[time-nuts] GPS Selective Availability. Is it On or Off?
Tom Clark, K3IO (ex W3IWI)
K3IO at verizon.net
Mon Mar 13 16:44:51 EST 2006
> 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
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.
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