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

John Day johnday at wordsnimages.com
Tue Mar 14 13:34:08 EST 2006

If you send a copy to me I can post it at nm2.org for anyone to access.


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
>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
>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.
>time-nuts mailing list
>time-nuts at febo.com
>time-nuts mailing list
>time-nuts at febo.com

More information about the time-nuts mailing list