From: higgins@fnala.fnal.gov (Bill Higgins-- Beam Jockey)
Subject: Galileo's antenna (was Re: Gallileo's antenna)
Organization: Fermi National Accelerator Laboratory
Date: Fri, 16 Oct 1992 00:17:19 GMT

In article <9210140332.AA04848@etu2.educ.ucalgary.ca>, dstevens@educ.ucalgary.ca (Douglas Stevens) writes:
> Does anyone know if NASA has managed to get Gallileo's antenna
> open yet?

Boy, this is becoming a Frequently Asked Question in sci.space... I'll
have to prepare something to add to the FAQ files.

The answer is "no."  They are still trying cooling turns.  Below is an
excerpt from a Galileo mission summary I recently wrote for science
teachers.  (Keep it around, and we can all mail hundreds of copies to
the next person that asks this question...)  It owes a lot to Jon
Leech's notes from the 11 June briefing, and to the videotape that
Ross Doyle of Fermilab made for me.  Thanks again, guys.

     THE ANTENNA PROBLEM
     
A number of "cooling turns" have been performed, where the antenna
mast is pointed so it's in the shadow of the spacecraft.  As it cools
off, the mast shrinks a little, and the antenna ribs have to bend
more.  Putting a strain on the ribs like this, over and over again,
may "walk" the stuck pins out of the sockets on the antenna mast.  At
least, that's what the computer models say. So Galileo will continue
to perform cooling turns every couple of months.  

In December 1992 and January 1993, Galileo will be closer to the Sun
than it ever will be again, and hence warmer.  At this time cooling
turns will have their maximum effect.  Galileo engineers will combine
these with "hammering" the deployment motor, turning them on and off
thousands of times.  This will increase the force on the stuck ribs,
and gives the best hope of success in nudging them loose.
     
If the HGA still fails to open by next March, planners will begin to
follow a plan to complete the mission without it.
     
     WHAT HAPPENS IF IT DOESN'T OPEN?
     
The High-Gain Antenna can transmit from Jupiter and Earth at rates up
to 110,000 bits per second.  The Low-Gain Antenna is designed to
transmit at only 10 bits per second.  If Galileo has to get along
without the HGA, its operators will have to get very clever.  Here are
the steps planned:
     
--Improvements in ground antennas:  The Deep Space Network will
install better amplifiers at the Canberra, Australia station, better
receivers at all stations, and will combine several dishes together to
support a bit rate of up to 100 bits/second through the LGA.
          
--Data compression:  Galileo has multiple computers aboard. 
"Stealing" the spare attitude control computer, JPL can program it to
process image data and "squeeze" it so that a picture can be
represented by 5 to 10 percent of the usual number of bits.  Tricky
algorithms allow them to do this without serious loss of image
quality.  Data compression is a hot topic in computer science on Earth
today-- for instance Compuserve's Graphical Interchange Format (GIF)
makes use of it.
     
--Error-correcting codes:  Software will be installed aboard the
spacecraft to encode data in better formats that are resistant to
errors or noise. This will enable Galileo to transmit at a higher data
rate where errors are more likely. (The modem on your computer
probably uses error-detecting or error-correcting codes.)
     
--Flyby vs. coasting:  It has always been planned that Galileo will
spend  short periods doing close flybys of the Galilean moons,
followed by long, slow loops (typically one or two months, sometimes
longer) around Jupiter before the next flyby.  If data are crammed
into the tape recorder during a week or so during a satellite
encounter, Galileo can take several weeks to process and transmit the
results to the ground.  It will be possible to get nearly as much
science done as with the original mission, where the satellites are
concerned. But some other aspects will not do as well.
     
The first major Galileo mission objective is direct measurement of
pressures, temperatures, chemistry, and other properties of Jupiter's
atmosphere when the probe parachutes into the giant planet.  This will
happen on Day One of the orbiter's Jovian orbit-- 7 December 1995. 
The probe will transmit its findings to the orbiter during its
75-minute lifetime.  The orbiter can manage to store all this
information on its recorder and play it back using only the Low-Gain
Antenna, so this part of the mission can be accomplished despite the
handicap.
     
The major sacrifice will be frequent coverage of Jupiter itself--
atmospheric scientists were hoping to make detailed and extensive
movies of the planet's weather, wind circulation, color changes, and
so forth.  Since the total number of pictures will be reduced, these
studies will suffer.  
     
The majority of Galileo's instruments produce a fairly modest stream
of bits, compared to the cameras.  So data that are not pictures, such
as magnetic and particle measurements or ultraviolet spectra, should
be fairly manageable, and most of the original objectives of these
experiments should be met.
     
The Galileo science team estimates that despite the loss of the HGA,
about 70% of the scientific objectives of the mission could still be
met-- given hard work by many people on planet Earth over the next few
years.  And who knows?  The antenna might open after all.  Keep an eye
on the news.

     O~~*           /_) ' / /   /_/ '  ,   ,  ' ,_  _           \|/
   - ~ -~~~~~~~~~~~/_) / / /   / / / (_) (_) / / / _\~~~~~~~~~~~zap!
 /       \                          (_) (_)                    / | \
 |       |     Bill Higgins   Fermi National Accelerator Laboratory
 \       /     Bitnet:     HIGGINS@FNAL.BITNET
   -   -       Internet:  HIGGINS@FNAL.FNAL.GOV
     ~         SPAN/Hepnet:      43011::HIGGINS