[CentOS] System Time Source

[Robert Moskowitz]

On 05/24/2017 11:37 AM, Tate Belden wrote:
> Warren, one slight correction on an other wise nicely written bit of info:
>
> The time transmitted from WWV is not Mountain Time. Even though the WWV
> transmitter farm is located in the Mountain time zone, the signals are
> transmitted as "Coordinated Universal time", UTC, or 'Zulu' time.
>
> Here, you can listen to a recording made at the transmitter site for the
> 5Mhz signal:
> https://ia802605.us.archive.org/24/items/WWV5MHz/WWV-5MHz.MP3

I remember listening to this on a shortwave radio back around '62...

>
> On Wed, May 24, 2017 at 8:36 AM, Warren Young <warren at etr-usa.com> wrote:
>
>> On May 24, 2017, at 7:53 AM, Chris Olson <chris_e_olson at yahoo.com> wrote:
>>> One of our STEM interns recently observed that there are
>>> inexpensive clocks that sync via radio to standard time
>>> services.
>> There are two major types:
>>
>> 1. WWVB and its equivalents in other countries:
>>
>>      https://en.wikipedia.org/wiki/WWVB
>>
>> 2. GPS clocks.
>>
>>
>> WWVB has several problems:
>>
>> 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!
>>
>>      https://mm.icann.org/pipermail/tz-announce/2016-November/thread.html
>>
>> b. It’s a weak signal.  Unless you’ve got a big antenna or are positioning
>> the receiving device near a window, you probably can’t receive the WWVB
>> signal reliably.
>>
>> c. Computers have major RFI shielding problems, which is why they’re
>> typically placed in metal boxes.  (Even plastic-cased laptops have metal
>> boxes inside.)  That means you have to have an external antenna even in the
>> best case.  Now apply what you know about Wifi reliability to the problem
>> of receiving a signal from a different *time zone*.
>>
>> I happen to be in the Mountain time zone, and it doesn’t take too much to
>> shield our WWVB clocks from the signal.  I can only imagine how much easier
>> it is out on the coasts.
>>
>>
>> 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.
>>
>> The GPS transmitters probably have a higher received signal strength than
>> WWVB, but cinderblock walls and grids of 42U equipment racks block the GPS
>> signal quite well.  This is why data centers with such clocks generally
>> have to run an antenna to the outside for their clock.  That makes it far
>> more expensive than just connecting to an upstream NTP server.
>>
>>> When I was a student, such questions would have earned me
>>> extra homework assignments.
>> So do I get an “A”? :)
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>
>