Viking Lander Image Data Dictionary
Below are definitions for keywords in the PDS label of the Viking
Lander EDR image files. The definitions are based on the standard PDS
data dictionary definition, but are, in some cases, customized or
expanded for the specific application to the Viking Lander images.
The major sections within the PDS label for Viking Lander EDR images are:
The Viking Lander cameras were facsimile scanning systems. They had a
mirror that rotated around a horizontal axis to scan about 100 degrees
in elevation. They also rotated around a vertical axis to scan nearly
360 degrees of azimuth. Note that a scan line refers a vertical line
of data collected at a given camera azimuth.
There are several coordinate systems used for the elevation and
azimuth coordinates of Viking Lander image data. Azimuth and
elevation values in the PDS labels are given in the Camera-Aligned
Camera Coordinate System (CACCS). In this coordinate system, azimuths
increase in the clockwise direction when viewed from above. The 0
degree azimuth for a camera is in the general direction of the other
camera. The 0 degree elevation is perpendicular to the camera's
azimuth rotation axis. Elevation is negative below this direction and
File Format and Length Keywords
- The PDS_VERSION_ID element indicates the version number of the PDS
standards that apply to a data product label. The value for the
Viking Lander image archive is PDS3, which indicates that version 3 of
the PDS standards apply.
- The RECORD_TYPE element indicates the format of records in a file.
Viking Lander image files have fixed length records.
- The RECORD_BYTE element indicates the number of bytes
in a record.
- The FILE_RECORDS element indicates the number of records, including
PDS label records and data records.
- The LABEL_RECORDS element indicates the number of file records that
contain PDS label information. The number of data records in a file
is determined by subtracting the value of LABEL_RECORDS from the value
Pointers to Data Objects in File
- The ^HISTOGRAM keyword gives the starting record number
of the histogram object.
- The ^IMAGE keyword gives the starting record number of the
Image Description Keywords
- The DATA_SET_ID element is a unique alphanumeric identifier for a data
set. The DATA_SET_ID is a combination of identifiers for the
mission, target, instrument, data type, and version of the data set.
In most cases the DATA_SET_ID is an abbreviation of the
- The SPACECRAFT_NAME element provides the full name of a
spacecraft. Values for the Viking Landers are VIKING_LANDER_1
- The INSTRUMENT_NAME element provides the full name of an
instrument. Values for the Viking Lander cameras are CAMERA_1 and
- The PRODUCT_ID element is a unique identifier assigned to a data
product by the data product producer. For Viking Lander images,
PRODUCT_ID has the form LCXSSS-FFF, where L is the lander number, C is
the camera number, X is a letter (starting with A for the first images
and ending with J for Lander 1 and I for Lander 2), SSS is a sequence
number from 000 to 255, and FFF is a code for the filter name. Note
that when the sequence number reaches 255, it was then reset to 000
and the letter was incremented. An example PRODUCT_ID is 12A001-BB1
for the first image acquired by Viking Lander 1.
- The MISSION_PHASE_NAME is a commonly-used identifier for a portion of
time during a mission. Examples are PRIMARY MISSION and EXTENDED MISSION.
- The TARGET_NAME identifies a target. The target may be a planet,
satellite, ring, region, feature, asteroid, or comet. For the Viking
Lander image data set, TARGET_NAME values include MARS, SUN, PHOBOS,
DEIMOS, and N/A. The N/A values are associated with calibration
- The START_TIME element provides the date and time in UTC
of the beginning of an image. Format is YYYY-MM-DDThh:mm:ssZ.
- The STOP_TIME element provides the date and time in UTC of the end of
an image. For Viking Lander images STOP_TIME may be
approximate. Format is YYYY-MM-DDThh:mm:ssZ.
- The LOCAL_TIME element provides the local time of day at the start of
the image, measured in local hours from midnight. A local hour is
defined as one twenty-fourth of a local solar day. A local
hour for Mars, by definition, is longer than an hour because a Mars solar day
is greater than 24 hours. Also, the value of LOCAL_TIME is given as a
decimal number (NOT as hour and minutes).
- The PLANET_DAY_NUMBER element indicates the number of days elapsed
since the landing day (landing day number is zero). This keyword is
often referred to as sol number for the Viking Landers.
- The START_AZIMUTH is the azimuth at which an image starts. It is
given in degrees in the camera-aligned camera coordinate system.
- The STOP_AZIMUTH is the azimuth at which an image stops. It is given
in degrees in the camera-aligned camera coordinate system.
- The CENTER_ELEVATION is the elevation at the center line of an image.
Depending on SAMPLING_PARAMETER_INTERVAL, the camera scanned either
approximately 20 degrees or 60 degrees in elevation.
- The OBSERVATION_TYPE element identifies the general type of an
observation. For Viking Lander images, some examples are
HIGH RESOLUTION SINGLET, COLOR TRIPLET, and INFRARED TRIPLET.
- The FILTER_NAME element provides the common name of the
instrument filter used for a given image. The Viking Lander cameras
had twelve filters: BB1, BB2, BB3, BB4, BLUE, GREEN, RED, IR1, IR2,
IR3, SURVEY, and SUN.
- The SAMPLING_PARAMETER_INTERVAL element identifies the spacing between
data points. In this case, SAMPLING_PARAMETER_INTERVAL is the spacing
between pixels of an image. Possible values for Viking Lander cameras
are 0.04 and 0.12 degrees in azimuth and elevation.
- The OFFSET_NUMBER indicates the offset value used in the analog
to digital conversion. The offset was a constant voltage added to the
measured voltage signal before digitization. Viking Lander cameras
had 32 commandable offsets with equal voltage steps.
- The GAIN_NUMBER indicates the gain value used in the analog to
digital conversion. The gain value was a multiplicative factor used
in the analog to digital conversion. There were 6 commandable gains
for the Viking Lander cameras.
- The DETECTOR_TEMPERATURE is the average detector temperature during
the time of image acquisition. Values are given in degrees Celsius.
- The START_RESCAN_NUMBER is the scan line number at which the rescanning
mode begins. The rescan mode consists of scanning vertically
repeatedly at the same azimuth. A value of 0 for START_RESCAN_NUMBER means
there was no rescanning.
- The TOTAL_RESCAN_NUMBER is the total number of rescan lines
- SCAN_RATE is the rate at which the instrument scanned while acquiring
a data frame. For Viking Lander images, the SCAN_RATE value is
given in bits/second.
- The DATA_PATH_TYPE element describes the telemetry path that data
traversed from spacecraft to the ground. Values for Viking
Lander images are RECORDED UHF LINK, REALTIME UHF LINK,
RECORDED S-BAND LINK, and REALTIME S-BAND LINK.
- The DUST_FLAG parameter indicates whether a dust sequence was
executed in association with an image. It is TRUE if a Viking
Lander camera was commanded to discharge a stream of CO2 gas across
the protective outer window of the camera prior to acquiring an
image. It is FALSE otherwise.
- The MISSING_SCAN_LINES element is the total number of scan lines missing
from an image when it was received on Earth. Missing scan lines are
filled with zeros in the archive image file.
- The NOTE element provides miscellaneous notes or
comments about a data product. Notes in Viking Lander image
labels indicate the reason for acquiring the image or are a description
of the image content.
Histogram Object Keywords
- The ITEMS element defines the number of multiple occurrences
of a single object, such as a column. In this case there are 256
items in the image histogram.
- The DATA_TYPE element supplies the internal representation and/or
mathematical properties of a value being stored. For example,
MSB_INTEGER means an integer number stored in most significant byte
- The ITEM_BYTES element gives the size in bytes of an item
within a data object.
Image Object Keywords
- The LINES element is the total number of data instances along
the vertical axis of an image. For Viking Lander EDR images, the
number of lines is always 512 lines.
- The LINE_SAMPLES element is the total number of data instances along
the horizontal axis of an image. The number of samples varies with
each Viking Lander image.
- The SAMPLE_TYPE element indicates the data storage representation of
sample value. For example, UNSIGNED_INTEGER is a positive integer
- The SAMPLE_BITS element indicates the number of bits
used to store one data sample value. The value
is always 8 for Viking Lander EDR images.
- The SAMPLE_BIT_MASK element identifies the active bits in a
sample. The value 2#11111100# means that the original 6-bit data value was
multiplied by 4 to generate the data value in the archive product.
- The CHECKSUM element is sum of all data
values in a data object.