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Text Data Descriptions
The International Halley Watch agreed early in the project that all data
would be submitted from the individual Disciplines to the Lead Center using
the FITS format (Wells et al., 1981). When the decision was made to
distribute this information on CD-ROM, it was determined that the data had to
have even broader accessibility. For this reason the original FITS files,
with contiguous headers and data, were split into separate files
distinguishable by their filename extensions (.HDR for headers). The file
sizes were preserved as multiples of 2880 bytes, allowing the original FITS
byte stream to be recovered by concatenating the appropriate header and
datafile. PDS labels were constructed to allow definition of the datafiles
for the Planetary Data System. For each datafile there must always be an
associated FITS header.
The convention for naming files on the IHW CD-ROMs was proposed by the
Lead Center and NASA/Goddard Space Flight Center (GSFC) personnel to include a
unique data qualifier for the data. Specifically, a set of subnet codes was
established to enable identification of the IHW Discipline/subdiscipline from
the filename itself. A CD-ROM running number and file extension complete the
filename. A short list of this convention for the Astrometry Discipline
follows:
PDS Object FITS Discipline Subnet File Ext
(description) NAXIS = Code
______________________________________________________________________
text 1 Astrometry ASTR .dat
______________________________________________________________________
The file extensions follow suggestions by the Planetary Data System
(SPIDS v1.1; Martin et al., 1988) for tabular and image data. In addition,
for IHW FITS, the original headers and data were split into separate files,
with filename extensions as listed below.
.DAT - other non-image or non-table data
.HDR - FITS header records
.LBL - detached PDS stream format
These PDS labels are metadata (as headers describing data submitted to the
archive). There has been no effort to duplicate the documentation contained in
the full FITS headers because the PDS and FITS headers for a given data file
differ only in the filename extension. Instead we have attempted to use the
power of the PDS label syntax to fully describe the data structures and thus
gain access to software by that group. Standards for the Preparation and
Interchange of Data Sets, Martin, T. Z., et al, Document D-4683, Jet Propulsio
Laboratory, California Institute of Technology, Pasadena, CA document version
1.1 was the primary reference to the Object Description Language (ODL)
necessary to create the PDS labels. (We acknowledge R. Borgen and M. Martin
PDS-CN, JPL, for assisting the IHW through version 1.1 of the ODL
implementation for SPECTRUM.) The basic PDS descriptors such as SFDU_LABEL,
RECORD_TYPE, RECORD_BYTES, and FILE_RECORDS are explained in the SPIDS
document. The RECORD_TYPE for all data files is FIXED_LENGTH. The PDS labels
have been formed as fixed length (78 byte) plus an embedded CR and LF.
The TEXT object (which is used for the Astrometry Discipline's data with
extension .DAT) is an 80 byte fixed length record that contains only ASCII
values. In the FITS formulation, the 80-byte records are strung together,
typically as 4 or 5 "card" images with no delimiters and padded to fill the
2880 byte record structure. It can be recognized in the FITS formulation by
the NAXIS=1 statement, which indicates that a byte stream follows usually
carrying "text" description. The Astrometry data has been separated into its
own subdirectory (ASTROM) throughout the Archive discs.
REFERENCES
Martin, T.Z., Martin, M.D., Davis, R.L., Mehlman, R., Braun, M., Johnson
M.: October 3, 1988, Standards for the Preparation and Interchange of
Data Sets, Version 1.1, JPL D-4683.
Wells, D.C., Greisen, E.W., and Harten, R.H.: 1981, Astron. Astrophys.
Suppl. Ser. 44, 363.