astTimeAdd - Add a time coordinate conversion to a TimeMap

**Description:**- This function adds one of the standard time coordinate
system conversions listed below to an existing TimeMap.
When a TimeMap is first created (using astTimeMap), it simply performs a unit (null) Mapping. By using astTimeAdd (repeatedly if necessary), one or more coordinate conversion steps may then be added, which the TimeMap will perform in sequence. This allows multi-step conversions between a variety of time coordinate systems to be assembled out of the building blocks provided by this class.

Normally, if a TimeMap's Invert attribute is zero (the default), then its forward transformation is performed by carrying out each of the individual coordinate conversions specified by astTimeAdd in the order given (i.e. with the most recently added conversion applied last).

This order is reversed if the TimeMap's Invert attribute is non-zero (or if the inverse transformation is requested by any other means) and each individual coordinate conversion is also replaced by its own inverse. This process inverts the overall effect of the TimeMap. In this case, the first conversion to be applied would be the inverse of the one most recently added.

- This function adds one of the standard time coordinate
system conversions listed below to an existing TimeMap.
**Synopsis:**`void astTimeAdd( AstTimeMap this, const char cvt, const double args[] )`

**Parameters:****this**- Pointer to the TimeMap.
**cvt**-
Pointer to a null-terminated string which identifies the
time coordinate conversion to be added to the
TimeMap. See the
`"`Available Conversions`"`section for details of those available. **args**-
An array containing argument values for the time
coordinate conversion. The number of arguments required, and
hence the number of array elements used, depends on the
conversion specified (see the
`"`Available Conversions`"`section). This array is ignored and a NULL pointer may be supplied if no arguments are needed.

**Notes:**- When assembling a multi-stage conversion, it can sometimes be
difficult to determine the most economical conversion path. A solution
to this is to include all the steps which are (logically) necessary,
but then to use
astSimplify to simplify the resulting
TimeMap. The simplification process will eliminate any steps
which turn out not to be needed.
- This function does not check to ensure that the sequence of coordinate conversions added to a TimeMap is physically meaningful.

- When assembling a multi-stage conversion, it can sometimes be
difficult to determine the most economical conversion path. A solution
to this is to include all the steps which are (logically) necessary,
but then to use
astSimplify to simplify the resulting
TimeMap. The simplification process will eliminate any steps
which turn out not to be needed.

**Available Conversions**- The following strings (which are case-insensitive) may be supplied
via the
`"`cvt`"`parameter to indicate which time coordinate conversion is to be added to the TimeMap. Where arguments are needed by the conversion, they are listed in parentheses. Values for these arguments should be given, via the`"`args`"`array, in the order indicated. Units and argument names are described at the end of the list of conversions, and`"`MJD`"`means Modified Julian Date.`"`MJDTOMJD`"`(MJDOFF1,MJDOFF2): Convert MJD from one offset to another.`"`MJDTOJD`"`(MJDOFF,JDOFF): Convert MJD to Julian Date.`"`JDTOMJD`"`(JDOFF,MJDOFF): Convert Julian Date to MJD.`"`MJDTOBEP`"`(MJDOFF,BEPOFF): Convert MJD to Besselian epoch.`"`BEPTOMJD`"`(BEPOFF,MJDOFF): Convert Besselian epoch to MJD.`"`MJDTOJEP`"`(MJDOFF,JEPOFF): Convert MJD to Julian epoch.`"`JEPTOMJD`"`(JEPOFF,MJDOFF): Convert Julian epoch to MJD.`"`TAITOUTC`"`(MJDOFF): Convert a TAI MJD to a UTC MJD.`"`UTCTOTAI`"`(MJDOFF): Convert a UTC MJD to a TAI MJD.`"`TAITOTT`"`(MJDOFF): Convert a TAI MJD to a TT MJD.`"`TTTOTAI`"`(MJDOFF): Convert a TT MJD to a TAI MJD.`"`TTTOTDB`"`(MJDOFF, OBSLON, OBSLAT, OBSALT): Convert a TT MJD to a TDB MJD.`"`TDBTOTT`"`(MJDOFF, OBSLON, OBSLAT, OBSALT): Convert a TDB MJD to a TT MJD.`"`TTTOTCG`"`(MJDOFF): Convert a TT MJD to a TCG MJD.`"`TCGTOTT`"`(MJDOFF): Convert a TCG MJD to a TT MJD.`"`TDBTOTCB`"`(MJDOFF): Convert a TDB MJD to a TCB MJD.`"`TCBTOTDB`"`(MJDOFF): Convert a TCB MJD to a TDB MJD.`"`UTTOGMST`"`(MJDOFF): Convert a UT MJD to a GMST MJD.`"`GMSTTOUT`"`(MJDOFF): Convert a GMST MJD to a UT MJD.`"`GMSTTOLMST`"`(MJDOFF, OBSLON, OBSLAT): Convert a GMST MJD to a LMST MJD.`"`LMSTTOGMST`"`(MJDOFF, OBSLON, OBSLAT): Convert a LMST MJD to a GMST MJD.`"`LASTTOLMST`"`(MJDOFF, OBSLON, OBSLAT): Convert a GMST MJD to a LMST MJD.`"`LMSTTOLAST`"`(MJDOFF, OBSLON, OBSLAT): Convert a LMST MJD to a GMST MJD.`"`UTTOUTC`"`(DUT1): Convert a UT1 MJD to a UTC MJD.`"`UTCTOUT`"`(DUT1): Convert a UTC MJD to a UT1 MJD.`"`LTTOUTC`"`(LTOFF): Convert a Local Time MJD to a UTC MJD.`"`UTCTOLT`"`(LTOFF): Convert a UTC MJD to a Local Time MJD.

- Julian epochs and offsets: Julian years
- Besselian epochs and offsets: Tropical years
- Modified Julian Dates and offsets: days
- Julian Dates and offsets: days

- MJDOFF: The zero-point being used with MJD values.
- JDOFF: The zero-point being used with Julian Date values.
- BEPOFF: The zero-point being used with Besselian epoch values.
- JEPOFF: The zero-point being used with Julian epoch values.
- OBSLON: Observer longitude in radians (ve westwards).
- OBSLAT: Observer geodetic latitude (IAU 1975) in radians (ve northwards).
- OBSALT: Observer geodetic altitude (IAU 1975) in metres.
- DUT1: The UT1-UTC value to use.
- LTOFF: The offset between Local Time and UTC (in hours, positive for time zones east of Greenwich).

- The following strings (which are case-insensitive) may be supplied
via the

Starlink User Note 211

R.F. Warren-Smith & D.S. Berry

25th February 2013

E-mail:starlink@jiscmail.ac.uk

Copyright (C) 2014 Science \& Technology Facilities Council