Space

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