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

NE8S ne8s at earthlink.net
Tue Mar 14 11:47:23 EST 2006


I for one would like a soft copy of this document as well. Three (3) MB is 
okay by me at:

csutc.observatory at earthlink.net

Many thanks,

Gar   NE8S
ne8s at earthlink.net

----- Original Message ----- 
From: "Rob Kimberley" <time.bandit at btinternet.com>
To: "'Discussion of precise time and frequency measurement'" 
<time-nuts at febo.com>
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: 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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