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

Bill Hawkins bill at iaxs.net
Tue Mar 14 10:41:43 EST 2006

Rob, I'd like a copy of that. 3 MB is no problem.

Please send to bill at iaxs.net

Bill Hawkins

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
Behalf Of Rob Kimberley
Sent: Tuesday, March 14, 2006 3:36 AM
To: 'Discussion of precise time and frequency measurement'
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

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.


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