[time-nuts] GPS Selective Availability. Is it On or Off?

John Day johnday at wordsnimages.com
Tue Mar 14 18:09:08 EST 2006


johnday at wordsnimages.com

At 02:54 PM 3/14/2006, you wrote:
>John,
>
>What email address do you want me to send to?
>
>Rob
>
>-----Original Message-----
>From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at 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 at febo.com [mailto:time-nuts-bounces at febo.com] On
> >Behalf Of Magnus Danielson
> >Sent: 13 March 2006 22:32
> >To: K3IO at verizon.net; time-nuts at febo.com
> >Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
> >
> >From: "Tom Clark, K3IO (ex W3IWI)" <K3IO at 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 at 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
> >
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> >
> >
> >
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