[time-nuts] Coalition to Save GPS

J. Forster jfor at quik.com
Sun Mar 13 23:06:09 UTC 2011


> 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

==============





More information about the time-nuts mailing list