Date: 30-Jun-90 12:05
From: Al Quaglieri
Subj: SHUTTLE COMMS

I recently ran into a gent named G. Gordon Apple on Compuserve;
he mentioned in passing that he helped design the communication
system for the Space Shuttle. Since monitoring the Space Shuttle
seems to be a recurring topic on this Echo, I asked Mr. Apple to
explain, for the benefit of this forum, why it's unlikely any of
us will ever hear the shuttle comms directly. For the sake of
clarity, TDRSS are geosynchronous satellites which relay shuttle
comms back to an earth station.

  "Since you asked why you can't receive Shuttle Air/Ground
comm., I will try to give you all the reasons I can think of at
the moment.

"1.  Part of my involvement was as systems engineer on a unit
called the Network Signal Processor (NSP).  This unit does voice
encoding (a type of digitization, but not encryption) for two
voice channels, multiplexes other data, provides framing, error
correction encoding (convolutional) and decoding (Viterbi), and
bit synchronization.  The audio is not exactly "digitally
scrambled" but would be difficult to extract without a lot of
time and expense.  It's not that complicated by today's
standards.  The actual NSP is all 7400 series chips, the most
complex of which is probably a 4-bit full adder.  (Remember, this
sucker was designed 15 years ago and required space qualified
parts.)  The (digital) bit sync was done by my friend Dr. Bill
Lindsey (of PLL fame) from USC and his company LINCOM.  There is
nothing classified about the NSP.  However, there is a pair of
jacks which goes to a ComSec box for encryption on classified
missions.

"2.  Primary Comm is on S-band and was done by TRW (where I
worked at the time).  This system can communicate directly with
ground station when passing overhead of a tracking station.
TDRSS (another TRW satellite on which I did some system
engineering) was designed to give world-wide coverage (well,
maybe 90%) using two geo-synchronous satellites with relays to
White Sands, NM.  It was intended to replace the worldwide
network of tracking stations.  After an incredible rescue of the
first of these from a non-re-firing (Boeing) IUS, and dropping
the second one into the Atlantic (with Challanger), we finally
have them in operation.  Therefore the shuttle antennas are now
always pointed up -- not down.

"3.  The secondary (but higher bit-rate) Comm is at Ku-band and
was done by Hughes.  This system operates only through TDRSS and
not directly with the ground."

[I mentioned that a dish of the size necessary to pluck out
Shuttle comms is much larger than most hobbyist could afford.]

"4.  I think your comment about the required dish size is
basically correct but, as you can see from the above, it is also
irrelevant.

  "BTW, don't even think about intercepting TDRSS relays.  The
difficulty goes up another couple of orders of magnitude.

  I hope this helps some.

G. Gordon Apple, PhD (NM6F)"
--- TBBS v2.1/NM
 * Origin: ANARC BBS-Assoc.of North American Radio Clubs (913)345-1978
(280/3)