[EAS] EAS time stamp usage

[Eric Adler]

However, with propogation delay due to digital encoding (and sometimes decoding) at every hop, possible satellite transmission, text-to-speech rendering (if applicable), you could very easily add almost a minute of delay even for a regional message.  Creating a tolerance of +/- 2 minutes to begin with you may very easily have a 1 minute difference in clock simply due to how an operator sets the clock and then you will have only 1 minute for tolerance.  I don't think this is enough to simply account for normal operation.  With digital delays, encoders, and decoders everywhere, I don't think updating a clock based on an incoming radio feed would be intrinsically useful.  And no, I will not update my EAS clock to be the post-delay time, as it needs to be good for my logs and the post-delay time depends on the receiver/decoder being used.  If you want to set your clock to an RF source, there's GPS, WWV, and a slew of other services from other countries.  Perhaps a delay-less subcarrier RF time source could be added to a LP (local primary, not low power) or NWS transmitter in each market as well, but then you are entrusting that source with your clock.  Also, while RWTs need not be forwarded, the possibility that they are forwarded exists and you may receive one from a nefarious source (or just a misaligned source -- especially near DST changes); if it is used to set your clock, this could cause your clock to be grossly misaligned.  
In short, no one else's clock is reliable.  We have determined our appropriate reference for our facility and will use that reference for all time-critical clocks.  I expect that others do the same.  

Eric

-----Original Message from Sean Donelan [sean at donelan.com] -----
  - Warn operator if incoming RWT message timestamp from a required
    monitoring source is more than +/- 2 minutes different from local
    clock.
        Either the local clock or remote clock could be incorrect and
        needs investigation

        Optionally update local clock based on RWT message timestamp
        if remote clock source is reliable, which would help keep EAS
        decoder clocks synchronized with area required monitoring sources.

        Only RWT (required weekly test) is always single-hop transmission,
        with a timestamp near the time of reception.  One RWT a week
        would keep even low quality clock chips synchronize within +/- 2
        minutes without external clock sources at every EAS box.