[EMAIL PROTECTED] At 02:54 PM 3/14/2006, you wrote: >John, > >What email address do you want me to send to? > >Rob > >-----Original Message----- >From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On >Behalf Of John Day >Sent: 14 March 2006 18:34 >To: Discussion of precise time and frequency measurement >Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off? > >If you send a copy to me I can post it at nm2.org for anyone to access. > >john > > >At 04:36 AM 3/14/2006, you wrote: > >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]; [email protected] > >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 > >[email protected] > >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > > > > >_______________________________________________ > >time-nuts mailing list > >[email protected] > >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > >_______________________________________________ >time-nuts mailing list >[email protected] >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > >_______________________________________________ >time-nuts mailing list >[email protected] >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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