[time-nuts] WWVB Response

[Clint Turner]

While there could have been a few things to make the WWVB transmissions 
easier to recover with low S/N, keeping them compatible with the 
"legacy" time-only receivers was somewhat of a hindrance.

Unlike the DCF77 signal - which has a digital phase modulation that does 
NOT really lend itself to improved SN (it's at a much higher rate, but 
since a WWVB-ish signal is much stronger in Germany owing to its 
small-ness) and was really intended for use with strong signals, anyway 
- the vast majority of users ignore the BPSK, anyway and rely on the AM.

As for how the new WWVB format allows improved recovery of frequency 
reference and time signals:

- I really don't see how this new scheme makes it worse - just 
different.  A very narrow bandwidth loop on the carrier recovery such as 
a Costas (e.g. as slow as you want!) allows the carrier to be recovered 
at very low S/N.  At least in part, it depends on how long you want to wait!

- With typical antennas and receivers, the fairly narrow bandpass is 
still wide enough to allow reasonable recovery of the signal.  Note 
that, at worst, the phase changes at a rate of 0.5 Hz and to have 
recovered the 1-second time signalling at all with any degree of 
accuracy, it would be wider than that 0.5 Hz!

- The phase change occurs in a pre-determined manner.  Once you recover 
the carrier, you *know* that you can window the filtering on 1 second 
blocks and that there are only two possibilities of what the phase could 
be within that block.  On a computer, you can use "past" data and slide 
back and forth until good correlation is received:  This would then get 
one close to 1-second timing in that same operation!  (A bit harder to 
do this on very low-power receivers, but easy on a PC.)

- The use of the same preamble (to indicate time or message) can allow 
one to review many minutes of past-stored data to achieve a correlation 
of the minute frame.  Using multiple blocks of past data can greatly 
improve the effective S/N of that particular aspect - the degree 
depending on how long you are planning to wait!  (Again, tougher on a 
low-power receiver, but fine on a PC.)

- Once you have "minute" frame correlation (and second-timing as well) 
you can then use several techniques to further-recover the time data:
   - You *know* that the DST/Leap Second information isn't going to 
change much so that can be used for further correlation.
   - The time data is incrementing at a known rate.  In the event that 
the data is consistently "ratty" (e.g. you don't get the full time frame 
without errors) you could further-correlate those bits that are NOT 
changing very quickly over several minute frames.
   - Furthermore, if you have an "approximate" time (which can be known 
from the user if not from long-term correlation of that time data that 
changes very slowly over several frames) you can then go back in old 
data and apply assumptions based on best guesses of time and see if that 
data "fits" as well.
   - Don't forget that the use of the Hamming code (the ability to 
correct one bit of data and to detect if two bits are in error) also 
provides a degree of recoverability - particularly if applied over a 
number of minute frames.

With a low-bandwidth antenna (assuming that it's not too narrow - that 
is, it has 2-3 Hz bandwidth or more) the difficulty will be determining 
when, exactly, the 1-second intervals are occurring to within a few 10's 
of milliseconds - even knowing the amount of ("fixed") delay in the 
filter/detection scheme, but this uncertainty is almost guaranteed with 
varying noise and signal level conditions and the vagaries associated 
with a very narrow filter.

If, however, the filter within the receive system is extremely narrow 
(<1-2 Hz) then the BPSK does make things more difficult.

73,

Clint
KA7OEI

> The AM just makes the situation in low S/N areas worse. The BPSK wipes out
> the possibility of any very narrow band prefiltering, because of filter
> time response.
>
> I suspect, although have not tested, that active antennas with either
> mechanical or crystal filters in their preamps will be rendered useless.
>
> -John