Winds of Mars: Aeolian Activity and Landforms
Appendix
Mariner and Viking Image Nomenclature
Mariner 9 Photographs
Mariner 9 images are identified by their DAS (Data Automation
Subsystem) time number. DAS time is an eight-digit integer incremented
each 1.2 seconds. The DAS time at which the camera was shuttered
is listed in the main data file, the SEDR (Supplementary Experiment
Data Record). The final processed photographs, the RDR (Reduced
Data Record) version, use a DAS number that is five counts larger
than that in the SEDR, representing the DAS time of the arrival
of the first line of data.
Viking
Orbiter Photographs
A much simpler system is used to identify the Viking orbiter
photographs. Each frame has a unique picture number with the
format mmmxyy, where mmm denotes the orbit number and yy denotes
the number in the photo sequence taken in that orbit. X can
be A, B, or S: A denotes Viking 1 orbiter, B denotes Viking
2 orbiter, and S denotes Viking 1 orbiter for orbits greater
than 999. (The Viking spacecraft were designed for only a 90-day
lifetime! Both orbiters eventually ran out of altitude control
gas and both landers died a natural death. VL-1 was the last
to go, hanging on until November 1982, completing over 75 months
of service on the surface of Mars.)
Two
processed versions of Viking images are in common use, SCR2
and NGF. The raw image often lacked contrast, so the SCR2 (Shading
Corrected) version stretched the image contrast to make the
darkest thing in the image completely black and the brightest
thing in the image completely white, with the intermediate brightness
values stretched accordingly. The other common version, NGF
(Non Gradient, high-pass Filtered), used what is called an asymmetric
boxcar filter for image processing. Two projections were used:
rectilinear (as seen by the spacecraft) and orthographic (a
geometrically processed image of the scene as it would appear
from directly overhead). Viking pictures would be taken using
an array of filters: clear, red, green, blue, violet, and minus-blue.
Minus-blue is a filter that passes all but blue light; the other
filters pass only the color indicated. Color images can be constructed
by combining black-and-white images taken with different color
filters, usually red, green, and blue.
For
more information on Mariner 9 photographs, see Mutch et
al (1976); for more on Viking orbiter photographs, see
Carr (1981).
Viking
Lander Photographs
The Viking lander photographs are uniquely identified by a number
with the following format: abxmmm/nnnn. The mission number is
designated by a = 1 for VL-1 or a = 2 for VL-2. There are two
cameras on each lander, designated by the b number. The frame
number is designated by the xmmm character string, with x being
a letter and mmm being a number. The framing sequence, regardless
of the camera used, starts at AOOO and counts sequentially to
A255, then the next frame is BOOO and so on. The nnnn number
is the sol on which the photograph was taken.
Viking
Lander Images Processed at Washington University in St. Louis
The high-resolution, color-enhanced slides of the martian surface
(slides #27, #28,
and #30) were made by combining
high-resolution black-and-white images with lower-resolution
three-channel (red, green, and blue) images. Care was taken
in image selection to ensure similar illumination geometry between
images.
The
color triplets were processed as follows. The first step was
to remove random noise spikes from the image, then expand them
in size by a factor of three to match the high-resolution frame
scale. The three-channel image was then converted into intensity,
hue, and saturation images (see a basic text on image processing
for a more detailed explanation of the processing described
in this Appendix). In this usage, hue is the peak wavelength,
intensity is the amount of energy at that wavelength, and saturation
is a measure of how much energy is received at wavelengths near
the peak value. The high-resolution images were substituted
for the intensity image derived from the color triplet and subjected
to a linear contrast enhancement (similar to the SCR2 stretch
for Viking orbiter photographs). Linear contrast enhancement
was performed on the saturation image and a piecewise contrast
enhancement was performed on the hue image. The last step was
to convert back into red, green, and blue images and to generate
a color composite image. The resulting image provides a detailed
color portrayal of the surface, and one in which colors can
be enhanced to show variations from place to place.
The
designated slides in this set were produced at the Earth and
Planetary Remote Sensing Laboratory, Department of Earth and
Planetary Sciences, McDonnell Center for the Space Sciences,
Washington University, St. Louis, headed by Dr. Raymond Arvidson.
They were generously made available by Mary Ann Dale-Bannister.
My thanks to all involved in their preparation.