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

Rob Kimberley time.bandit at btinternet.com
Tue Mar 14 14:54:27 EST 2006


What email address do you want me to send to?


-----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.


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