Rob, I'd like a copy of that. 3 MB is no problem. Please send to [EMAIL PROTECTED]
Thanks, Bill Hawkins -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Rob Kimberley Sent: Tuesday, March 14, 2006 3:36 AM To: 'Discussion of precise time and frequency measurement' Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off? I have a 75 page PDF briefing from Zyfer on SAASM P/Y which has loads of useful information on GPS signal structure, acquisition, jamming, spoofing etc. Can either post it to the group (approx 3MB) or send it on request. Rob Kimberley -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Magnus Danielson Sent: 13 March 2006 22:32 To: [EMAIL PROTECTED]; time-nuts@febo.com Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off? 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 time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts