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!die
lauren@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--