[time-nuts] GPS "tests" by the DoD

Chris Albertson albertson.chris at gmail.com
Fri Jan 21 17:33:37 UTC 2011

> That's actually quite difficult.  Consider that for GPS, each receiver is
> seeing a different set of signals (in terms of relative time/phase) from SVs
> that are moving.  The code phase and doppler have to be consistent, etc.
> I spent some time over the last few weeks thinking about how hard it would
> be to build a cheap GPS simulator with an FPGA for testing a single radio.
>  Generating the NAV messages and the spread codes is easy.  Sliding them
> around and keeping range, range rate, and Doppler consistent, not so easy.
>  (some sort of funky chain of NCOs was what I came up with, but I don't know
> if that would work)

(1) You are not going to jam the entire Earth.  If the receiver is
reasonably close by both you and the receiver to be jammed are seeing
the same sats. (2) I doubt the company that built the Navy's GPS
jammer was constrained by the same budget you were.  I imagine they
had on order $100M to spend on the project    (3) the simplest way to
"jam" the signal is to turn the un-encrypted signal off or to
re-enable SA.  They have the ability to do that selectively over just
parts of an orbit but that effects a huge area
Chris Albertson
Redondo Beach, California

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