LORAN C reference question]
Fri, 30 Jun 2000 09:18:31 -0400
Stewart Nelson wrote:
> Hi all,
> IMO, WWVB is not a good source for a frequency reference, unless
> you are close enough to Fort Collins to consistently receive
> a predominantly surface wave signal. Otherwise, the QSB associated
> with skywave will occasionally (typically once per night) create
> a signal which dips in amplitude and recovers with a phase shift of
> approximately 180 degrees. When this happens, it is hard to tell
> if you have gained or lost half a cycle.
> As a result, you can't expect accuracy/stability better than about
> 1E-9, even if your LO is oven controlled and you are using a long
> time constant.
> With LORAN or GPS, it's easy to do a factor of 100 better.
> WWVB is not the best way to tell time, either.
> It is difficult to precisely measure transitions of the
> low frequency ASK. You are lucky if you can synchronize
> within a few hundred microseconds. With LORAN or GPS,
> it is easy to get within a few hundred nanoseconds, and a
> good system can average over several hours and achieve
> accuracy of a few nanoseconds.
> Stewart KK7KA