Viking Lander Navigator
Terminology

Camera-aligned camera coordinate system

The coordinate system primarily used [here] is the camera-aligned camera coordinate system (CACCS). The origin of this system is at the intersection (nominal) of the rotation axes of the camera mirror. In the CACCS, azimuth angles are measured clockwise (viewed from above) from a vector pointing in the general direction of the other camera. For camera 1 this vector passes in back of camera 2 but for camera 2 it passes slightly in front of camera 1. (See fig. 5(a).) The CACCS is the reference for the start azimuths and stop azimuths presented in the parameter lists for VL-1 and VL-2. The cameras are commanded by using the CACCS notation. This system is the reference for the notation which appears on the horizontal scale through the center of each camera view in the VL-1 and VL-2 skyline drawings. The CACCS system is also referenced by the fiducial annotation on the EDR photographic products. However, this fiducial annotation presents two azimuth angles separated by a slash (/). The first references the CACCS, as just discussed; the second references the lander-aligned camera coordinate system as described in the next section.

The camera elevation angle is measured from the direction perpendicular to the azimuth axis of the camera as described in the section "Center elevation".

Lander-aligned camera coordinate system

The lander-aligned camera coordinate system (LACCS) is referenced in only two situations. The azimuth entries which appear following the slash (/) at the top of the EDR photographic products reference the LACCS as do the azimuth entries which appear at the top and bottom of each camera view on the VL-1 and VL-2 skyline drawings. In the LACCS system, azimuth angles are measured clockwise (viewed from above) from a vector perpendicular to the inter-camera baseline and the intersection (nominal) of the rotation axes of the camera mirror. LACCS derives its name from its similarity to a commonly used general lander coordinate system, the lander-aligned coordinate system (LACS), which has an origin 1.1m below the center of the top surface of the lander. (See fig. 5(b).)

Lander tilt

The landers are tilted slightly relative to the local gravity vector. VL-1 is tilted 3° downward in the direction 285.18° clockwise from north. (See fig. 5(c).) VL-2 has a tilt of 8.21° downward in the direction 277.9° from north. (See fig. 5(d).)

Frame count

Frame count is a lander assigned sequence number which increases by 1 for each camera event. Its starting value is 1 and recycles to 0 after reaching 255.

Camera event

Camera event (CE) refers to a single executed camera command which is identified by a frame count. It results in the collection of vertical scan lines of camera data.

Camera event label

The camera event label is a 10-character identifier used to designate a camera event. The first character (1 or 2) indicates the lander (VL-1 or VL-2), the second character (1 or 2) indicates the camera on the lander, and the third character is the frame count cycle. This third character takes on the values A, B, C, . . . and is included because the frame count (the three digits following the frame count cycle) resets to 0 after counting to 255. The three digits following the slash (/) are the sol on which the camera event occurred.

Mars solar day

The Mars solar day (sol) is considered to have a length of 24 hours, 39 minutes, and 35.25 seconds; it is usually abbreviated as sol. Sol 0 is the particular spacecraft's landing day. Sol 0 for VL-1 is July 20, 1976, and sol 0 for VL-2 is September 3, 1976.

Local lander time

Local lander time (LLT) refers to the time after local midnight at the lander on Mars. Generally, it refers to the beginning of a camera event. It is designated in hours, minutes, and seconds (Earth units) in the format H:M:S and often appears preceded by sol separated by a slash.

Diode

There are 12 photodiodes in the camera. These diodes are generally referred to by name: BB1, BB2, BB3, and BB4 for the high-resolution broadband diodes; SURV for the low-resolution broadband diode; BLU, GRN, and RED for the visual color diodes; IR1, IR2, and IR3 for the infrared diodes; and SUN for the diode used in Sun imagery. When the diodes are used in triplet mode (nominally BLU, GRN, RED or IR1, IR2, IR3), a /T is generally appended (e.g., BLU/T). Calibration images are identified by "CAL" in this field, sometimes preceded by the first diode used in the calibration and followed by the light source level.

Azimuth start/stop

The azimuth start and stop entries are the azimuth limits, in degrees, of a camera event. Sometimes, there are missing camera scan lines which result in no image data at points within this range.

Center elevation

Camera events are commanded by specifying the center elevation (ELEV) pointing angle (EPA) of the resulting image. The direction perpendicular to the camera's azimuth axis of rotation is the 0° reference (negative is below, positive above). This value is sometimes followed by the lower and upper limits of the scan in parentheses. Note that this is the commanded elevation and must be adjusted for nonnominal modes.

Step size

The camera step size may be either 0.04° or 0.12° and expresses the angular separation in azimuth between successive scan lines and the angular separation in elevation between successive picture element centers.

Channel

The channel (CHAN) is the camera parameter which determines the diode used in the camera event. The value range is from 0 to 15 but 6, 7, 12, and 15 are left undefined. (See table 2.)

Mode

The mode is a camera command parameter which selects the camera scan step size and the choice between a single or triple scan (at each azimuth position). The mode also selects one of four intensity levels of the internal calibration light source. (See table 2.)

Nonnominal modes

Table 2 displays the diode channels and the camera modes. The term nonnominal mode refers to a mismatch between the step size used in a camera event and the instantaneous field of view of the diode channel selected. (See table 1.) It is important to note that high-resolution diodes (BB1, BB2, BB3, and BB4) used with a step size of 0.12° will cause the elvation of the image to be increased by approximately 5.6°. A low-resolution diode used with a step size of 0.04° will cause the elevation of the image to be decreased by approximately 5.6°.

The term nonnominal mode is also sometimes used to refer to triplet images other than the normal: BLU, GRN, RED, or IR3, IR2, IR1 scanning. However, no such nonnominal triplet scanning was commanded during the primary mission phase.

Offset

There are 32 commandable offsets of equal voltage steps available which can be applied to the sensed radiometric measurements prior to digitization at the camera. The offsets are identified by the numbers 0 to 31. (See figg. 3.)

Gain

The six commandable camera gains are designated by the integers 0 to 5 and represent decreasing amounts of gain (by factors of 2) as the numbers increase from 0. (See fig. 3.)

Data path

Two indicators separated by a slash are used to describe the data transmission path from the cameras to Earth. The first is either REC or RT indicating, respectively, that the data were recorded on the lander for later transmission or transmitted in real time as the camera scanned. The second (UH or SB) indicates whether the data went to the orbiter via the UHF link or directly to Earth via the S-band link.

Scan rate

The camera has two scan rates: 16,000 bps and 250 bps. The parameter relects this rate with an entry of 16K or 250.

Photosensor array temperature

The photosensor array temperature (PSA TEMP) is a value in the range 0 to 63 (DN) which indicates the temperature of the camera's photosensor array. It is expressed in the EDR data as an average over the duration of a CE and may be shown either on a scale of 0 to 63 or converted to degrees Celsius by the following formula:

°C = 1.98DN - 60.95

Solar azimuth and elevation

The azimuth and elevation of the Sun (SOLAR AZ/EL) are expressed in the "local horizon system" wherein the zenith is at 90° elevation and the horizon (perpendicular to the local gravity vector) is at 0° elevation. Azimuth is measured in the clockwise direction (viewed from above) with 0° directed toward the north.

Antisolar azimuth and elevation

The azimuth and elevation of the antisolar vector (ANTI-SOLAR AZ/EL) is given in the CACCS which acquired the image referenced. The direction is that of a vector from the Sun to the camera.

Event time

The day of year and time of the beginning of a camera event are listed on most photographic product labels. Although labeled as GMT, the value indicated is that of UTC. The elements - day, hour, minute, and second - are in the format DDD/HH:MM:SS. On some photographic products, the colons are replaced by periods. The year is not included in the label.

Dust

The capability exists on the camera to direct a discharge of CO₂ across the outer surface of the protective outer window of the camera prior to executing a camera event. This dusting (D) was done several times in the early days of the Primary Mission.

Rescan

There are three different conditions which can result in rescan (R) as follows:

1. Rescan command bit: A rescan command bit can be set for a camera event; this results in the camera rescanning at the stop azimuth for a time (RESCAN.DT) determined by a command stored in the data base of the lander's on-board computer. Such CE's are identified by the appearance of the word RESCAN (or R) in their parameter entries.
2. RTI camera events: If the event duration listed in the lander's real-time imaging tables is longer than the time required to scan the commanded azimuth range, the extra time is consumed in rescanning of the last azimuth position.
3. Recorded camera events: For the recorded CE's, rescanning was used in connection with the assignment of an extra time allocation in recorded imagery to allow for the tape recorder run up and reversal. This generally resulted in less than 12 rescan lines. Most photographic products list the first camera scan line which represents a rescan line and the total number of such rescan lines. The total is calculated by subtracting the line number of the first rescan line from the last rescan line and adding one. This step uses the designated line numbers returned with the data. Transmission errors may have reduced the actual amount of data received in some cases.

Data record

The final imaging data may come from either the system data record, the intermediate system data record, or a composite (COMP) of the two. An entry is made under data record to reflect this source.

Data link

Some photographic products list (under data link) the processing stage, or path, of an image during the development of the EDR data base. This is specified as "RAWEDR" for the final EDR versions of the images.

Scan lines

The total number of vertical scan lines stored in the data system is listed under the heading SCAN LINES or LINES TOTAL. It is determined by dividing the commanded azimuth range by the step size and then adding the total number of rescan lines.

Missing lines and gaps

Camera scan lines not received due to transmission errors appear as black lines in positive photographic products (Pixel value = 0). The total number of missing lines and the number of image gaps they cause are listed on some photographic products.

Average data number value

The average data number value (AVE DN VALUE) is the arithmetic mean of the digital value of all received pixels for an image.

Standard deviation

The standard deviation entry is the standard deviation (STAND DEV) of the distribution of digital values of all received pixels for an image.

EDR tape and file number

The identification number of the VICAR format magnetic tape containing the EDR data for each image is specified on most photographic products. The data file position is included with it. In some cases EDR tape and file number (EDR TAPE/FILE) identification is referred to as the VICAR tape and file number.

Segment

Many images have been divided into segments (SEG) to facilitate film conversion. In certain contexts the entire image may be called a segment, such as for small images not requiring division for film conversion.

Segment azimuth, elevation, and step size

The segment azimuth, elevation, and step size (SEGMENT AZ/EL/STEP SIZE) entry indicates the azimuth and elevation of the upper left corner of a segment of an image (or the entire image if unsegmented). The azimuth is expressed in degrees in the CACCS.

Image Processing Laboratory picture identifier

The Image Processing Laboratory picture (photograph) identifier (IPL PIC ID) is a 15 character date-related descriptor which uniquely identifies a photographic product processed at the JPL Image Processing Laboratory [now the Multimission Image Processing System]. It contains four subfields which are separated by slashes: year, month, day of month, and time of day. Where an image has been subdivided to facilitate film conversion each segment has its own IPL PIC ID.

PDS Navigator Development Team Web Page Curator: Karen Boggs Last modified January 14, 2003.