***** File TEXTFORM.TXT                                                       
                                                                              
                          Text Data Descriptions                              
                                                                              
                                                                              
     The International Halley Watch agreed early on in the project that       
all data would be submitted from the individual disciplines to the Lead       
Center using the FITS format.  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. The         
original FITS byte stream could then be recovered by concatenating the        
appropriate header and data files.   PDS labels were constructed to allow     
definition of the data files for the Planetary Data System.   For each        
data there must be a file indicating a FITS header.                           
                                                                              
     The convention for naming files on the CD-ROM was proposed by the        
IHW Lead Center to include a unique data qualifier for the data.   A          
specific set of rules was established to identify the net and subnet for      
each discipline (given by a letter code).  A CD-ROM running number relates    
the information contained in the various indices to the files containing      
the actual data.  A short list of this convention for Astrometry follows:     
                                                                              
  PDS Object              FITS      Discipline    Letter    File Ext          
 (description)         NAXIS =                     Code                       
                                                                              
text                       1        Astrometry     ASTR       .dat            
                                                                              
     The file extensions follow suggestions by the Planetary Data System      
(SPIDS v1.1) 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 for Astrometry.                                    
                                                                              
                 .DAT - other non-image or non-table data                     
                                                                              
    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 Propulsion 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 thankfully acknowledge the help of M. Martin,    
JPL, for assisting the IHW through version 1.1 of the ODL that allows         
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) with 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.