die@x.UUCP (Dave Emery) (10/15/84)
Does anybody know if any of the 1200 baud diphase psk Navy fleet broadcasts from the geosynchronous UHF fltsatcom spacecraft carry UPI, AP or weather channels in some format that an ordinary mortal (very far from NSA level as a cryptanalyst) can recover with a suitable computer program ? In the old days (late 60's to mid 70's) the Navy fleet broadcasts on hf (and lf) 85 hz shift 16 channel fsk fdm ssb used to carry 100 wpm weather circuits; sometimes special fdm transmissions to particular fleet units could be found with AP or UPI broadcast wires (from either Baltimore Md. or Norfolk Va.) on at least one tone. From time to time the UPI low speed A newspaper wire would be included also. In all cases these channels would be 7.42 (or 7.50 for AP) unit baudot code start stop at the same baud rate as they are on the wire services themselves. More recently it has been reported that certain hf fdm mux tty transmissions carry a 100 wpm AFRTS circuit which retransmits national news from both the AP and UPI radio wires slightly delayed. It is presumably true that the signals were transmitted on hf just as they came off the wire so as not to compromise crypto security by broadcasting an encipherment of a known text. Perhaps if the Navy supplies the same stuff to it's (larger) fleet of ships these days it also sends it in the clear on the fleetsatcom psk broadcasts that are now the primary method of transmitting to ships at sea ? I've always assumed that nothing else on the Navy fleet transmissions including the most routine UNCLAS stuff was breakable by an amateur (or even most governments), as I suppose it is enciphered by more or less the same crypto as used for high security stuff but set up from another key list. It is true, however that US low speed tty crypto systems use a start-stop to synchronous transceiver (more properly isochronous as no separate clock is transmitted) and it is at least remotely possible that certain very routine unclassified traffic is transmitted in synchronous format without benefit of a real key from the key generator (perhaps some simple sequence is used simply to keep transition density high for the clock pll). (If anyone knows, and can say anything about low security modes for unclassified traffic I would appreciate hearing from them.) The uhf satellite transmissions are relatively high power (nearly whole transponder) signals on various frequencies between 249 and 261 mhz, they carry a 24 hour a day 15 channel time division multiplexed stream of mostly crypto circuits. The multiplexing is straight tdm with a 16 bit wide frame, each frame consists of a framing bit and one sample (bit) of each channel sent sequentially starting with channel 1. Frame sync is obtained by detecting and locking onto a continuously repeating pattern in the 16th bit position of each frame. 1200 baud divided by 16 yeilds a channel bit rate of 75.0 baud which is the standard rate for 100 wpm enciphered baudot tty traffic. It appears that the 1200 baud rate is exactly that (out to several places) so no bit stuffing is being done (understandably enough). The modulation is differential diphase mark (phase shift of 180 degrees for every mark (one bit) and 0 degrees (no shift) for every space (zero bit)). Recovery of the data requires little more than a simple Costas loop to recover the bit stream, and a bit clock recovery circuit to regenerate the 1200 baud clock. TDM frame and bit clock recovery could be easily implemented using a Zilog 8530 SCC chip which incorperates a transition tracking digital phase lock loop. The 8530 SCC chip could be fed data directly from the phase detector on the Costas loop, as it has a NRZI mode which will allow it to recover data as it would appear at that point. Obviously the 8 bit data coming out of the SCC would represent alternate halves of the 16 bit frame, and would not necessarily be in frame sync. The signals are powerful enough so a modified scanner (or one of the new scanners which cover the military uhf band) ought to suffice as a receiver, although the best reception might be had with a down converter and a hf receiver with medium (say 4 or 8 khz) if bandwidth. The Costas loop would connect to the receiver at it's low IF (455 khz for most receivers and scanners) after the selectivity to reduce the noise bandwidth. Antenna requirements are pretty minimal, but a small crossed yagi or helix aimed at the right point in the sky for the particular satellite of interest would be ideal. Has anyone explored these signals ? Are there any other military satcoms (say the X-band DSCS spacecraft) which carry readable traffic ? Please reply by either US mail or posting to the net (net.ham-radio on usenet for those who may see this via gateways to other nets), as I have had no luck whatsoever in receiving email. -- ---- David I. Emery Charles River Data Systems 983 Concord St. Framingham, MA 01701 Tel: (617) 626-1102 uucp: ...!decvax!frog!die