[time-nuts] Time Transfer

[Didier Juges]

Tim,

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Tim Shoppa
> Sent: Sunday, December 16, 2007 7:29 AM
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] Time Transfer
> 
> "Didier Juges" <didier at cox.net> wrote:
> > I would have thought the GPS receiver averages the readings when it 
> > locks on multiple satellites. Are you saying it only uses 
> one at a time?
> 
> Average is perhaps too simple of a word.
> 
> The good GPS receivers (here I'm assuming we aren't talking 
> about Trimble SV6!) look at all the satellites in view and 
> in mask, throw out obvious outliers, and average among the 
> remaining.
> 

That's what I meant I suppose. The interesting part is that the timing GPS
receivers don't continualy try to select the best satellites from all those
it can see, so other than eliminating outliers, there may still be some that
are not far out enough to be eliminated, but will negatively affect the
average. What is the criterion to determine an SV is an outlier? How far off
should it be?

An algorithm could select the best 3 at all times for instance, even if 5
are visible and unmasked. I am not sure that would be better, but it's a
possibility. On the other hand, I suppose that's what the mask if for. Maybe
we need a dynamic mask so that we could force the receiver to never look at
more than the best 3 or 4? 

> > If so,
> > what's the point of a self survey?
> 
> ??? If you're wrong by 1000 feet about where you are, then 
> that could be hundreds of nanoseconds in time error. And that 
> hundreds of nanoseconds would jump around like the dickens as 
> you chose different satllites or the chosen satellite pans 
> across the sky.
> 

I agree, but to do a self-survey, you need to look at more than one
satelite, which was my point.

> > I talk about WWV because most ham operators have a receiver that 
> > receives WWV, so it is an inexpensive and convenient way to get an 
> > absolute reference, even though it is not as good as GPS.
> 
> WWV is great. But again, you gotta know where you are to have 
> the right time. Fluctuations in propogation will change path 
> length and if you don't know the path length, it is NOT an 
> absolute reference in the nanosecond or even microsecond 
> range. For typical conditions the propogation delay for WWV 
> cannot be estimated much better than the tens of 
> microseconds; combining WWV with a better "absolute"
> clock in fact lets you study propogation by studying the 
> shifting of the various ionospheric layers. (There have been 
> ham operators who do this, including me!)
> 
> Tim. (N3QE)
> 

OK, once again I confused "frequency standard" and "time standard". I
promise that's the last time :-)

Yes, it's interesting to look at the WWV signal wander on the HP 3586,
compared to it's OCXO, and while listening to it on the speaker or on a
receiver. I need to plot it over a 24 hour period or so.

Didier KO4BB