[CentOS] System Time Source

Wed May 24 16:20:58 UTC 2017
Valeri Galtsev <galtsev at kicp.uchicago.edu>

On Wed, May 24, 2017 10:45 am, Warren Young wrote:
> On May 24, 2017, at 8:52 AM, Chris Adams <linux at cmadams.net> wrote:
>> Once upon a time, Warren Young <warren at etr-usa.com> said:
>>> a. It’s transmitting from a fixed location in a time zone you
>>> probably aren’t in — US Mountain — being the least populous of
>>> the lower 48’s four time zones.  You therefore have to configure time
>>> zone offset and DST rules, which means additional software if you want
>>> it to track changes to these things.  There were 10 batches of such
>>> changes last year!
>> This really has no bearing on time source; none of the commonly-used
>> time sources (satellite, terrestrial radio, or network) carry time zone
>> information
> In editing my prior reply, I removed the observation that GPS solves
> problem “a” by telling you where you are in the world, as well as what
> the UTC time is.
> It still has problems b and c, though.
>> (although WWVB does carry a bit to indicate if US DST rules
>> are in effect).
> …which is of no help when the DST rules are hard-coded into the clock,
> as they are so frequently.  I had to discard a few WWVB clocks when the
> last DST rules went into effect.
>>> GPS time is a much better solution, but it’s power-hungry, as you
>>> probably know from running GPS on your smartphone.  This rules it out
>>> for laptops.
>> Not exactly; laptop batteries' capacity is an order of magnitude larger
>> than phone batteries.
> Sure, but it’s still a market where people buy based on benchmarks.  The
> laptop that gets 20 minutes less battery life but has great time accuracy
> even when not on the Internet will lose in the market.
>>> The GPS transmitters probably have a higher received signal strength
>>> than WWVB
>> No, GPS is lower signal strength than WWVB, at least for most of the
>> continental US (although WWVB signal strength varies significantly based
>> on the time of day, because it is a low frequency signal).
> I went looking, and you’re right.  GPS satellites transmit at 0.5 kW and
> WWVB at 70 kW, and WWVB has the additional advantage of less distance to
> transmit.
> (21000 km from orbit to Earth surface vs ~3000 km from Fort Collins to
> either Bangor, Maine or Miami.)

It is insightful, yet... There are a bunch of other factors that may need
to be taken into account. Angular transmission pattern of satellite (horn?
or is it yagi? antenna) vs ground based (monopole? or dipole? antenna -
which one is used there to transmit in HF?). Ground effect (attenuation)
along the whole path or propagation for ground based HF vs ground effect
only at the receiption point, but much higher for much higher frequencies
of GPS; pre-amplifier Signal to Noise ratio (S/N; which can technically be
achieved to be much better at much higher GPS frequencies...). So, 70 kW
vs 0.5 kW and the distance advantage still may not necessarily allow
ground system beat hands down satellite based one (again, speaking only
about S/N ratio for one vs another). It would be good to ask someone who
can measure S/N for both (using the same $$ receiving radios) - which one
is better.

I would expect one disadvantage factor for GPS, as you have to receive and
decode much wider signals for it as opposed to much slowed stuff for time
purely ground based one...


> Regardless, that's another reason not to do this as a matter of general
> policy.
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Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247