This week I was in Rotterdam for RIPE meeting. On Friday morning I gave a lightning talk on the topic “Where does my computer get its time?” The RIPE meeting website has copies of my slides and video of the talk; This is the blog version and not an exact transcript.
I wrote a note: “Where did the message ‘Where does my computer get time come from?’?” There were some things I didn’t mention in the conversation.
Where does my computer get its time?
From NTP – here is a representation of an NTP packet.
Here’s a photo of David Mills, who invented NTP.
Simple question, simple answer, end of conversation? No!
Let’s peel off a few layers…
Layer 3 NTP servers get time from Layer 2 NTP servers, Layer 2 NTP servers get time from Layer 1 NTP servers, Layer 1 NTP servers get time from some reference clock.
Here is a GPS time receiver.
And here is the GPS satellite.
Where does GPS get time?
Schriever Space Force Base in Colorado
They look after a lot of different top secret satellites and stuff in Schriever, as you can see from the logos.
So you won’t be able to get close enough to take a good photo.
Where does Schriever Space Force Base get its time?
The alternate master clock for the US Naval Observatory is located at Schriever in Colorado.
The alternate master clock of the US Naval Observatory receives its time from the US Naval Observatory in Washington, DC.
There are three answers
First answer: atomic clocks, many atomic clocks
There are dozens of cesium beam clocks in the background.
In the foreground, black boxes contain hydrogen masers.
These shiny cylinders – rubidium fountains
The US Naval Observatory has so many atomic clocks that entire buildings are dedicated to them.
As I was preparing this talk, I noticed on Apple Maps that there was a huge construction site in the middle of the US Naval Observatory campus. It turns out they are building a new fancy house with a clock; The main limitation to the accuracy of their clocks is the stability of the environment: temperature, humidity, etc., so the new building will have significant ventilation.
The second answer is that UTC is a terrible compromise between atomic clock time and earth rotation time.
Therefore, the US Navy Observatory receives time from the International Earth Rotation Service, which is based at the Paris Observatory.
Twice a year MSVZ sends out Bulletin C, which tells you whether there will be a leap second in six months; leap seconds are added (or possibly removed) from UTC to synchronize it with the Earth’s rotation.
The ISA extends to several organizations that contribute to its scientific work.
For example, you can subscribe to Bulletin A from ISVZwhich is a weekly notification with precise details about the Earth’s orientation parameters.
Bulletin A is distributed by the US Naval Observatory.
They need to know the exact orientation of the Earth under GPS satellites so that they can provide accurate positioning.
Third answer: How does the US Navy Observatory know that its atomic clock is working well?
This information comes from the International Bureau of Weights and Measures in Paris, which maintains the global UTC standard.
How does the BIPM define what UTC is?
The BIPM collects time measurements from national time laboratories around the world and uses these measurements to determine official UTC.
Periodically they send out Circular Twhich provides information on discrepancies between official UTC and UTC from various national time laboratories.
The BIPM is responsible for maintaining the international system of units, which is determined by the General Conference on Weights and Measures.
The GCVM is an international treaty organization created by the Meter Convention of 1875.
UTC is an SI implementation of the unit of time based on quantum measurements of cesium atoms.
Where does this magic number of about 9.2 GHz come from?
In 1955, Louis Essen (right) and Jack Parry (left) built the first cesium-based atomic clock.
The current definition of the second is derived from the calibration of this watch
Before the advent of atomic clocks, the definition of the second was based on astronomy, so Essen and Parry needed the help of astronomers to figure out how fast their clocks were ticking according to the existing standard of time.
They were helped by astronomers from the US Navy Observatory.
It worked like this: William Markowitz measured time by looking at the sky, and Louis Essen measured time by looking at his atomic clock, and to compare their measurements, they both listened to the WWV radio time signal broadcast by the National Bureau of Standards to Washington.
This project took 3 years, 1955 – 1958.
Markowitz measured the “ephemeris second.”
In 1952, the International Astronomical Union changed the definition of time so that it was based not on the Earth’s rotation on its axis, but on the Earth’s orbit around the Sun.
In the 1930s, they discovered that the Earth’s rotation is not entirely uniform: it slows down and speeds up slightly.
Clocks were now more accurate than the Earth’s rotation, so the ephemeris second became a new, more accurate time standard.
The second ephemeris is based on the astronomical ephemeris, which is a mathematical model of the solar system.
Standard ephemeris was created by Simon Newcombe in the late 1800s.
He collected a huge amount of historical astronomical data to create his mathematical model.
It remained the standard until the mid-1980s
Here’s a photo of Simon Newcombe
He’s a handsome Victorian gentleman
Where did he work?
At the US Navy Observatory!
(and US Nautical Almanac office)
Now I’ve run out of layers: up until this point, the clock was set more simply by watching the stars cross the sky.
So, to summarize my conversation, where does my computer get its time?
He doesn’t get it from the Royal Greenwich Observatory!