Message-ID: <MZvVxt-00VcJQ50k4r@andrew.cmu.edu>
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From: space-request+@Andrew.CMU.EDU
To: space+@Andrew.CMU.EDU
Date: Fri,  2 Mar 90 02:23:38 -0500 (EST)
Subject: SPACE Digest V11 #100

SPACE Digest                                     Volume 11 : Issue 100

Today's Topics:
			    Re: Cheap DSN?
		       Giotto Update - 02/26/90
----------------------------------------------------------------------

Date: 1 Mar 90 05:49:36 GMT
From: unmvax!nmtsun!nraoaoc@ucbvax.Berkeley.EDU  (NRAO Array Operations Center)
Subject: Re: Cheap DSN?

In article <1990Feb28.163917.24091@utzoo.uucp>
  henry@utzoo.uucp (Henry Spencer) writes:
>(Yes, there are techniques for using multiple dishes as if they were one
>still-bigger one, but if you thought the technology for *one* dish was
>difficult to do in your basement, multi-dish arraying is a whole new order
>of magnitude.)

I think that in general, Henry is correct about a field of satellite
dishes being a bad way to build a DSN.  However, I would guess that
the limiting problem is probably the receiver temperatures.  The
important thing to know about a phased array is that you do _not_ need
a complex correlator.  The signals are added together, not multiplied.
This saves a whole lot in terms of electronics.  You still need delay
lines and fringe rotators to track your source, but these aren't all
that bad to build.  This is especially true if you want to pack all of
your dishes elbow to elbow, so that the baselines aren't very long.
If the baselines are short, then the delay and fringe rates are small
and the electronics are (comparatively) cheap.  Plenty of early radio
telescopes were built with techniques like this, (albeit with 19060's
technology).

However, one of the reasons that they aren't built that way today is
that it is not economical to do it that way.  (In addition, you get
considerably more flexibility for astronomy with more sophisticated
techniques.)  However, let's stay with our field of satellite dishes.
Just off the top of my head, the system temperature of a K-band
satellite receiver, (read block converter if that makes more sense),
is about 1200 K.  Note that such things do not exist for the
frequencies that NASA is most interested in tracking.  (Voyager used
X-band, for example.)  Let's say that one could get a 1000 K X-band
receiver for a reasonable price.  The output signal-to-noise of a
phased array scales with the square root of collecting area, and
inversely with the system temperature for each individual element.  In
our case here at the VLA, our X-band receivers are about 30 to 35 K,
and each element is 25 m in diameter.  In other words, you need
(25^2)/(2^2) = 156 small dishes to match the collecting area of one
big one.  One top of that, you take a hit of, say, 1000 / 35 = 28.5
due to the difference in system temperatures.  So now we need maybe
4500 small dishes to get the same point source sensitivity as one big
one.  Even with economies of scale entering in, I think you'd be lucky
to build such an array for anything less than $1000 per element.  At
least half that would be capital cost, probably more.  Add in
development cost, and $1000 per is cheap!  So the sum total for our
array is now some 4.5e6 dollars so far.  For comparison, a single
fully outfitted antenna here costs about $2.0e6.  Now consider that by
NASA standards, a 25 m dish is small.  The VLA helped considerably
during the Voyager encounter, but only by virtue of the fact that it
has 27 antennas.  The primary Goldstone dish is, what 76 m or so?  It
also a couple of outriggers in the 35 m class, too.  By the time you
start trying to match these sorts of numbers, you're talking a minimum
of hundreds of millions of dollars.  At this point, it has long since
become worth your while to do a careful cost analysis of your new
array, in order to get the biggest bang for the buck.  It turns out
that TV satellite dishes just aren't the way to do it.

The single point I want to hammer on, is that anytime to try to make
up with numbers what you lack in size, you have to make the individual
elements fairly cheap.  It often turns out that the performance hit
you take by making the elements cheap is worse that the benefits you
gain by making lots of them.

As far as the orbiting DSN is concerned, I have long thought that a
free flying Arecibo type dish with a free flying feed is an incredibly
sexy idea!  (Note that this is a spherical reflector.  Take up a large
load of soap bubble stuff, and then coat it with metal vapor.)
However, I have been reluctantly convinced that _for astronomical
purposes_, the moon is a much better idea.  Maybe different arguments
apply for the DSN case, but I doubt it.  I'm afraid that for the
immediate future, this will remain a pipe dream.

-----
This is a shared guest account, please send replies to
dbriggs@nrao.edu (Internet)
Dan Briggs / NRAO / P.O. Box O / Socorro, NM / 87801  (U.S. Snail)

------------------------------

Date: 26 Feb 90 22:39:44 GMT
From: zaphod.mps.ohio-state.edu!brutus.cs.uiuc.edu!jarthur!elroy.jpl.nasa.gov!jato!mars.jpl.nasa.gov!baalke@tut.cis.ohio-state.edu  (Ron Baalke)
Subject: Giotto Update - 02/26/90


                              Giotto Update
                            February 26, 1990
 
     Reactivation attempts on Giotto continues. On February 23 the
European Space Operations Center (ESOC) attempted to perform a
spacecraft attitude maneuver as well as test the despin motor of the
High Gain Antenna (HGA). The downlink of the Low Gain Antenna (LGA) was
left on during these attempts. The doppler signature confirmed the manuever
did take place successfully. The doppler signature also indicated
that ESOC was able to control the HGA despin motor. ESOC's first command
sequence (to turn off the spacecraft downlink) did not work. ESOC
had to retransmit the sequence to turn the spacecraft off. There was
no indication of a cause for the first command sequence not being
accepted.
 
     On February 24 the HGA acquired earth point. The tests started with
a series of telemetry on/off commands to check the command system
responses. Following this, a maneuver was performed to sweep the HGA
past the earth. This manuever was successful. The maneuver was stopped
and a series of small reverse maneuvers was performed to bring the HGA
back to earth point, and was achieved with telemetry in-lock.

 Ron Baalke                       |    baalke@mars.jpl.nasa.gov 
 Jet Propulsion Lab  M/S 301-355  |    baalke@jems.jpl.nasa.gov 
 4800 Oak Grove Dr.               |
 Pasadena, CA 91109               |

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End of SPACE Digest V11 #100
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