📚 PDS_IHW.TXT
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261 lines***** 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.