admin@hydra.UUCP (admin) (03/10/85)
I have recently learned that the UPI is using spread
spectrum transmissions from one of the Westar or Galaxy satellites
to feed news to small earth stations (2-3 feet in diameter).
I understand that the technology used was developed by
a company called Equatorial Communications. I further undertand
that the spreading is done by diphase modulating a carrier with
a pseudo random sequence at a much greater rate than the actual
data stream being transmitted so as to convert it into a 2-mhz
wide noise like signal which is uplinked to occupy part of the
bandwidth (and power) of some transponder.
It seems to me that it might be a challenging and fun task
(with the understanding of course that the recovered data would not
be used for other than my incidental amusement and definately not
for benefit or gain) to demodulate the signal and recover the news.
In order to do this I need some more details of how the system works.
Specifically :
1. Which transponder of which bird carries the UPI feed ? What
is the exact carrier frequency of the signal ? How powerful
is it ?
2. What is the chip rate (PRN sequence bit rate) used ?
3. What is the actual data bit rate (IE how many chips to a bit) ?
4. What is the PRN sequence used ? How is synchronism with the
local correlator sequence generator obtained ? I have been told
that there is some sort of reset pattern, is this true ?
Is the sequence ROM based or generated by a feedback shift
register circuit ? If I recover the chip sequence with a
large enough dish and a PSK demod could I figure out
how to generate it (given that some parts of the sequence I
recovered were inverted by data) ? Does anybody know a good
method of figuring out the generating polynomial for a P/N
sequence given a sample of that sequence EXOR'd with random
data ?
4. Does the data modulate the sequence by simply EXORing with
it or does it select entirely different patterns (one pattern
for a 1 a different one entirely for a zero). (The later scheme
seems slightly unusual to say the least but some literature
on the system described it that way.) Is NRZI coding used
for the data or straight NRZ ?
5. Am I correct in assuming that the underlying data is sent
in a bit synchronous format (with fixed length characters)
with synchronism established by some mechanism similar to
that used in BSC protocols (a SYN character) ? That information
is packetized with headers and some sort of CRC (a fire code ?)
that can be used to correct errors in the data (same sort of
thing that a disk controller does when it gets an ECC error).
(The company advertises forward error correction, but they
don't say what rate or mechanism is used).
6. Is the actual data (the news) encrypted in any way or should
I expect to see sequences of characters (once I acquire character
framing) that seem to resemble ASCII or Baudot or TTS news stories?
If the data is lightly encrypted could I break the cipher
given lots of plaintext in the form of stories from the same
wires acquired from other sources (such as SCPC carriers) ?
[I might say I very much doubt I would have the time or talent to
to break such a cipher, but I can always hope to find
someone who likes such challenges to help me ]
7. Is there a publiclly available filing with the FCC or
any other public record description of the details of
the modulation and coding scheme used ? (I am told that
most such things - even TV scrambling schemes - are available
as public documents if you know where to look)
8. Does anybody know of any features of the system that would
make it impractical for a hacker who is a hardware/software
design engineer by trade to receive news with reasonable
effort ? I assume that one can cheat by using a somewhat bigger
dish and better LNA (there are some 60 K ones now) and not
do quite as optimal a correlator. I assume also that determining
all the details of the packet protocol is something one can do
over time (and only to the extent that one needs to to recover
the "wires" one is interested in). The real idea is to
see a 3 foot dish pointed out the picture window receive
some mostly intelligable news when connected to a breadboard
full of cicuitry one designed and built controlled by a program
one wrote.
Any help would be greatly appreciated.die@hydra.UUCP (Dave Emery) (03/10/85)
Due to an anomaly in the mail system at CRDS (and the fact I had
superuser on) my posting requesting information on UPI spread spectrum
signals on satellites wasn't properly signed.
What follows is my address ...
--
David I. Emery Charles River Data Systems 617-626-1102
983 Concord St., Framingham, MA 01701.uucp: decvax!frog!dielauren@vortex.UUCP (Lauren Weinstein) (03/12/85)
Equatorial considers their spread-spectrum encoding details to be very highly proprietary. They have not been published. They also currently scramble much of the data and will probably be scrambling it all (most likely via DES) rather shortly. --Lauren--