Re: GPS SA and Network Timing

[Marshall Eubanks <tme () 21rst-century com>]

Eric Germann wrote:

> During the recent attacks, there was a brief discussion as to whether the
> USG had or would invoke GPS SA.  We have a client who is building a broad
> ATM network and had planned to use GPS based timing solutions from TrueTime
> for a clocking source, in addition to a feed from the LEC.
>
> If GPS SA is invoked down the road during a crisis, what effect is there, if
> any, on networks which rely on GPS for timing, not positioning?
>
> Eric
>
> ==========================================================================
>   Eric Germann                                        CCTec
>   ekgermann () cctec com                                 Van Wert OH 45801
>   http://www.cctec.com                                Ph:  419 968 2640
>                                                       Fax: 603 825 5893
>
> "It is so easy to miss pretty trivial solutions to problems deemed
> complicated.  The goal of a scientist is to find an interesting problem,
> and live off it for a while.  The goal of an engineer is to evade
> interesting problems :)"  -- Vadim Antonov <avg () kotovnik com> on NANOG

Dear Eric;

Basically the same as your vertical position error, if you express position in terms of
timing. SA is accomplished by varying  (i.e., introducing an error into)
the clock available from the spacecraft on an
few minute basis over a few 100 nanoseconds peak to peak or ~ 100 meters peak to peak.
Because of the way that the dilution of precision occurs, the vertical and the clock are
highly correlated, and the error cannot be beaten down much.
SO, your derived (local clock - GPS Master Clock) will vary by a few hundred nanoseconds
peak to peak.

If you have common view of the satellites with another GPS receiver, you can remove this error in doing
(local clock - remote clock) almost entirely, although you'll still have it in the
(either clock - GPS MC). This is enough for many purposes.

A plot of the effects with and w/o SA

In clocks
http://maia.usno.navy.mil/gpst/sa_off.ps
In the vertical
http://maia.usno.navy.mil/gpst/sa_trans.gif

Links to lots more info than you probably ever want to know
http://maia.usno.navy.mil/gpst.html

If you have really particular needs, contact me off list and I'll get you
in touch with the responsible people (I used to work in the USNO Directorate of Time).

--
                                 Regards
                                 Marshall Eubanks



T.M. Eubanks
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