Didier, Uploaded this morning.
Rob K -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Didier Juges Sent: 15 March 2006 03:57 To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off? Rob, Could you upload it to ftp.ko4bb.com, login: manuals, password: manuals so I could put it with the other manuals at www.ko4bb.com/ham_radio/Manuals? Thanks in advance Didier KO4BB Rob Kimberley 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
