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: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On >Behalf Of Magnus Danielson >Sent: 13 March 2006 22:32 >To: K3IO@verizon.net; time-nuts@febo.com >Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off? > >From: "Tom Clark, K3IO (ex W3IWI)" <K3IO@verizon.net> >Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off? >Date: Mon, 13 Mar 2006 16:44:51 -0500 >Message-ID: <4415E7D3.8000106@verizon.net> > > > 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

John, What email address do you want me to send to? Rob -----Original Message----- From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] 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: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On >Behalf Of Magnus Danielson >Sent: 13 March 2006 22:32 >To: K3IO@verizon.net; time-nuts@febo.com >Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off? > >From: "Tom Clark, K3IO (ex W3IWI)" <K3IO@verizon.net> >Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off? >Date: Mon, 13 Mar 2006 16:44:51 -0500 >Message-ID: <4415E7D3.8000106@verizon.net> > > > 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