**3. Time measurement systems**

Time measurement is one of the most important tasks of modern metrology, astrometry and physics. In order to fulfil this task, it is necessary:


It is easy to use periodic processes based on natural life cycles and characterized by high stability over long time periods as standard time units. Among all processes used for determining time units there are three basic ones:


Thanks to these natural processes, three independent time scales were developed; these are universal (astronomical), ephemeris and atomic time scales. Universal or worldwide time scale was so called as it is directly connected to the nychtemeral cycle that is the main cycle of people's lives. In terms of a type of a process that forms a universal time scale, there are three universal time systems.

1. *UT*0 is universal time registered directly as a result of astronomical observations. Technically, it is not universal, as it depends on where on the Earth the observatory is located.

2. *UT*1 is universal time which was corrected as a result of change of longitudes of observatories under the influence of movement of poles, it is calculated according to the formula:

$$\mathsf{LIT1} = \mathsf{LIT} + \mathsf{ΔX},\tag{1}$$

where is a correction to the observatory longitude, for movement of the North Pole.

3. *UT*2 is universal time, which also takes into account seasonal variations in the Earth's rotation speed. 2 *UT* is the most uniform time, which can be received from astronomical observations of diurnal rotation of the Earth. 2 *UT* and 1 *UT* time scales differ in the value of seasonal variations *Ts* :

$$
\Delta IT \, \mathbf{2} = \mathbf{L}IT \mathbf{1} + \Delta T\_s \,. \tag{2}
$$

The ephemeris time scale was introduced in 1952 by the decision of Paris international conference on fundamental astronomical constants. Ephemeris time ( *ET* ) is an independent variable of celestial mechanics equations and is defined as difference between observed and calculated, according to celestial-mechanical theories, coordinates of the Moon, the Sun and planets. As *ET* standard a second, equal to 1/31556925,9747 part of a tropical year, was taken. Uniformity of *ET* scale was influenced by observation errors and uncertainties of adopted theories of the Moon's and planetary movement (Bakulin & Blinov, 1977).

In 1967 by the decision of the XII General Conference of Weights and Measures the atomic time scale *TAI* was introduced, the scale unit is equal to a time span, in which the caesium atom goes through 9192631770 emission periods. The atomic scale is characterized by the highest stability of all mentioned ones and that is why it is the main time standard in the world. It is connected to the ephemeris time scale by the formula:

$$ET = TA1 + \text{32}/18 \text{ c} \tag{3}$$

Universal Coordinated Time scale *UTC* is a combined one; it is based on the atomic time scale, indications of which are corrected taking into account data concerning the Earth's rotation. From time to time *UTC* indications are corrected to 1 second, so that difference between 1 *UT UTC* does not exceed 0,9 second in modulus. *UTC* signals are transmitted in broadcasting networks and used in daily life. The *UTC* advantages are high uniformity that is characteristic for atomic time, and connection to natural processes (sunrise, sunset) that is characteristic for solar time. Some countries form and maintain their own *UTC* scale. For example, in the U.S. the *UTC* (USNO) scale is based on the assembly of (about 50) caesium standards, its indications are not more than 50 ns in error compared to the international standard *UTC* . In Russia the national scale *UTC* (SU) is maintained which is offset by +3 hours from *UTC* (Belotserkovsky & Kaufman, 1972).

The possibility of independent pulsar time scale formation has lately been the subject of wide speculation. Pulsars are speed-up neutron stars that represent sources of high-stable radio pulses with frequencies of the order of 1-1000 Hz. In view of this property, pulsars are regarded as potential time keepers, located in extraterrestrial environment. This remarkable property of pulsars is especially important in view of creation and development around the globe of Global Positioning Systems that are based on metric unity of four-dimensional

2. *UT*1 is universal time which was corrected as a result of change of longitudes of observatories under the influence of movement of poles, it is calculated according to the

3. *UT*2 is universal time, which also takes into account seasonal variations in the Earth's rotation speed. 2 *UT* is the most uniform time, which can be received from astronomical observations of diurnal rotation of the Earth. 2 *UT* and 1 *UT* time scales differ in the

The ephemeris time scale was introduced in 1952 by the decision of Paris international conference on fundamental astronomical constants. Ephemeris time ( *ET* ) is an independent variable of celestial mechanics equations and is defined as difference between observed and calculated, according to celestial-mechanical theories, coordinates of the Moon, the Sun and planets. As *ET* standard a second, equal to 1/31556925,9747 part of a tropical year, was taken. Uniformity of *ET* scale was influenced by observation errors and uncertainties of

In 1967 by the decision of the XII General Conference of Weights and Measures the atomic time scale *TAI* was introduced, the scale unit is equal to a time span, in which the caesium atom goes through 9192631770 emission periods. The atomic scale is characterized by the highest stability of all mentioned ones and that is why it is the main time standard in the

Universal Coordinated Time scale *UTC* is a combined one; it is based on the atomic time scale, indications of which are corrected taking into account data concerning the Earth's rotation. From time to time *UTC* indications are corrected to 1 second, so that difference between 1 *UT UTC* does not exceed 0,9 second in modulus. *UTC* signals are transmitted in broadcasting networks and used in daily life. The *UTC* advantages are high uniformity that is characteristic for atomic time, and connection to natural processes (sunrise, sunset) that is characteristic for solar time. Some countries form and maintain their own *UTC* scale. For example, in the U.S. the *UTC* (USNO) scale is based on the assembly of (about 50) caesium standards, its indications are not more than 50 ns in error compared to the international standard *UTC* . In Russia the national scale *UTC* (SU) is maintained which is offset by +3

The possibility of independent pulsar time scale formation has lately been the subject of wide speculation. Pulsars are speed-up neutron stars that represent sources of high-stable radio pulses with frequencies of the order of 1-1000 Hz. In view of this property, pulsars are regarded as potential time keepers, located in extraterrestrial environment. This remarkable property of pulsars is especially important in view of creation and development around the globe of Global Positioning Systems that are based on metric unity of four-dimensional

adopted theories of the Moon's and planetary movement (Bakulin & Blinov, 1977).

world. It is connected to the ephemeris time scale by the formula:

hours from *UTC* (Belotserkovsky & Kaufman, 1972).

is a correction to the observatory longitude, for movement of the North Pole.

, (1)

2 1 *UT UT T <sup>s</sup>* . (2)

*ET TA c* 1 32,18 (3)

formula:

where

1 *UT UT*

value of seasonal variations *Ts* :

space-time continuum. (GRED, 2003). But there are natural factors that influence radiofrequency radiation stability of pulsars and are related to their inherent properties and difference in conditions of signal transmission in interstellar medium. So, for actual use of pulsars as time keepers it is necessary to minimize accidental variations and systematic deviations of observed radio frequencies. This is achieved by formation of a pulsar time group scale and application of parameterization methods. In the long term integration of pulsar time with a group atomic keeper for the purpose of atomic time scale correction will enable to carry out comparative measurement and determine corrections to bring indications of all atomic keepers in line with high-stable pulsar time scale, using it as the key standard. In European Radio Astronomical Observatories systematic observations of 25 pulsars are carried out. In the U. S. observation of an assembly of 25 pulsars are carried out as part of the NANOGrav program (North American Nanohertz Observatory for Gravitational Waves) by means of the largest radio telescopes: Arecibo (300m) - 13 MP, RT-100 etc. (Ilyasov and others, 2010).
