***** File PDS_IHW.TXT
PDS LABELS
The International Halley Watch agreed early in the project that all data
would be submitted from the individual scientists 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
reference for this project are as follows: SPIDS, Standards for the
Preparation and Interchange of Data Sets, Martin, T. Z., et al, V1.1, JPL
D-4683, 10/3/88; Planetary Data System, Data Preparation Workbook, Volume 2,
Standards, V2.0, JPL D-7669, 05/03/91; and Planetary Data System, Standards
Reference, V3.0, JPL D-7669, part 2, 11/20/92.
There are three data objects in this archive: ARRAY (COLLECTION), IMAGE,
TABLE (TIME_SERIES as a subset). Our aim was to construct a 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.
Most keywords were already in the Planetary Science Data Dictionary but a
few dealing with the ARRAY and TIME_SERIES were introduced to specifically
describe the IHW data. A listing of these keywords with definitions follow.
BYTES
The number of bytes constituting a single data item
COLUMNS
The number of items of information in each row of a data table
CONTAINER
The container object allows for description of 2-d array data.
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 occurring in the file for a given field
DERIVED_MINIMUM
The minimum value occurring in the file for a given field
DESCRIPTION
Text describing an object. Sometimes this is expressed as a pointer
to another file containing the descriptive text; e.g., FITS header.
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
The Fortran 77 representation of the format statement needed to read
a data item in an ASCII file
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 file; either "ASCII" or "BINARY"
ITEMS
Elements held in any arbitrary variable
ITEM_BYTES
Number of bytes per item
LINES
The number of lines in an image
LINE_SAMPLES
The number of samples contained in each image line
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 type of a data object. It is used by ODL
to indicate the start of a data object definition.
OBSERVATION_TIME
As used by the International Halley Watch, the time at the midpoint of
an observation.
OBSERVATION_ID
A unique number which identifies each archived measurement gathered by
the International Halley Watch.
OFFSET
A shift in zero point required to properly calculate 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.
REPETITIONS
The number times a repeating data item recurs
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_INTERVAL
Unit of change of the independent variable (assumed to be constant)
SAMPLING_PARAMETER_ITEMS
Number of elements along independent axis
SAMPLING_PARAMETER_NAME
Name associated with the independent variable
SAMPLING_PARAMETER_UNIT
Unit associated with the 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. The first item will have a START_BYTE of 1.
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
TEXT
The object which contains an ASCII string with no formatting characters
TIME_SERIES
The object in a form of a table that stores a sequence of measurements
taken at regular intervals of time
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.
Planetary Data System. Data Preparation Workbook, Volume 2, Standards,
V2.0, JPL D-7669, May 3, 1991.
Planetary Data System, Standards Reference, V3.0, JPL D-7669, part 2,
November 20, 1992.
Wells, D.C., Greisen, E.W., and Harten, R.H.: 1981, Astron. Astrophys.
Suppl. Ser. 44, 363.