[CentOS] System Time Source

Wed May 24 15:29:38 UTC 2017
Pete Biggs <pete at biggs.org.uk>

On Wed, 2017-05-24 at 13:53 +0000, Chris Olson wrote:
> One of our STEM interns recently observed that there are
> inexpensive clocks that sync via radio to standard time
> services.  This begged a question about why every computer
> would not have a radio module to receive time.  Our senior
> staff did not have a good answer or if time from such a
> radio module would be supported by the operating system.

In addition to what everyone else has said ...

The terrestrial radio clocks are actually not that accurate. They are
not designed for keeping things like a system clock "correct".
Commercial solutions only keep to within about +/- 0.5s per day, with
resynchronisation happening about once a day.

The GPS time system is also notoriously very precisely wrong. The time
was set when the first satellite was sent up and has never been
corrected since - so hasn't taken account of leap seconds or
relativistic effects. All that matters for GPS is that the time on each
satellite transmission is identical, and to that end you can get a
precision of about 3ns (which is what you need to get metre GPS
accuracy) and which you then have to correct for all the various
artefacts since inception. Lower cost GPS receivers get about 50ns
accuracy, which is probably still OK for a system clock. The good thing
is that the corrections necessary are well known and updated

The bottom line is that NTP is still by far the best way of obtaining
an accurate clock with a computer. If you need GPS accuracy, then it's
usually an enterprise solution with a single good GPS receiver
providing a stratum 1 NTP source on the local network.