Basic Concepts of Synchronisation

TIME, TIME SCALES AND DATES

The two meanings of the word time:

1. Time as the date of an event on a time scale.
2. Time as a time interval between two events.

Time scales :
A time scales is defined by:

1. a time unit
2. a time origin

Dates:
A date is a number of units on the time scale.



THE SECOND

The second is the time unit of the International System of Units (SI.

Definition:
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the fundamental state of the Cesium 133 atom.



CLOCK

A clock consists of:

1. a periodic phenomenon which can be observed
   
2. a counter which count the number of periods
3. a means for setting the counter to a preset value
4. a display of the registered count
   

ATOMIC TIME SCALES I

Origin of Atomic Time Scales:
1 January 1958, on 0 h 0 min 0 s UT2

International Atomic Time (TAI):
Time scale based on the definitions of the second and of the origin of Atomic Time Scales (as mentioned above), and implemented by a network of atomic clocks located all over the earth and operated by Bureau International de I'Heure (BIH) in Paris.



Atomic Time Scales II

Coordinated Universal Time (UTC):
Time scale based on the time unit of TAI, but adjusted by means of leap seconds so as to keep within 0.9 seconds of UT1;
UTC differ from TAI by an integer number of seconds;
leap seconds are added or subtracted when necessary at the end of the last day of a month.


UT1: based on the rotation of the earth around its axis. (old time standard before atomic standard inverted)
UT2: Based on the rotation of the earth around the sun.