[time-nuts] WWVB format change in 2012

[Bob Camp]

Hi

WWVB and WWV (like any radio uncorrected radio system) has fairly predictable shifts
associated with the day / night ionosphere. One *could* fix that issue with a table
based on station location. I do not know of any library of code that does that already. 

The next “layer” of trouble comes from how the low cost receivers are implemented. The
common issue is local noise. The common solution is a narrowband crystal filter in front
of the receiver. The bandwidth of that filter (and to some extent it’s temperature dependance) place
a “best case” limit on performance in the 10’s to 100’s of ms range depending on the 
exact details. There are higher performance receivers (but not a lot of them) that do get into
the single digit ms range. At that point the propagation issue mentioned above needs some
work. 

Further complicating things is the distance factor. A user in Denver with ground wave “view” 
of the transmitter will do *much* better than the numbers above. A user in Miami or Bangor ME 
may be very lucky to get close to the numbers above on an intermittent basis. 

For time transfer, you have “carrier phase ambiguity” due to the day night propagation shifts. Simply
put the time delay to the transmitter caused the received signal to vary by more than one cycle. That 
makes it a less than ideal source of time. For precision use, a WWVB system often does a 
carrier measure at a single time per day. The phase data is averaged over may days to make
a precision estimate. This works ok for a frequency based (think GPSDO) type system). For autonomous 
timing it’s not a practical solution. 

Bob


> On Feb 28, 2016, at 11:15 AM, Sanjeev Gupta <ghane0 at gmail.com> wrote:
> 
> On Sun, Feb 28, 2016 at 10:57 PM, Bob Camp <kb8tq at n1k.org> wrote:
> 
>> The new WWVB format is troublesome for older gear that looks at carrier
>> phase as a source of precision timing. The NTP driver does not do this.
>> 
>> The new WWVB format is fine for any gear that recovers time from the
>> AM modulation on the carrier. This is what the NTP driver *does* do.
>> 
> 
> This is a very clear phrasing, thanks.
> 
> My understanding is that existing commercially-available equipment that
> recovers time from the AM carrier provides an accuracy on the order of a
> milli-second.  Anything better required tracking phase.
> 
> So, what would the (NTP with current WWVB equipment) accuracy and jitter be?
> 
> I appreciate that we seem to be moving towards a GPS-monoculture, but how
> close is the (NTP with WWVB AM) to the 50 microseconds number?
> 
> -- 
> Sanjeev Gupta
> +65 98551208     http://www.linkedin.com/in/ghane
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