[time-nuts] Time Transfer

[Tim Shoppa]

"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.

> 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 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)