[time-nuts] WWV / WWVH / WWVB
Lux, James P
james.p.lux at jpl.nasa.gov
Wed Jan 14 16:05:20 EST 2009
> -----Original Message-----
> From: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Tom Van Baak
> Sent: Wednesday, January 14, 2009 12:58 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] WWV / WWVH / WWVB
> > Kind of a shame they didn't use the unused bits in the WWVB
> stream for
> > some kind of checksum or error correcting code.
> Not really. At just 1 baud and with questionable reception in
> many parts of the country, checksum bits aren't the answer.
> The usual WWVB trick is to collect as many minutes of data as
> you want in order to increase confidence in your decoded
> result. It allows you to pull a very weak signal out of the noise.
> This might be as little as 1 or 2 minutes; or depending on
> time and location, it might be 5 or 10 or more minutes. With
> this level of patient oversampling there are a number of
> software tricks one can employ to pull out an error-free
> timecode from incredibly weak or noisy signals.
It *is* effectively coded with forward error correction. Integrate for 10 minutes and you have a rate 1/10th code (considering a whole frame as one symbol) with a very simple coder (code symbol(i+1) = code symbol(i) + 1). (well, the code's a bit more complex, because it's BCD, etc.)
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