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 positive above.

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 of FILE_RECORDS.

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 object.

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 DATA_SET_NAME.

The SPACECRAFT_NAME element provides the full name of a spacecraft. Values for the Viking Landers are VIKING_LANDER_1 and VIKING_LANDER_2.

The INSTRUMENT_NAME element provides the full name of an instrument. Values for the Viking Lander cameras are CAMERA_1 and CAMERA_2.

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 images.

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 acquired.

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 first order.

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 number.

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.