[time-nuts] What position is measured?

Tom Holmes tholmes at woh.rr.com
Wed Sep 8 14:18:49 UTC 2010

Thanks, Jim.

I assume that neither the satellite nor the receiver knows what the
variation in the light time delay is, so it must be small enough to allow
the claimed nanosecond accuracy of the PPS edge. 

Although one sat is sufficient for time work, would using more improve the
PPS accuracy? Seems like having more inputs would help with the light delay
and other corrections, but it probably is no different than having multiple
Rb's in the lab (the guy with two is never quite sure and all that).

Mostly just curious, as my Z3801 is quite good enough for my needs. 

Tom Holmes, N8ZM
Tipp City, OH

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of jimlux
> Sent: Wednesday, September 08, 2010 9:42 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] What position is measured?
> Tom Holmes wrote:
> > One other delay contributor  would appear to be processing delay in the
> > receiver, which thus begs the question of how the PPS signal is actually
> > synchronized to the GPS system.
> >
> The GPS nav messages is synchronized to the seconds, so it's a matter of
> making sure the output pulse is synced to the appropriate time in the
> GPS signal.  The delay in the receiver is (reasonably) constant, so the
> mfr essentially calibrates it out.
> It's not done precisely like this, but conceptually, you have a 1pps on
> the spacecraft driven by a Cs clock, you receive the signal in your
> receiver (some time later than the actual "change of second") and
> subtract out the light time delay from satellite to you. (or, more
> accurately, delay the signal from the receiver to the "next" second).
> It's controlling for that "light time delay" that's the tricky part,
> since it varies depending on the degree of ionization of the ionosphere.
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