Dynamic Time System

1. Dynamic Time System Dynamic time is a special time scale that is derived based on movement Earth, Moon, and planets in the solar system. In 1991, International The Astronomical Union (IAU) defines several types of time in a consistent way with the general theory of relativity. Each time type is adapted to four reference frames certain dimensions. Pict 1. Evolution of Time Scale a. Ephemeris Time (ET) Ephemeris Time (ET) is the first dynamic time scale in history defined by the International Astronomical Union (IAU) in the 1950s and superseded by Barycentric Dynamical Time (TDB) in 1984. Ephemeris Time can actually be obtained by observing the orbital positions of planets or any satellite that lists an orbital position that can be calculated as a function time. However, the orbital position of the Earth regarding the Sun, as developed mathematically by the American astronomer, Simon Newcomb, in the Table of Sun (1898), chosen as the standard for determining the numerical measure of Ephemeris Time. This is seen from the Earth and the Sun are 180° apart i.e., opposite each other in plane ecliptic, so that the Sun's observations of the stars provide orbital positions Earth. Ephemeris Time values are also obtained from observations of the Moon with using lunar ephemeris. However, the lunar ephemeris contains non-empirical terms gravitational (needed to correct the tidal effects that occur on Earth by Month). The moon is generally used to determine Ephemeris Time due to its movement its fast orbit. The accurate position of the Moon is obtained visually through observation occultation of stars by the Moon. The Time Ephemeris has served two important purposes before replaced in 1984, namely: • Definition of seconds Ephemeris Time (one tropical year in the epoch 1900.0 is 31,556,925, 9747 ephemeris seconds) became the basis for the redefinition in 1967 from the SI second on the atomic time scale. • ET is a reference scale used for comparison with rotation time to determine variations in the speed of the Earth's rotation from about 700 BC to 1955. b. Barycentric Dynamical Time (TDB) and Terrestrial Dynamical Time (TDT) In 1976 the IAU established two scales for the theory of dynamics and ephemerides that will be used in 1984, namely TDB and TDT. Barycentric Dynamical Time (TDB) is the independent variable in the equation motion of celestial bodies (including the term relativity). The solution to this equation gives the coordinates rectangle of these objects relative to the barycentre (center of mass) of the system sun. In this system, the use of the theory of relativity is generally not specified specific, so that the TDB scale can be formed but with differences in coordinates small. Terrestrial Dynamical Time (TDT) is a time system that refers to the center earth (geocentric) and is an additional scale determined by the equation: TDT = TAI + 32.184 s. The units are SI seconds. The constant difference between TDT and TAI makes TDT continuously connected with ET before TAI was established in mid-1955. TDT is a time system quasi-inertial replacement for Ephemeris Time and commonly used in integration. Differential equations for the movement of satellites orbiting the earth. Another definition of TDT is TDT = TDB - R Where, R = Number of periodic relativistic terms not included in TAI. The two TDT equations above are only valid if dynamic time and atomic time are equivalent. c. Terrestrial Time (TT) Terrestrial Time is the time scale used to calculate position geocentric Solar System bodies as estimates of time on a relativistic time scale Barycentric Dynamical Time (TDB). TT is a continuation of Ephemeris Time, as a uniform time scale that would be measured by an ideal clock on the surface of the geoid sea average). The TT was introduced in 1984 under the name Terrestrial Dynamical Time (TDT) but was changed to Terrestrial Time in 1991. The basic unit is TT is the day 86,400 SI seconds. TT is realized with International Atomic Time (TAI) through the TAI 1.0 definition January 1977 corresponds to 1.0003725 January 1977 TT. This means that TT is running smoothly permanent 32.184 seconds ahead of TAI, but behind UTC, which is moving further from TT each time a leap second is introduced. The relationship between TT and TAI can be formulated as follows: TAI = TT – 32.184” d. Terrestrial Coordinate System (TCG) Terrestrial Coordinate System is the time scale used in Geocentric Celestial Reference System with SI units of seconds. TCG is also referred to as relativity time coordinates of a 4D geocentric frame. TCG is different from Terrestrial Time because the clock on the earth's surface moves about the geocentric frame and influenced by the Earth's gravitational field. Speed difference between TCG and Terrestrial Time is about 22 ms per year. e. Barycentric Coordinate Time (TCB) Barycentric Coordinate Time (TCB) is a standard coordinate time used as the independent variable of time for all related calculations with orbits of planets, asteroids, comets, and interplanetary spacecraft in the solar system. TCB was designated in 1991 by the International Astronomical Union, as one replacement for the problematic definition of Barycentric Dynamical Time (TDB) in 1976. TCB is defined in the context of the general theory of relativity. Because of the TCB's terms of reference unaffected by the gravitational potential caused by the solar system, the TCB beats faster than a clock on the Earth's surface by 1.550505 × 10 −8 (about 490 milliseconds per year). Time coordinates on the TCB scale use Julian and calendar dates Gregorian Atomic Time System a. International Atomic Time (TAI) TAI is a time scale calculated by BPIM using data from around 500 atomic clocks at more than 700 national laboratories. TAI is determined and maintained by BPIM in Paris and has been officially in effect since January 1972. Pict 2. Distribution of Laboratories Contributing to TAI as of April 2013 (Source: bipm.org) Basically there are several types of atomic clocks (oscillators), the majority of which are atomic clocks used is a Cesium clock. These clocks will be synchronized using a signal GNSS and satellite-based two-way time and frequency transfer methods. b. Universal Time Coordinates (UTC) Universal Time Coordinate (UTC) is an international based atomic scale for standard times and frequencies. UTC was introduced on January 1, 1960 with The SI unit of seconds, namely the atomic second defined by the resonance of the Cesium-133 atom, and UTC is widely broadcast by radio signal. These signals at the end provides the basis for setting all public and private clocks. Since January 1, 1972, UTC adds "leap seconds" when necessary. UTC functions to accommodate differences in timeliness that arise between atomic time (which comes from atomic clocks) and solar time (which comes from astronomical measurement of the Earth's rotation on its axis relative to the Sun). With Thus UTC has the same speed as the TAI atomic clock, but different an interger number of seconds (leap seconds) and has an interval of 0.9 seconds from the time indicator astronomical UT1. Due to the irregular deceleration of the earth's rotation speed by tidal friction and other forces, now there is about one second more (clock derivative atomic) in one solar year than there are seconds UT1. To improve this difference, UTC is maintained within 0.9 seconds of UT1 by adding second jump to UTC as needed. The slowdown in the Earth's rotation varies drastically regularly, so the number of leap seconds that UTC must inhibit to maintain it staying in an epoch with UT1 cannot be predicted years in advance. Pict 3. Relationship between TT and TAI (Source: Satellite Geodesy Teaching Materials) The relationship between Atomic Time and Star Time or Solar Time can be formulated as follows: TAI = UTC + 1” . n With, |UT1 - UTC | < 0,9” If |UT1 - UTC | > 0,9” then an additional (leap second) will be added to the time UTC. Since January 1 2017, the accumulated value of lead second (n) is determined by IERS it has reached 37 seconds. c. GPS Time GPS time is a time system used by GPS (Global Positioning System). The relationship between GPS time and atomic time is: TAI = GPS time + 19,000" (constant offset) The relationship between GPS time and UTC is: Where, n = leap second GPS time = UTC + 1.00". n - 19,000" From the relationships above it can be seen that the principles of UTC and GPS time are atomic time system. Pict 4. Relationship between UTC, GPS, TAI and TT