***** File PDSINFO.TXT PDS LABELS 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. 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. In addition, detached PDS labels were constructed to allow parallel definition of the data files for the Planetary Data System. The SPIDS (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. There is only one data class considered on the IMAGE volumes of the Comet Halley Archive. Our aim was to construct a basic PDS label for each data file on the CD-ROM. These PDS labels contain pointers to the actual data files (or to headers describing data not digitized for 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 the powerful software already supported by that group. The basic PDS descriptors such as SFDU_LABEL, RECORD_TYPE, RECORD_BYTES, and FILE_RECORDS are explained in the SPIDS document. We had discussed with the PDS the definition of a specific keyword (TYPE = FITS) to indicate the presence of a FITS header and the byte ordering of the data (MSB_INTEGER) required by FITS. The description of compressed images is still under active discussion. Unlike previous PDS efforts with compressed images, we chose not to compress the header (or label) and as such have included a keyword to describe the type of compression (ENCODING_TYPE = PREVIOUS_PIXEL) used. The label for compressed images also contains information to permit software to skip over the data if the decoding algorithm is unknown (ITEMS, ITEM_TYPE, and ITEM_BITS). We have included some code for decompressing the images but no general software package is anticipated. A full set of the templates used for the IMAGE labels follow: IHW Sample PDS Label for Browse Data NJPL1I00PDS100000000 = SFDU_LABEL /* File Format and Length */ RECORD_TYPE = FIXED_LENGTH RECORD_BYTES = 256 FILE_RECORDS = 256 /* Record Pointer to Major Object */ ^IMAGE = 'LSPN0059.IBG' /* Description of Object in File */ OBJECT = IMAGE LINES = 256 LINE_SAMPLES = 256 SAMPLE_TYPE = UNSIGNED_INTEGER SAMPLE_BITS = 8 NOTE = "LARGE SCALE PHEN, SUBSAMPLED IMAGE" END_OBJECT END IHW Sample PDS Label for Compressed data NJPL1I00PDS100000000 = SFDU_LABEL /* File Format and Length */ RECORD_TYPE = FIXED_LENGTH RECORD_BYTES = 2880 FILE_RECORDS = 424 /* Record Pointer to Major Object */ ^IMAGE = 'LSPN0059.IMQ' /* Description of Object in File */ OBJECT = IMAGE ENCODING_TYPE = PREVIOUS_PIXEL ITEMS = 1221120 ITEM_TYPE = UNSIGNED_INTEGER ITEM_BITS = 8 LINES = 1024 LINE_SAMPLES = 1024 SAMPLE_TYPE = MSB_INTEGER SAMPLE_BITS = 16 NOTE = "LARGE SCALE PHEN, COMPRESSED IMAGE" END_OBJECT END Martin, T.Z., Martin, M.D., Davis, R.L., Mehlman, R., Braun, M., and Johnson, M.: October 3, 1988, Standards for the Preparation and Interchange of Data Sets, Version 1.1, JPL D-4683.