Rob, I for one would like a soft copy of this document as well. Three (3) MB is okay by me at:
[EMAIL PROTECTED] Many thanks, Gar NE8S [EMAIL PROTECTED] ----- Original Message ----- From: "Rob Kimberley" <[EMAIL PROTECTED]> To: "'Discussion of precise time and frequency measurement'" <[email protected]> Sent: Tuesday, March 14, 2006 3:36 AM 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]; [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
