Документ взят из кэша поисковой машины. Адрес оригинального документа : http://xmm.vilspa.esa.es/sas/6.0.0/doc/earthbarycen.ps.gz Дата изменения: Fri Mar 19 00:45:39 2004 Дата индексирования: Tue Oct 2 11:13:34 2012 Кодировка: Поисковые слова: earth

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earthbarycen
March 18, 2004
Abstract
Convert time tags from the local satellite frame to earth-based Barycentric Dy-
namical Time
1 Instruments/Modes
not applicable
2 Use
pipeline processing no
interactive analysis yes
3 Description
3.1 Introduction
The earthbarycen task converts times expressed in the local satellite frame to Barycentric Dynamical
Time (TDB) at a given point on the Earth. In more detail, the task requires as main input a table in a
data set which contains a column with times in XMM MET (Mission Elapsed Time) frame[1]. On this
table, the following operations are carried out:
1. Check whether the earthbarycentric conversion has been performed already (based on value
of attribute TIMEREF) and, if found true, exit
2. Correct all time tags stored in a specied column of the table
3. If a Good Time Interval extension exists, convert all interval start and stop times to TDB
as well
4. Update the values of the attributes TIMEREF, TSTART, TSTOP, and TELAPSE where necessary
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In accordance with NASA/OGIP recommendations, the value of the TIMEREF keyword is either LOCAL
or SOLARSYSTEM depending on whether the times are expressed in local satellite time or TDB. Note that
the conversion to TDB does not aect the actual time system in use, i.e., all times keep being referred to
the xed mission reference time 1998-01-01T00:00:00 UTC [1].
The conversion of a satellite time tag to TDB requires the knowledge of the satellite's position at that
time. Therefore, the task needs access to the orbit le in the ODF which covers the time period in
question. This access is done transparently via the ODF access layer which, however, needs to be pointed
to the directory containing the ODF.
In addition to the satellite position data, the time conversion algorithm in use by earthbarycen requires
the barycentric vectors for the time period in question to be known a priori. This information is comprised
in the JPL ephemeris data set in FITS format which is part of the barycen package.
4 Parameters
This section documents the parameters recognized by this task (if any).
Parameter Mand Type Default Constraints
withtable yes boolean true Whether to convert ta-
ble in a dataset
should be set to true if a table containing events to correct has been specied and to false otherwise.
table yes string name of existing table
a specier which must point to a table in a data set. It must be in either of the two forms setname:, or
setname:tableid where setname must be the name of an existing data set and tableid an identier of
a table in that data set. If the rst form, setname:, is used, the table data are sought in the rst block
of the named data set.
timecolumn no string TIME name of existing col-
umn
the name of an existing column of type real64 in the table given via the parameter table (see Sect. 4). The
values in the column are interpreted as XMM MET tags and corrected for earthbarycentric propagation
delays. Please note: This process is irreversible - if the original time column is to be kept a copy of the
data set should be made a priori.
withsrccoordinates no boolean false true|false
The earthbarycentric correction algorithms needs the coordinates of the celestial source. This boolean
parameter determines from where these are obtained. If set to false, the coordinates are attempted to
be read from the attributes RA OBJ/DEC OBJ in the table or the associates set. If this fails, the attributes
RA NOM/DEC NOM are read instead. If this fails as well a fatal error condition is raised. If withsrccoordinates
is set to true, the user has to provide the coordinates through the parameters srcra/srcdec.
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srcra/srcdec no string source coordinates
If withsrccoordinates is set to true the coordinates of the celestial source have to be specied through
these parameters in the equatorial, earth-centered J2000 reference frame. Recognized formats are for
srcra (Right Ascension)
1. 123.45678 (decimal degrees)
2. 23 59 59.1223
3. 23h59m59.1234s
and for srcdec (Declination)
1. 123.45678 (decimal degrees)
2. 20 18 59.1223
3. 20o18'59.1234"
processgtis no boolean true true|false
Boolean switch determining whether any present GTI tables should be corrected as well or not. The
parameter has no eect if the set does not contain any GTI tables.
basetime no real 0
Base time for MET frame
time no real 0
A single time tag in MET frame on which to perform the earthbarycentric correction. Only used if
withtable is false.
obslong no real 0
The longitude (East positive) of earth-bound observer
obslat no real 0
Geodetic latitude of earth-bound observer
obsalt no real 0
Altitude above NN of earth-bound observer [m]
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5 Errors
This section documents warnings and errors generated by this task (if any). Note that warnings and
errors can also be generated in the SAS infrastructure libraries, in which case they would not be docu-
mented here. Refer to the index of all errors and warnings available in the HTML version of the SAS
documentation.
ThisIsNotXMM (error)
No XMM data set given
NoCorrectionNecessary (error)
The specied table has already been processed with earthbarycen
NoEphemerisFile (error)
The JPL ephemeris le was not found
NoPositionData (error)
There is no position data available for a particular time | set time to NaN
6 Input Files
1. a data set with a table containing a time column to be processed
7 Output Files
1. the input data set with modied time column
8 Algorithm
subroutine earthbarycen
check if earthbarycentering has been applied already - if yes, exit
locate ODF orbit file
locate earth ephemeris file
foreach row in table
compute TBD from time value of current row
end
update timing related attributes
foreach GTI extension
forach row in GTI table
compute TBD of START and STOP time
end
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end
end subroutine earthbarycen
9 Future developments
References
[1] Time system standards in XMM science data sets, TN to be written by TBD
References
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