Gor#f@mit-eddie.ARPA (03/16/85)
I have no specific knowledge of the scheme used for UPI, but the NAVSTAR (GPS) satalite navagation system uses that form of spread spectrum. One of their modes used 10 Mega "chips" per second, and a PRS with a repeat period, at that clock rate, of 287 days. (They actually only use one week long subsets of it, asigning a different subset to each bird, and they reset the sequence once a week.) The badwidth they claim to use is 20MHz, so I would guess that if the UPI system claims to use 2MHz that their clock rate is about 1MHz. An interesting feature of the NAVSTAR system is that the spread signal is below the noise as received by the typical earth station. If I had to guess, I'd say that the UPI system tries for that too, and considers it a feature. When you unspread the spectrum at the receiver, the signal comes back up, but the noise stays down. However, with a decent FIRE code, corelation drops to zero by the time you are 1 "chip" timeout of phase with your unscrambling PRS. Even with a reasonably short PRS, if you don't know what is, and they are running below the noise, you don't have much of a chance of finding it, even if you know the carrier frequency. (You could camp next to the uplink transmitting antenna, it should be well above the noise there on the uplink frequency.) As to encryption, you might get lucky. If I were using such a system for secure purposes I wouldn't use any. I'd just make the PRS longer and/or change it frequently. FIRE codes are a dime a dozen.