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

Rob Kimberley time.bandit at btinternet.com
Wed Mar 15 04:10:02 EST 2006


Didier,

Uploaded this morning.

Rob K 

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Didier Juges
Sent: 15 March 2006 03:57
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?

Rob,

Could you upload it to ftp.ko4bb.com, login: manuals, password: manuals so I
could put it with the other manuals at www.ko4bb.com/ham_radio/Manuals?

Thanks in advance

Didier KO4BB

Rob Kimberley 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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