From: "Tom Clark, K3IO (ex W3IWI)" <[EMAIL PROTECTED]> Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off? Date: Mon, 13 Mar 2006 16:44:51 -0500 Message-ID: <[EMAIL PROTECTED]>
> Chuck said > > > I got the notion that it was turned off during Desert Storm, > > by virtue of being involved in the e-warfare effort that lead > > up to, and followed the event. > > > > I haven't been paying much attention since. I knew that they > > had intended to turn SA back on after production of the p-code > > units was up to speed, but I hadn't heard whether or not they > > did. > Yes, it was turned off for a brief period during DS, largely because the > DoD had to scurry around to buy mortal commercial units to fill the > need. Also during DS (and the present excursion) lots of parents sent > COTS GPS widgets to their kids. > > It turned out that one of the most important uses of cheap GPS receiver > in DS was by the food trucks. Troops were deployed in the desert all > along the Iraq & Kuwait border. The mess tents were behind the lines, > and hot meals needed to be delivered to the remote outposts. The > delivery trucks found they could navigate across the roadless desert > very well by using GPS receiver intended for navigating civilian boats. > > S/A is a dithering of the clock with a pseudorandom phase jitter. The > key to disentangling it was to have the same code generator available on > the ground. I use the analogy that DoD had a smart mouse in each > satellite running around on a phase resolver. To de-jitter it, you need > the mouse's clone inside the receiver. > > The dithering of S/A had nothing to do with the encryption of the P code > to make the Y code. The P-code is a LONNNNG code (37 weeks until a > repeat) at 10.23 Mbits/sec. Each of the satellites uses the same code > stream, offset by some integer number of weeks. The Y-code is an > additional secret code that uses a shorter code to (pseudo)randomly flip > the phase of the P-code. On the ground, the civilian "code crackers" > have found out that the convolution code is running at a rate ~500 > kbits/sec. This means that the Y-code may be the correct P-code for ~20 > bits, and then it (may|may not) flip phase to become "anti-P" code. > AFAIK, Ashtec's patented "Z-code" receivers generate a hardware estimate > of this code and (nearly) coherently demodulate the signal. Other brands > have similar tricks up their sleeve. The Y-code is the P-code xored with the A-code (sometimes also referred to as the W-code). The A-code is indeed ~500 kbis/sec. The first "codeless" receivers just squared out the A-code from the equation, but then they had a worse problem to fight regarding ambiguity. Also, it does not form a very good receiver. The Ashtec solution is to make the L1 handover from C/A-code to P-code and predict the A-code, delay that a suitable amount to the L2 Y-code and attempt to lock up to that. The delay is trimmed to match up with the L1-L2 delay in P(Y)-code. You could say that the Ashtec receivers cracks the code, but they really don't since they do not disclose the state of the A-code generator or its architecture. Infact, they don't even get it rigth all the time, but sufficiently often for a good lock since each success has a good quality. It is interesting that what they did to figure things out was hunting GPS satellites with a big parabol antenna tracking the satellite and getting a much better S/N than normal semi-omnidirectional antennas. With that they could make advanced guesses. Cheers, Magnus _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
