Time Reference Frames
CASA supported time reference frames:
Acronym  Name  Description 

ET  Ephemeris Time  The time scale used prior to 1984 as the independent variable in gravitational theories of the solar system. In 1984, ET was replaced by dynamical time (see TDB, TT). 
GAST  Greenwich Apparent Sidereal Time  The Greenwich hour angle of the true equinox ^{[1]} of date. 
GMST  Greenwich Mean Sidereal Time 
The Greenwich hour angle of the mean equinox ^{[1]} of date, defined as the angular distance on the celestial sphere measured westward along the celestial equator from the Greenwich meridian to the hour circle that passes through a celestial object or point. GMST (in seconds at UT1=0) = 24110.54841 + 8640184.812866 * T 
GMST1  GMST calculated specifically with reference to UT1  
IAT  International Atomic Time (a.k.a. TAI en Francais):  The continuous time scale resulting from analysis by the Bureau International des Poids et Mesures of atomic time standards in many countries. The fundamental unit of TAI is the SI second ^{[2]} on the geoid ^{[3]} , and the epoch is 1958 January 1. 
LAST  Local Apparent Sidereal Time 
LAST is derived from LMST by applying the equation of equinoxes ^{[1]} or nutation of the mean pole of the Earth from mean to true position yields LAST. 
LMST  Local Mean Sidereal Time 
Sidereal time is the hour angle of the vernal equinox, the ascending node of the ecliptic on the celestial equator. The daily motion of this point provides a measure of the rotation of the Earth with respect to the stars, rather than the Sun. It corresponds to the coordinate right ascension of a celestial body that is presently on the local meridian. http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html 
TAI  International Atomic Time (a.k.a. TAI en Francais)  see IAT 
TCB  Barycentric Coordinate Time  The coordinate time of the Barycentric Celestial Reference System (BCRS), which advances by SI seconds ^{[2]} within that system. TCB is related to TCG and TT by relativistic transformations that include a secular term. 
TCG  
TDB  Barycentric Dynamical Time  A time scale defined by the IAU (originally in 1976; named in 1979; revised in 2006) used in barycentric ephemerides and equations of motion. TDB is a linear function of TCB that on average tracks TT over long periods of time; differences between TDB and TT evaluated at the Earth's surface remain under 2 ms for several thousand years around the current epoch. TDB is functionally equivalent to Teph, the independent argument of the JPL planetary and lunar ephemerides DE405/LE405. 
TDT  Terrestrial Dynamical Time  The time scale for apparent geocentric ephemerides defined by a 1979 IAU resolution. In 1991 it was replaced by TT. 
TT  Terrestrial Time  An idealized form of International Atomic Time (TAI) with an epoch offset; in practice TT = TAI + 32s.184. TT thus advances by SI seconds on the geoid ^{[3]} 
UT  Universal Time  Loosely, mean solar time on the Greenwich meridian (previously referred to as Greenwich Mean Time). In current usage, UT refers either to UT1 or to UTC. 
UT1  UT1 is formally defined by a mathematical expression that relates it to sidereal time. Thus, UT1 is observationally determined by the apparent diurnal motions of celestial bodies, and is affected by irregularities in the Earth's rate of rotation.  
UT2 
Before 1972 the time broadcast services kept their time signals within 0.1 seconds ^{[2]} of UT2, which is UT1 with annual and semiannual variations in the earth's rotation removed. The formal relation between UT1 and UT2 is UT2 = UT1 + 0.022 * sin(2 * Pi * t)  0.012 * cos(2 * Pi * t)


UTC  Coordinated Universal Time  UTC is based on IAT but is maintained within 0s.9 of UT1 by the introduction of leap seconds when necessary. 
Footnote Number  1 

Footnote Text 
mean equator and equinox v. true equator and equinox: The mean equator and equinox are used for the celestial coordinate system defined by the orientation of the Earth's equatorial plane on some specified date together with the direction of the dynamical equinox on that date, neglecting nutation. Thus, the mean equator and equinox moves in response only to precession. Positions in a star catalog have traditionally been referred to a catalog equator and equinox that approximate the mean equator and equinox of a standard epoch.

FootnoteNumber 
2 

Footnote Text 
The Systeme International (SI) second is defined as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of caesium 133.

Footnote Number  3 

Footnote Text 
The geoid is an equipotential surface that coincides with mean sea level in the open ocean. On land it is the level surface that would be assumed by water in an imaginary network of frictionless channels connected to the ocean.
