***** File PDS_IHW.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 (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.                                                                     
                                                                              
     In addition, detached (PDS) labels were constructed to allow parallel    
definition of the data files for the Planetary Data System (PDS). 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   
Bthe Object Description Language (ODL) necessary to create the PDS labels.    
(We acknowledge R. Borgen and  M. Martin, PDS- JPL, for assisting the IHW     
through version 2.0 of the ODL that allows for description of HEADER, TEXT,   
and SPECTRUM.)                                                                
                                                                              
     There are five fundamental data objects in this archive: IMAGE, TABLE,   
TEXT, HEADER, and SPECTRUM.  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 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 the
powerful software already supported by that group.   A more full explanation  
of each object is carried in text (.TXT) files HDRFORM, IMAGFORM, SPECFORM,   
TABLFORM, and TEXTFORM.   In addition, a further description of the PDS       
detached label is listed in file LBLFORM.TXT.   Each of these files was used  
as a reference for the SFDU pointers in the VOLDESC.SFD.                      
                                                                              
     Most keywords were already in the Planetary Science Data Dictionary but a
few dealing with the spectral_image_qube were introduced to specifically      
describe the IHW data.   A listing of these keywrods with definitions follow. 
                                                                              
                                                                              
                                                                              
                                                                              
                           Keyword Definitions                                
                                                                              
     The definitions of the keywords used in the detached PDS labels on the   
International Halley Watch Arcive CD-ROMs are given below.  Where applicable  
the definitions are taken from the draft PSDD (Planetary Science Data         
Dictionary 1990) or the PDS Standards for the Preparation and Interchange of  
Data Sets (SPIDS) document (Martin et al., 1988). There are some differences  
from the PDS data dictionary in the definitions used on this CD-ROM, since the
definitions of some keywords in the current PDS data dictionary do not        
dBescribe comet data.  In addition, some keywords for processed (UVFITS) data 
do not exist in the current PDS data dictionary.                              
                                                                              
  B                                                                           
AXES                                                                          
    The number of independent variables in a data array.                      
                                                                              
BAND_STORAGE_TYPE                                                             
    The arrangement of data in a qube ordered by spectral bands.              
                                                                              
BYTES                                                                         
    The number of bytes containing a data item.                               
                                                                              
COLUMNS                                                                       
    The number of items of information in each row of a data table.           
                                                                              
CORE_DESCRIPTION                                                              
    The dependent variable expressed in a spectral_image_qube.                
                                                                              
CORE_ITEMS                                                                    
    The number of elements along an independent axis.                         
                                                                              
CORE_NAME                                                                     
    Identifying name for avariable along an independent axis.               

                                                                              
DATA_SET_ID                                                                   
    A unique alphanumeric identifier for a data set.  It is used as a         
    primary key in the PDS catalog.                                           
                                                                              
DATA_SET_PARAMETER_NAME                                                       
    The name of the physical parameter represented in an image.  Note         
    this definition differs from the PDS data dictionary definition.          
                                                                              
DATA_TYPE                                                                     
    The data type of a data item.  Valid values are INTEGER, REAL, DATE,      
    TIME, and CHARACTER.                                                      
                                                                              
DERIVED_MAXIMUM                                                               
    The maximum value held in a data record.                                  
                                                                              
DERIVED_MINIMUM                                                               
    The minimum value held in a data record.                                  
                                                                              
DESCRIPTION                                                                   
    Text describing an object.  Sometimes this is expressed as a pointer      
    to another file containing the descriptive text; e.g., FITS header.       
                                                                              
ENCODING_TYPE                                                                 
     Previous pixel compression of 16-bit data; also called first difference. 
                                                                            

END_OBJECT                                                                    
    This keyword is used by ODL to indicate the end of a data object          
    definition.                                                               
                                                                              
FILE_RECORDS                                                                  
    The number of physical records in a data file.                            
                                                                              
FORMAT B                                                                      
    The Fortran 77 representation of the format statement needed to read      
    a data item.                                                              
                                                                              
IMAGE                                                                         
    The data in an image file, expressed as a pointer to the record           
    where the data begins.  For example, ^IMAGE = ("filename",3)              
    indicates that image data begins in record 3 of file "filename".          
                                                                              
INTERCHANGE_FORMAT                                                            
    The type of data stored in a data table, such as ASCII or BINARY.         
                                                                              
ITEMS                                                                         
    Elements held in any arbitrary variable.                                  
                                                                              
ITEM_BITS                                                                     
    Number of bits per item.                                                  
                                                                              
ITEM_TYPE                                                                     
    The data type of an item.                                                 
                                                                              
LINES                                                                         
    The number of lines in an image.                                          
                                                                              
LINE_SAMPLES                                                                  
    The number of samples contained in each image line.                       
        B                                                                     
MINIMUM_SAMPLING_PARAMETER                                                    
    For the spectrum object, the first value along the fastest                
    varying axis.                                                             
                                                                              
NAME                                                                          
    The name of a column in a table.                                          
                                                                              
NOTE                                                                          
    Descriptive text about a data file, referring to IHW disciplines.         
                                                                              
OBJECT                                                                        
    This keyword specifies the name of a data object.  It is used by ODL      
    to indicate the start of a data object definition.                        
                                                                              
OBSERVATION_TIME                                                              
    A timBe associated with the midpoint of the International Halley Watch    
    set of observations.                                                      
                                                                              
OBSERVATION_ID                                                                
    A unique number held to identify each archived measurement gathered by    
    the International Halley Watch.                                           
                                                                              
OFFSET                                                                        
    A shift in zero point required to properly calculated the reduced         
    value represented in a FITS data record.                                  
                                                                              
PRODUCER_FULL_NAME                                                            
    The full name of those mainly responsible for production of               
    a data set.                                                               
                                                                              
RECORDS                                                                       
    The number of records in the object being described; for example,       

    the number of records in a header object.                                 
                                                                              
RECORD_BYTES                                                                  
    The number of bytes in each record of a data file.                        
                                                                              
RECORD_TYPE                                                                   
    The record structure type of a data file.  Valid values are               
    FIXED_LENGTH, VARIABLE_LENGTH, and STREAM.  Images and data tables        
    usually have fixed-length records, whereas text files have stream         
    format records.                                                           
                                                                              
ROWS                                                                          
    The number of logical records in a data table.                            
                                                                              
ROW_BYTES                                                                     
    The number of bytes in each row (i.e., logical record) of a data          
    table.                                                                    
                                                                              
SAMPLE_BITS                                                                   
    The number of bits of data comprising one sample or pixel in an           
    image or element in other objects.                                        
                                                                              
SAMPLE_TYPE                                                                   
    The data type of an image sample or pixel.  The table below lists         
    the values used on this CD-ROM:                                           
      UNSIGNED_INTEGER   An unsigned integer value.  Samples with a           
                         length of 16 bits are in most-significant-byte       
                         first order.                                         
      MSB_INTEGER        A signed integer with most-significant-byte          
                         leading as required by FITS format.                  
      COMPLEX_INTEGER    The value represented in a process step              
                         (UVFITS) for certain types of radio data.            
                                                                              
SAMPLING_PARAMETER_DESCRIPTION                                                
    Text explanation of independent variable.                                 
                                                                              
SAMPLING_PARAMETER_INTERVAL                                                   
    Smallest uniform change of independent variable.                          
                                                                              
SAMPLING_PARAMETER_ITEMS                                                      
    Number of elements along independent axis.                                
                                                                              
SAMPLING_PARAMETER_NAME                                                       
    Name associated with independent variable.                                
                                                                              
SAMPLING_PARAMETER_UNIT                                                       
    Unit associated with independent variable.                                
                                                                              
SCALING_FACTOR                                                                
    The factor that must be applied to the FITS data record to scale          
    the values as described by UNIT.                                          
                                                                              
START_BYTE                                                                    
    The byte position of the beginning of a data item within a row of       

    data.                                                                     
                                                                              
START_TIME                                                                    
    The date and time of the beginning of an event, such as data              
    collection, in PDS standard (UTC) format.                                 
                                                                              
TARGET_NAME                                                                   
    The name of a planetary body, such as a planet or satellite.              
                                                                              
TYPE                                                                          
    The type of header in a data file, such as a VICAR2 label embedded        
    in the image file.                                                        
                                                                              
UNIT                                                                          
    The units of measure of a data item.                                      
                                                                              
                                                                              
                                                                              
                                                                              
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.