This week I was in Rotterdam for a RIPE meeting.
On Friday morning I gave a lightning talk called where does my
computer get the time from?
The RIPE meeting website has a copy of my slides and a video of the talk;
this is a blogified version, not an exact transcript.
I wrote a follow-up note, “Where does ‘where does my computer get
the time from?’ come from?” about some things I left
out of the talk.
Where does my computer get the time from?
from NTP – here’s a picture of an NTP packet
and here’s a picture of David Mills who invented NTP
simple question, easy answer, end of talk? No!
let’s peel off some layers…
stratum 3 NTP servers get the time from stratum 2 NTP servers,
stratum 2 NTP servers get the time from stratum 1 NTP servers,
stratum 1 NTP servers get the time from some reference clock
maybe a radio signal such as MSF in Britain or DCF77 in Germany
but in most cases the reference clock is probably a GPS receiver
here’s a GPS timing receiver
and here’s a GPS satellite
where does GPS get the time from?
Schriever Space Force Base in Colorado
they look after a lot of different top secret satellites and other
stuff at Schriever, as you can see from all the mission logos
so you can’t get close enough to take a nice photo
Where does Schriever SFB get the time from?
the US Naval Observatory Alternate Master Clock
is on site at Schriever in Colorado
the US Naval Observatory Alternate Master Clock
gets the time from the US Naval Observatory in Washington DC
there are three answers
the first answer is atomic clocks, lots of atomic clocks
in the background there are dozens of rack mounted caesium beam clocks
in the foreground the black boxes house hydrogen masers
these shiny cylinders are rubidium fountains
the USNO has so many atomic clocks they have entire buildings dedicated to them
When I was preparing this talk I noticed on Apple Maps that there’s a
huge building site in the middle of the USNO campus. It turns out they
are building a fancy new clock house; the main limit on the accuracy
of their clocks is environmental stability: temperature, humidity,
etc. so the new building will have serious air handling.
the second answer is that UTC is a horrible compromise between time
from atomic clocks and time from earth rotation
so the USNO gets the time from the international earth rotation
service, which is based at the Paris Observatory
twice a year the IERS sends out
Bulletin C,
which says whether or not there will be a leap second in six months
time; leap seconds are added (or maybe removed) from UTC to keep it in
sync with earth rotation
the IERS is spread across several organizations which contribute
to its scientific work
for example, you can subscribe to IERS Bulletin A,
which is a weekly notice with precise details of the earth orientation paramet
