[time-nuts] BPSK decoder for WWVB

[Attila Kinali]

On Wed, 03 Jul 2013 19:15:40 +0000
Brian Alsop <alsopb at nc.rr.com> wrote:

> Apparently this modulation scheme is less prone to "jammers".
> There is is British station which "jams" east coast WWVB.

The way WWVB implements BPSK does not make it less prone to jammers
or noise. The idea, to get higher SNR would be to encode a known
signal onto the carrier which you can correlate against. WWVB does
not do that. The phase switches every second according to the bit
to be transmitted. As the bits are unkown (safe the 12 sync bits)
you have no additional information what the phase/signal is, actually
you have even less knowledge compared to a fixed phase signal (aka AM only
modulation), hence lower SNR (= less jamming protection).

Compare this modulation scheme to what DCF77 or GPS does. Both modulate a
long pseudorandom, but known bit string over the carrier against which you
can correlate. The effective data bitrate is much lower.

And just for clarity: The jamming resistance of such modulation schemes
is not inherent in the "known signal", but comes from spreading the signal
over a larger bandwidth. White noise and random narrow band noise (the
two most common noise sources over the air) can be "averaged" out by
the modulation if the bitstring is known. If designed right, you can detect
a signal that is several dB below the thermal noise limit, like GPS.


TL;DR version: The way how BPSK modulation is implemented in WWVB makes
it more prone to jammers and noise instead of less.


			Attila Kinali
-- 
The trouble with you, Shev, is you don't say anything until you've saved
up a whole truckload of damned heavy brick arguments and then you dump
them all out and never look at the bleeding body mangled beneath the heap
		-- Tirin, The Dispossessed, U. Le Guin