Explanation of Ancillary Data for MOC Images
The Planetary Data System (PDS) archive product created by Malin Space
Science Systems includes not only the images acquired by the MOC narrow-angle
and wide-angle cameras, but also ancillary data that describe
the images and enable scientific analysis. The data can be grouped into two
categories: acquisition parameters and derived values. Suppose you are
photographing a friend. The speed of the film that you load into the camera,
and the shutter speed and aperture that you choose (or that the camera
chooses for you) represent acquisition parameters. These are known values
specified by the photographer. The brightness of the ambient light, the
direction of illumination, and the distance between you and your friend
are not known to the camera but can be determined by other means. For
example you could use a light meter to measure the ambient light. Along
the same lines, the MOC is commanded with a set of acquisition parameters
that tell it when to start taking a picture, how long to expose each line,
how many lines to acquire, how wide the lines should be, etc. Geometric
information regarding the orientation of the spacecraft with regard to
the planet and so forth is tracked by NASA, JPL, and Lockheed-Martin
Astronautics, who operate the spacecraft. That geometric information is
required in order to create the map projections of the images that you see
on the MSSS MOC Image Gallery. We refer to such data as derived values.
Picture number (picno). Mars Orbiter Camera images are identified by
a 3-character
mission subphase
descriptor, followed by a hyphen, followed by a
5-digit numerical identification.
During the Aerobraking and Science Phasing Orbits portion of the mission
in 1997 and 1998, the 5-digit numerical identifier was based upon orbit
number (defined by periapsis point) and numerical sequence of image
commanded for that orbit. For example, an image identified as AB1-10905
was taken during the Aerobraking-1 Subphase on orbit 109 and it was the 5th
image taken by MOC during that orbit.
During the Mapping Phase of the mission -- including Calibration and
Fixed High Gain subphases and subsequent subphases designated Mxx --
the 5-digit identifier indicates the numerical order in which the image
was commanded during that subphase. For example, an image identified as
M03-00006 was the 6th image commanded during subphase M03.
Image IDs within a mission subphase are in numerical, time-sequential
order based upon the start time for each commanded acquisition. The
numbering scheme does not care which camera is used (narrow angle, red
wide angle, or blue wide angle) at any point within the sequence. Wide
angle context frames are not separately commanded, but rather are
autonomously commanded when the camera is told to acquire a context
frame for a commanded image. Wide angle context frames are given the
ID number immediately following the ID of the commanded image. Gaps
in the numbering sequence (i.e., a skip from image 01002 to
01009) indicate images that were commanded but never received back on
Earth. Loss of images can result from any number of problems,
including those that occur onboard the spacecraft as well as at
receiving stations on Earth.
In the Planetary Data System cumindex.tab file, this datum is called PRODUCT_ID and is found in column 3.
This is the Universal Time (i.e.,Greenwich Time Zone) at which the
first line of the image was acquired by MOC. It is often referred to
as the Spacecraft Event Time, or SCET.
Time is given in Year-Month-Date-hour-minute-second. (YYYY-MM-DD-HH:MM:SS.SS)
In the Planetary Data System cumindex.tab file, this datum is called IMAGE_TIME and is found in column 4.
This is the number of pixels across the raw (not map-projected) image.
In the Planetary Data System cumindex.tab file, this datum is called LINE_SAMPLES and is found in column 7.
This is the number of pixels down the raw (not map-projected) image.
In the Planetary Data System cumindex.tab file, this datum is called LINES and is found in column 8.
This is the amount of time it took for MOC to acquire each line (pixels
across) of the image. "Millisec" stands for "milliseconds." One millisecond
is 0.001 seconds.
In the Planetary Data System cumindex.tab file, this datum is called LINE_EXPOSURE_DURATION and is found in column 30.
This is the ratio of pixel scale down the image to across
the image. An image with an aspect ratio that is not 1.00 is an image
in which the pixel scale is different between the pixels across and
the pixels down. For example, an image in which the pixels across
have a resolution of 1.5 meters per pixel and pixels down have a
resolution of 2.25 meters per pixel has an aspect ratio of 1.5:1, or "1.5".
In the Planetary Data System cumindex.tab file, this datum is called PIXEL_ASPECT_RATIO and is found in column 12.
Images acquired at lower than full-resolution were summed onboard the
spacecraft. For example, an image with intrinsic resolution of 1.5 meters
per pixel that has been summed 4 times will have an actual resolution of
6.0 meters per pixel.
For narrow angle images, the crosstrack and downtrack
summing are always the same. For wide angle images, the crosstrack
and downtrack summing can be commanded to be different from each other.
For global images, the crosstrack summing is variable across the
detector. See the PDS data products for more details on the global
image summing.
In the Planetary Data System cumindex.tab file, this datum is called CROSSTRACK_SUMMING and is found in column 9.
Images acquired at lower than full-resolution were summed onboard the
spacecraft. For example, an image with intrinsic resolution of 1.5 meters
per pixel that has been summed 4 times will have an actual resolution of
6.0 meters per pixel.
For narrow angle images, the crosstrack and downtrack
summing are always the same. For wide angle images, the crosstrack
and downtrack summing can be commanded to be different from each other.
In the Planetary Data System cumindex.tab file, this datum is called DOWNTRACK_SUMMING and is found in column 10.
MOC images can be compressed before being transmitted to Earth. For example,
"NONE" means that the image was not compressed, "PRED" means that the image
was predictively compressed, "DCT" means discrete cosine transform
compression was used, and "WHT" is Walsh-Hadamard transform compression.
Numbers following compression type indicate lossless, lossy, and compression
factors used. Please consult PDS data product for additional information.
In the Planetary Data System cumindex.tab file, this datum is called COMPRESSION_TYPE and is found in column 34.
Exposure of the MOC is limited by the line-time. The MOC is only able
to covert this light into, at most, 256 shades of gray (black to white). "Gain"
is used to take the actual spread of brightnesses and scale it to 256
levels (8 bits). See the PDS products for details on the meaning of gain values
stated here.
In the Planetary Data System cumindex.tab file, this datum is called GAIN_MODE_ID and is found in column 31.
"Offset" is used in concert with "gain." It shifts the maximum number
of grays (256) into the range of the analog-to-digital converter. See PDS
products for details on the meaning of offset values stated here.
In the Planetary Data System cumindex.tab file, this datum is called OFFSET_MODE_ID and is found in column 32.
This is the approximate longitude location of the image center on the planet,
Mars.
In the Planetary Data System cumindex.tab file, this datum is called CENTER_LONGITUDE and is found in column 16.
This is the approximate latitude location of the image center on the planet,
Mars.
In the Planetary Data System cumindex.tab file, this datum is called CENTER_LATITUDE and is found in column 17.
This is the image resolution in meters per pixel at the center of the image.
For most narrow angle images, this value will be approximately the same over the
entire picture. For wide angle images, the pixel scale will vary over the
image.
In the Planetary Data System cumindex.tab file, this datum is called SCALED_PIXEL_WIDTH and is found in column 11.
This is the width of the image from left to right across the center of
the picture, in kilometers.
In the Planetary Data System cumindex.tab file, this datum is called SCALED_IMAGE_WIDTH and is found in column 35.
This is the height of the image from top to bottom across the center of
the picture, in kilometers.
In the Planetary Data System cumindex.tab file, this datum is called SCALED_IMAGE_HEIGHT and is found in column 36.
This is the position of Mars relative to the Sun measured in degrees from
the vernal equinox (start of northern Spring). This number is used as a
measure of martian seasons: Northern Spring/Southern Autumn start at
0°, Northern Summer/Southern Winter start at 90°, Northern
Autumn/Southern Spring start at 180°, and Northern Winter/Southern
Summer begin at 270°.
In the Planetary Data System cumindex.tab file, this datum is called SOLAR_LONGITUDE and is found in column 48.
This is the local time on Mars at the center of the image relative to
a division of the martian day into 24 equal parts. A martian day
is slightly longer than 24 hours and 37 minutes long. A time of 00:00
is "midnight" and "noon" is 12.00. An image with a local true
solar time of 14:00 would be "2 p.m."
In the Planetary Data System cumindex.tab file, this datum is called LOCAL_TIME and is found in column 49.
Measured from the center of the image, this is the angle between the
MOC and a "normal" drawn perpendicular to the planet's surface. In most
cases, the MOC is looking "straight down" and the emission angle is
thus close to 0°.
In the Planetary Data System cumindex.tab file, this datum is called EMISSION_ANGLE and is found in column 13.
Derived for the center of the image, this is the angle between the Sun
and a "normal" drawn perpendicular to the planet's surface at the time
the image was acquired. A higher incidence angle means that a person
standing on the ground would see the sun lower toward the horizon.
In the Planetary Data System cumindex.tab file, this datum is called INCIDENCE_ANGLE and is found in column 14.
This is the angle between the sun, the surface, and the MOC at the time
the picture was obtained.
In the Planetary Data System cumindex.tab file, this datum is called PHASE_ANGLE and is found in column 15.
In a raw or unprocessed MOC image, this is the angle in degrees clockwise
from a line drawn from the center to the right edge of the image to the
direction of the north pole of Mars. This number allows the user to determine
"which way is north/south? and which way is east/west?"
In the Planetary Data System cumindex.tab file, this datum is called NORTH_AZIMUTH and is found in column 41.
In a raw or unprocessed MOC image, this is the angle in degrees clockwise
from a line drawn from the center to the right edge of the image to the
direction of the sun at the time the image was acquired. This number
allows the user to determine "which way is the sun coming from in my
image?"
For images that have been map-projected so that north is to the top of
the frame, the sun azimuth can be determined relative to north by
subtracting the north azimuth from the sun azimuth. The resulting
number (positive clockwise) gives the sun azimuth relative to the top
of the frame.
In the Planetary Data System cumindex.tab file, this datum is called SUB_SOLAR_AZIMUTH and is found in column 42.
This number tells how high the MOC and MGS spacecraft were above the surface
of Mars when the center of the image was acquired.
In the Planetary Data System cumindex.tab file, this datum is called SPACECRAFT_ALTITUDE and is found in column 37.
This number is similar to the spacecraft altitude, but also takes into
account the emission angle...if the emission angle is 0 then this number
is the same as the spacecraft altitude. If the emission angle
is much greater than 0, then the "slant distance" to the surface
at the center of the image is also greater than the spacecraft
altitude.
In the Planetary Data System cumindex.tab file, this datum is called SLANT_DISTANCE and is found in column 39.
