dish antenna in some sort of scan
pattern will complicate the heck-n-darn out of the lives of the people
who have to keep that thing pointed in the right direction and not tumbling.
The way to use a directional dish antenna is to NOT move it, at
least not relative to the platform it is mounted on. Instead the whole
platform is set in motion, rolling or spinning or whatever, in order to
have the antenna pointing in a pattern (i.e. scanning). In other words,
if the platform is not spinning--it is danged unlikely that the thing is
swinging that dish back and forth mechanically. But even if the platform
is spinning, it still doesn't make much sense for imagery purposes.
The "bounce time" (time of flight at lightspeed from the platform
to earth and back) for the listed orbit for this bird is about 5
milliseconds (all numbers guessed at--feel free to correct), and that is
straight down. If the platform is spinning, then one could calculate what
the apparent maximum beamwidth of the antenna is by figuring out how many
degrees the antenna travels in the bounce time. It wouldn't make any
sense to send out a pulse and have it return too late with the antenna
now pointed in a new direction.
If the bounce time is approx 10 millisecs (we'll throw in some
process delays etc.) and the antenna picks up 3degrees, then one can
calculate that the spin would be once per 1.2 seconds. Of course, these
numbers are grabbed from the air and arbitrary. Likewise, if the bounce
time is only 5 millisecs, but the antenna is 1.5degrees narrow, then the
spin remains 1.2 seconds. One can make charts to ones hearts content.
Of note,however, you would presumably want to collect more than just one
pulse (or modulation, the principle is the same), and this throws a whole
new spanner into the works. Unless you simply want to 'detect' something
(like a ship on the water) you usually need to collect more data. I'll
avoid the miles of possibilities here, but I thought I'd throw the subject
open.
As to spacecraft size and weight and power and etc.? There
shouldn't really be any need for a large electrical supply from radar
demands alone. The RadarSat only utilizes 2.5kw of power. Why would the
bird on the FAS page, imagery or SIGINT, need those rather largish solar
panels? There are several possibilities.
If it is a SIGINT bird it would presumably be carrying a large
amount of parameter electronics for the measurement of collected signal.
Since the orbit is relatively low, it may also have a storage system for
use when the platform is out of the line-of-sight of ground stations. On
board processing systems, in the way of computers, could also be
considered quite likely. And many of these things (if it's a SIGINT bird
it could also have a cooled LNA) could use a good cryogenics unit. All
of this electricity is starting to add up--as compared to a simple radar
imagery system like RadarSat. Of course, if Lacrosse had storage and
other fancy features, they would add up in her as well.
But there is another good reason for hefty solar cells, and it
brings up the next big issue: Orbit.
The bird listed is in a 400mile 57degree inclination orbit. This
means that the platform is flying into and out of the sun all the time at
varying intervals. It not only needs to use a lot of electricity, but it
needs to recharge its batteries (there's more weight!) quickly under
certain situations, especially if it is operational a large amount of the
time. But this orbit business just gets worse and worse.
Why would anyone fly an imagery bird in a classic ELINT orbit?
For starters, IIRC, the maximum latitude under the ground track will
correspond to the inclination. Since known radar imagery systems (like,
once again, our canuck cousin RadarSat) image only relatively closely
along the ground track, this orbit makes no sense. For example,
Murmansk/Severomorsk/Plesetsk are all too far north, as is most of
eastern Siberia (with its plethora of imagery favorites). As for ocean
surveillance, the whole Northern fleet region down *below* the GIUK gap
is not going to be covered. This makes very little sense, the evil empire
may be rotting/dormant, but it is still worth a gander now and then.
On the other hand, if this is a SIGINT bird, then when it is at
its northernmost point in orbit, and with its antenna pointed out at an
angle, it will have coverage right up to the pole. The thing can cover
the whole world (because, as pointed out, imagery systems can only take
pictures relatively close to the ground track, while a SIGINT system
should be able to collect from anywhere within the line-of-sight).
Presumably, if one were to go to 57 latitude and go up approx 400 miles,
one would have a horizon that reached to the pole. Not much point in
scanning higher than that.
Why don't I think the bird on the FAS page is Lacrosse?
It has a high-gain dish antenna. Wouldn't an array make more sense?
It is big and uses juice. Well, Lacrosse could too. Weak.
Why is it in a SIGINT orbit?
Oh yeah. On that last point. If it doesn't make sense to put
Lacrosse into a SIGINT orbit, then what kind of orbit would you expect it
in? How about an imagery orbit? It makes even more sense than it sounds
like.
The 'regular' optical imagery satellites are typically in "polar"
orbits that take them over places at about the same time each day. For
example, one imagery bird will be the '9 AM' another the '1PM' and the
third the '4:30 PM' (I'm just guessing these, someone can fill in some
real values). This makes a great deal of sense since it doesn't work too
well to take pictures in the dark. On the other hand, the one great
advantage of a radar imagery bird is that it *can* take pictures in the
dark. In order to maximize its usefulness, wouldn't one expect it to be
placed into an orbit that mimics that of its compatriot optical imagery
birds, except that it arrives every time in the middle of the night?
I.E. one would expect a '2 AM' orbit or equivalent. This would allow for
periodic coverage around the clock (and managers *love* that) and would
maximize exploitation of the systems strength--the ability to do night
collection. If I were looking for a Lacrosse I would be looking for a
sun-synch orbit that comes over in the middle of the night, but that
naturally still puts the platform in sunlight for at least some part of
the day. This is worlds different than what is listed on the FAS page.
At any rate, that is a quick summary of why I think the satellite
listed on the FAS page is NOT a Lacrosse, but rather a SIGINT spinner.
Any other thoughts out there? Am I missing the proverbial boat here or
what? Is this a dead horse I am beating on this group (only recently
started reading it again...)?
regards,
----------------------------------------------------------------------
Steven J Forsberg at aufsj@imap2.asu.edu Wizard 87-01
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