The place does my pc get the time from? – Tony Finch

2023-10-05 08:42:37

This week I used to be in Rotterdam for a RIPE meeting.
On Friday morning I gave a lightning discuss known as where does my
computer get the time from?
The RIPE assembly web site has a duplicate of my slides and a video of the discuss;
it is a blogified model, not a precise transcript.

I wrote a follow-up observe, “Where does ‘where does my computer get
the time from?’ come from?” about some issues I left
out of the discuss.

The place does my pc get the time from?

from NTP – right here’s an image of an NTP packet

and right here’s an image of David Mills who invented NTP

easy query, straightforward reply, finish of discuss? 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

possibly a radio sign resembling MSF in Britain or DCF77 in Germany

however typically the reference clock might be a GPS receiver

right here’s a GPS timing receiver

and right here’s a GPS satellite tv for pc

the place does GPS get the time from?

Schriever Area Pressure Base in Colorado

they give the impression of being after a number of totally different high secret satellites and different
stuff at Schriever, as you’ll be able to see from all of the mission logos

so you’ll be able to’t get shut sufficient to take a pleasant photograph

The place does Schriever SFB get the time from?

the US Naval Observatory Alternate Grasp Clock
is on web site at Schriever in Colorado

the US Naval Observatory Alternate Grasp Clock
will get the time from the US Naval Observatory in Washington DC

there are three solutions

the primary reply is atomic clocks, numerous atomic clocks

within the background there are dozens of rack mounted caesium beam clocks

within the foreground the black containers home hydrogen masers

these shiny cylinders are rubidium fountains

the USNO has so many atomic clocks they’ve whole buildings devoted to them

Once I was making ready this discuss I observed on Apple Maps that there’s a
big constructing web site in the course of the USNO campus. It seems they
are constructing a elaborate new clock home; the principle restrict on the accuracy
of their clocks is environmental stability: temperature, humidity,
and many others. so the brand new constructing could have critical air dealing with.

the second reply is that UTC is a horrible compromise between time
from atomic clocks and time from earth rotation

so the USNO will get the time from the worldwide earth rotation
service, which relies on the Paris Observatory

twice a 12 months the IERS sends out
Bulletin C,
which says whether or not or not there can be a leap second in six months
time; leap seconds are added (or possibly eliminated) from UTC to maintain it in
sync with earth rotation

the IERS is unfold throughout a number of organizations which contribute
to its scientific work

for instance, you’ll be able to subscribe to IERS Bulletin A,
which is a weekly discover with exact particulars of the earth orientation parameters

Bulletin A is shipped out by the US Naval observatory

they should know the precise orientation of the earth below the GPS
satellites, to allow them to present exact positioning

the third reply is, how does the USNO know its atomic clocks are working properly?

that data comes from the worldwide bureau of weights and
measures in Paris, who keep the worldwide commonplace UTC

how does the BIPM decide what UTC is?

the BIPM collects time measurements from nationwide timing laboratories
world wide, and makes use of these measurements to find out official
UTC

periodically they ship out Circular T
which has details about the discrepencies between official UTC and
UTC from the varied nationwide time labs

the BIPM is liable for sustaining the worldwide system of
items, which is outlined by the overall convention on weights and
measures

the CGPM is a world treaty group established by the
metre conference of 1875

UTC is an implementation of the SI unit of time, based mostly on quantum
measurements of caesium atoms

the place did this magic quantity, about 9.2 GHz, come from?

in 1955, Louis Essen (on the proper) and Jack Parry (left) constructed
the primary caesium atomic clock

the present definition of the second got here from the calibration of this clock

earlier than atomic clocks, the definition of the second was based mostly on
astronomy, so Essen and Parry wanted assist from astronomers to seek out out
how briskly their clock ticks in line with the prevailing commonplace of time

they bought assist from the astronomers on the US Naval Observatory

the best way it labored was William Markowitz measured time by
the skies, and Louis Essen measured time by his atomic
clock, and to correlate their measurements, they each listened to the
WWV radio time sign broadcast by the nationwide bureau of requirements in
Washington DC

this venture took 3 years, 1955 – 1958

Markowitz was measuring the “ephemeris second”

in 1952 the worldwide astronomical union modified the definition of
time in order that as a substitute of being based mostly on the rotation of the earth about
its axis, it was based mostly on the orbit of the earth across the solar

within the Thirties they’d found that the earth’s rotation will not be
completely even: it slows down and quickens barely

clocks had been now extra exact than the rotation of the earth, so the
ephemeris second was a brand new extra exact commonplace of time