[time-nuts] Coalition to Save GPS

[Chris H]

Could this be a 'Good Thing' -- 
After all -- now no foreign power can use GPS to attack your country?

Or perhaps, the US is going to turn off GPS all together, after all it's
their network, built by them, designed by them....
Was Civilian  use ever condoned by the US government or was it just
'there' and people started to use it.. 
Or was it opened up like the internet after military then academic then
public use.. 

PS: All that was posted was said in jest, I intend to visit the US at
some point this year - I know I will get lost, and would LOVE GPS to
find my way around!

On Sun, 2011-03-13 at 16:06 -0700, J. Forster wrote:

> > Subject: Re: [time-nuts] Coalition to Save GPS
> > From:    bg at lysator.liu.se
> > ...I suspect that military receivers use L1 C/A,
> > L1 P(Y) and L2 P(Y). This is the exact same signals civilian geodetic
> > receivers has been using since about the Ashtech Z12 and its z-tracking.
> 
> 
> Once upon a time, all military receivers acquired the C/A code modulated
> components of the L1 signals transmitted by the satellites, read the NAV
> message(s) embedded in these signals, and used certain portions of these
> NAV data in order to acquire the P(Y) code modulated components of the
> signals transmitted in both bands.
> 
> However, some years ago the US DoD developed receivers that could acquire
> the P(Y) signals directly.  Even if jamming signals in the L1 band are so
> strong that all available means of rejecting the jamming fail and the L1
> channel of a receiver is overloaded beyond redemption, a direct-to-P(Y)
> receiver can acquire the P(Y) code modulated components of the L2 signals.
> 
> I don't know what portion of all the GPS receivers deployed by the DoD now
> have direct-P(Y)-acquisition capability.
> 
> Fundamental to direct-P(Y) acquisition is a priori clock synchronization. 
> If too much time has elapsed since a receiver had a GPS satellite in view,
> then the receiver's local clock may have drifted so far that the receiver
> would need an unacceptably long time to search far enough in epoch-offset
> to find a P(Y) signal.  Therefore, a direct-P(Y) receiver may need to be
> synchronized by external or extraordinary means, such as a portable atomic
> clock or a cable or radio link to an atomic clock.  Nowadays, the military
> is so extensively networked (for a variety of reasons) that the
> requirement for external synchronization is not terribly burdensome.
> 
> =====
> From a friend,
> 
> -John
> 
> ==============
> 
> 
> 
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