[CentOS] System Time Source

[Warren Young]

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”? :)