Edward A. Guinness
Department of Earth and Planetary Sciences
McDonnell Center for the Space Sciences
Washington University
St. Louis, Missouri 63130
The two Viking Lander spacecraft were the first spacecraft
to operate successfully for an extended period of time on the
surface of Mars. Both spacecraft operated from 1976 through
April 1980 and Viking Lander 1 (VL1) continued to operate until
November 1982. In 1981, Viking Lander 1 was renamed the Thomas
A. Mutch Memorial Station in honor of Tim Mutch, Lander Imaging
Team Leader and later NASA Associate Administrator for Space
Sciences. Tim Mutch was killed in a climbing accident in 1980.
With respect, the Mutch Memorial Station will be referred to as
VL1 throughout most of this document to be consistent with other
documentation and labels in this archive. The primary scientific
objective of the lander mission was to test for the presence of
life on Mars. Secondary objectives were embodied in the
seismology, meteorology, inorganic chemical analysis, imaging,
magnetic, and physical properties investigations. Among the
instruments aboard the landers was an imaging system that
consisted of two identical cameras. These cameras operated
throughout the mission and returned nearly 6600 images. This
archive on two compact disks (CD-ROM) includes the Experiment
Data Record (EDR) version of all available images acquired on
Mars by the Viking Lander imaging systems.
This EDR dataset is the primary record of lander image data
as it was received on Earth. EDR images were originally
distributed to lander imaging team members and to the National
Space Science Data Center (NSSDC) as photoproducts and digitally
on 9-track magnetic tapes. This CD-ROM archive was produced by
the Geosciences Discipline Node of NASA's Planetary Data System
(PDS) to transfer the EDR image dataset to more stable media and
to provide wide distribution of the dataset to interested
scientific research organizations and universities. The images
in this archive have not been processed in any form other than
generating and attaching PDS labels and histograms to the
original EDR data. This archive also includes documentation
about the imaging system and radiometric and geometric
calibration of the image data; browse versions of the EDR images;
and metadata about the images in a format suitable for loading
into spreadsheet or data base management programs. Derived
Viking Lander image datasets will be included in future PDS
archives.
The following material provides the primary documentation of the EDR
image archive. Included in this document are sections on: A) the
Viking Mission; B) the Viking Lander spacecraft; C) the lander imaging
system; D) the processes for generating the original EDR images and
this archive; E) the volume and PDS image file structure; F) image
calibration procedures; and G) the major types of imaging sequences.
Other documentation files are included elsewhere in this archive.
Detailed explanations of file formats are found in the CDVOLSIS.TXT file. Full radiometric and
geometric calibration information are found in the CALINFO.TXT and GEOMINFO.TXT files, respectively. The
ERRATA.TXT file contains a list of
comments and anomalies about the archive. Finally, the AAREADME.TXT file in the top-level directory
has an introduction to the archive and a description of the archive
structure.
The Viking Mission consisted of four spacecraft: two identical
orbiters and two identical landers [Soffen, 1977]. The landers were
attached to the orbiters during cruise to Mars. After orbit insertion
around Mars and landing site selection, the landers separated from the
orbiters for descent to the surface and a soft landing. The primary
scientific objective of the mission was to search for life on Mars.
Other science questions addressed by the mission were the composition
and physical properties of the atmosphere, the distribution of
atmospheric water vapor, global and local meteorology, the composition
and physical properties of the surface, nature of Mars seismicity, and
gravity field of Mars. The combined orbiter and lander spacecraft
supported thirteen science investigations. The orbiters had three
mapping experiments (imaging, infrared thermal mapper, and water vapor
mapper). An atmospheric investigation was conducted during lander
descent to the surface. The landers had eight scientific experiments
(see Section 3). In addition, a series of radio
science investigations were done using both the lander and orbiter
radio systems [Snyder, 1977; Snyder and Moroz, 1992].
The Viking 1 and 2 spacecraft were launched on August 20 and
September 9, 1975, respectively, and the cruise to Mars phase
lasted 10 to 11 months. Below is a table showing major events of
the mission.
Viking Lander 1 (VL1) landed at 22.480 deg N latitude and 47.968
deg W longitude planetographic [Kieffer et al., 1992; Michael,
1979] in the western portion of Chryse Planitia, which is a
region of smooth plains and impact craters. Viking Lander 2
(VL2) landed at 47.967 deg N latitude and 225.737 deg W longitude
planetographic [Kieffer et al., 1992; Michael, 1979] about 200 km
west of the Crater Mie in Utopia Planitia, which consists of
plains with broad swales and bulges [Moore et al., 1987].
The four spacecraft operated independently after lander
separation. The orbiters served as communication relay stations
to transfer data from lander to orbiter, and then on to Earth,
but data were also transferred directly from the lander to Earth.
Each spacecraft operated for a number of years and long after
their primary missions were completed. Viking Orbiter 2 (VO2)
was the first of the four spacecraft to end its mission. It
developed a leak in its propulsion system and lost its attitude
control gas. VO2 was turned off after 706 orbits around Mars.
Viking Orbiter 1 (VO1) consumed the last of its attitude control
gas and was turned off after 1485 orbits around Mars. VL2
operated on the surface for 1281 Mars days and was turned off
when its batteries failed. VL1 operated the longest of the four
Viking spacecraft. It returned data for nearly 6.5 Earth years
(2252 Mars days or over 3 Mars years). Its mission ended when
communications with the spacecraft were lost [Arvidson et al.,
1983].
The Viking Landers were identical to each other and had the
same instrument packages. The main lander structure was a
hexagonal prism body that housed the spacecraft computers, tape
recorder, batteries, several science instruments, and controls
for the surface sampler, thermal system, and data handling
system. The spacecraft body was supported above the surface by
three legs, each with a saucer-shaped footpad. The legs were
arranged in a triangle pattern with two at the front of the
lander and one at the rear. Mounted to the sides of the
spacecraft were three terminal descent engines, two propellant
tanks, and the extendible surface sampler arm with a collector
head and backhoe [Moore et al., 1987]. Also mounted on the
spacecraft body were two cameras, two radioisotope thermoelectric
generators (RTG) with covers, sample entry ports for the biology,
organic chemistry and inorganic chemistry instruments, the
seismometer, the meteorology boom, a magnifying mirror, a magnet,
and three imaging reference test charts. The spacecraft had
three antennas for communications; a high-gain S-band antenna
(large dish antenna), a low-gain S-band antenna, and a UHF
antenna. The two S-band antennas were used to communicate with
Earth, whereas the UHF antenna communicated with the orbiters.
Each lander was about 1.5 m across. Nominal clearance
between the spacecraft body and the surface was about 22 cm.
After landing the spacecraft mass was about 610 kg. Power for
the spacecraft was supplied by rechargeable batteries; the
batteries were recharged by the two RTGs. Excess heat from the
RTGs was used to heat the instruments and control systems in the
spacecraft body [Soffen, 1977].
Lander science investigations employed seven instruments in
addition to the cameras. The meteorology instrument was mounted
on a mast and was capable of measuring atmospheric temperature,
pressure, wind speed, and wind direction. Each lander had a
three-axis, short-period seismometer to measure Mars seismic
activity. The seismometer on VL1 failed to deploy and no data
were returned, while the one on VL2 operated as planned. An X-ray
fluorescence spectrometer (XRFS) measured the inorganic
elemental composition of soils at the landing sites. A gas
chromatograph mass spectrometer (GCMS) measured the composition
of the atmosphere and searched for organic compounds in the
soils. The biology investigation consisted of three experiments
(and instruments) to search for biological metabolism, growth, or
photosynthesis: carbon assimilation, labeled release of carbon-14,
and gas exchange. The physical properties investigation used
information from many lander operations, such as sampling
activities, digging trenches, pushing rocks, forming soil piles,
and footpad penetration during landing to characterize the
properties of rocks and soils at the landing sites. The magnetic
properties investigation employed two magnets on the backhoe of
the sampler collector head and one magnet array on the center
reference test chart on the lander body [Hargraves et al., 1979].
The reference test charts mounted on the lander also contained
two patches coated with paint that degraded when exposed to
ultraviolet radiation. Finally, lander communication systems
were used for radio science experiments [Snyder and Moroz, 1992].
The lander mission can be divided into a number of periods
(mission phases) in terms of activity level and the types of
observations. The table below lists the lander mission phases
and their dates. Snyder [1979] provides a narrative of the
activities for most of these mission phases. The time frames for
these mission phases apply to both Viking Landers.
The Primary Mission began with VL1 landing and continued
until the solar conjunction in November, 1976. The Primary
Mission had a high-level of activity and was highlighted by the
collection and analysis of soil samples and characterization of
the landing site and atmosphere. The Extended Mission began
after the solar conjunction with a lower activity level due to a
smaller staff. However, the Extended Mission provided an
opportunity to monitor the surface of Mars through a complete
Mars year and to perform a variety of experiments that were not
possible during the Primary Mission. During the Extended
Mission, additional soil samples were collected for biological
and chemical analyses. Three deep holes (ranging from 8 to 23 cm
in depth) were dug at both landing sites. The physical
properties investigation executed many experiments with the
surface sampler system [Moore et al., 1987]. The imaging
experiment studied the surface and atmosphere through the cycle
of Mars seasons.
During the first Mars winter of the Extended Mission, VL2
was programmed to operate in an automatic manner designed to
allow the spacecraft to survive the cold winter temperatures and
still return data (mainly imaging, meteorology, and seismology
data). VL1 continued to operate normally during the winter due
to its more equatorial location. VL2 returned to full operation
after the winter passed. A major limitation was placed on VL2
when a portion of its communication system failed in the Extended
Mission, leaving VL2 with no direct downlink to Earth.
At the end of the Extended Mission, all lander instruments
were turned off except for imaging, meteorology, and XRFS. These
three instruments continued to operate during the Continuation
Mission in a fully automated manner. Observation sequences
collected data on a 37 sol (Mars day) cycle that repeated
throughout the Continuation Mission. There was an Interim Period
after the Continuation Mission ended where communications with
the Viking spacecraft were severely limited because of Voyager
encounters with Jupiter. During the Interim Period, imaging and
meteorology data were collected and returned when possible. A
final VL2 surface sampler sequence was conducted during this
period as an engineering test in the cold temperatures of mid
winter.
The final phases of the lander mission were the Survey and
Completion Missions. The plan was to collect image and
meteorology data for as long as possible. For VL2 this meant for
as long as VO1 provided a relay link because VL2 no longer had a
direct downlink capability. Relay opportunities with VO1
occurred every seven weeks during the Survey Mission. VL2 was
finally turned off after its batteries could no longer hold a
charge. VL1 operated in a cyclic and automatic mode by returning
data about once a week with the image sequences repeating every
37 Mars days. The VL1 high-gain antenna was programmed to track
the Earth until December, 1994. However, communications with the
Mutch Memorial Station (VL1) were lost in November, 1982 after a
command sequence uplink, and thus, ended the Viking Lander
mission [Arvidson et al., 1983].
The Viking Lander camera design was very different from
vidicon framing or CCD array cameras. The lander camera was a
facsimile camera with a single, stationary photosensor array
(PSA), and azimuth and elevation scanning mechanisms. A lander
image was generated by scanning the scene in two directions
(elevation and azimuth) to focus light onto the photosensor
array. The Viking Lander cameras were built by Itek Corp. A
number of published papers described the characteristics and
performance of the lander cameras. The scientific rationale and
early design of the cameras were described in Mutch et al. [1972]
and a detailed description of the flight cameras was given in
Huck et al. [1975b]. Huck and Wall [1976] discussed image
quality and Patterson et al. [1977] described camera performance
during the Primary Mission. A summary of the information from
these papers is given here as a high-level description of the
camera and its operating modes.
Each camera was bolted to the top of the lander body. The
camera had a lower stationary section containing electronics and
an upper section that rotated in azimuth (i.e., around a vertical
axis). The other prominent exterior component was a post
assembly that protected the camera window from the Mars
environment when the camera was not in use. Major internal
camera components were two fused silica windows, a mirror,
elevation and azimuth rotation assemblies, a lens, the
photosensor array, electronics, and pinlights for calibration.
The overall camera height was 55.6 cm. The lower assembly
diameter was 25.6 cm and the upper assembly diameter was 14.4 cm.
The mass of the camera was 7.26 kg. Below is a table that lists
the serial numbers of the cameras and photosensor arrays as they
were assigned to the flight cameras [Huck et al., 1975a].
The camera had several features to deal with possible dust
abrasion or obscuration. When the camera was not in use, the
upper assembly was rotated so that the window was behind the post
assembly to prevent exposure to dust. Mounted in the post was a
nozzle for releasing carbon dioxide gas to blow dust off the
window. In addition, the outer window, known as the
contamination cover, was designed to move out of the optical path
if it became abraded or dust coated. The contamination cover
window was hinged and spring loaded so that it could be moved
aside by rotating the camera behind the post with enough force to
release the spring lever. During the Extended Mission the
contamination cover windows of camera 1 on lander 1 and camera 2
on lander 2 were opened. One drawback of the contamination cover
window was that its frame caused a vignetting effect at elevation
angles above 25 deg. [Patterson et al., 1977].
Light entered the camera through the windows, reflected off
the mirror toward the lens, passed through the lens, and was
sensed by one of the photodiodes. The light generated a voltage
in the selected photodiode that was digitized by an analog-to-digital
(A/D) converter. Below the lens was a black shutter that
could be closed to sample photodiode dark current and to perform
internal radiometric calibration. Between the lens and
photosensor array was a light baffle to minimize internal
reflections. The lens had a 0.95 cm aperture diameter and 5.37
cm focal length. For details on the transmittance and
reflectivity of the optical components, see the documentation and
data in the CALIB directory.
Mechanical scanning was done by rotating the mirror around a
horizontal axis to scan each vertical line in an image. The
entire upper camera assembly rotated around a vertical axis to
scan successive vertical lines. Image data were collected while
the mirror rotated upward through the elevation range in 512
steps (i.e., pixels were sampled from the bottom of the image to
the top). The mirror rapidly rotated down to the start position
while the camera turned in azimuth for the next scan. The mirror
could rotate from 60 deg below a plane perpendicular to the
azimuth rotation axis to 40 deg above it for a total range of 100
deg. The camera could see 342.5 deg in azimuth with the range
limited by the post assembly. Image commands specified both a
start and stop azimuth so that the widths of images varied from
image to image. On receiving a command to acquire an image, the
camera first rotated to a preset azimuth and then rotated to the
start azimuth. The camera stepped in azimuth after each vertical
scan until the stop azimuth was reached. The sequence ended by
the camera moving back to the preset azimuth position and then to
the park position with the window behind the post. The preset
position at the beginning and end of the sequence prevented the
camera from turning through mechanical stops. Internal
radiometric calibration could be done by closing the shutter
while the camera was in the preset position either before or
after the image data were acquired. The camera had two scanning
rates so that data could be collected at 250 or 16,000 bits/sec.
The photosensor array consisted of 12 silicon photodiodes (or diodes)
sensitive to light between 0.4 and 1.1 micrometers. The diodes were
arranged in a 2x6 array. There were four broad band, high resolution
(0.04 deg angular resolution) diodes, known as BB1, BB2, BB3, and BB4.
The distance between the lens and each high resolution diode was
different to vary the in-focus distance of each diode. In-focus
distances were 1.9, 3.7, 4.5, and 13.3 meters for BB1, BB2, BB3, an
BB4, respectively. There was a low resolution (0.12 deg angular
resolution), broad band diode known as the SURVEY diode. There were
also six narrow band, low resolution diodes for color (BLUE, GREEN,
and RED) and infrared (IR1, IR2, and IR3) images. The narrow
bandwidth was generated by covering diodes with a set of interference
filters, chosen to survive the spacecraft sterilization process. The
interference filters had significant out-of-band leaks (see CALINFO.TXT). Also, the infrared
filters were known to degrade due to neutron radiation from the lander
RTGs [Patterson et al., 1977]. The twelfth diode was also a low
resolution diode covered by a red filter for imaging the Sun. The
in-focus distance for the low resolution diodes was about 3.7 meters.
Diodes in the camera generated a voltage proportional to
incoming radiance. Each diode, except for the SUN diode, had an
amplifier to enhance the voltage signal. The SUN diode did not
need an amplifier because of the strong signal when directly
viewing the Sun. Voltage from the amplifier was digitized as a 6-bit
number in an A/D converter that also performed automatic dark
current subtraction. Dark current was sampled after every line
in slow scan mode and after every 64th line in rapid scan mode.
The A/D converter had 6 gains and 32 offsets so that the full
dynamic range of the diode output could be stored in 6-bits.
Gain defined the voltage range sampled and its resolution. Low
gain (high gain number) covered a wide range in voltage and had
low voltage resolution. The offset could be varied from a
slightly negative voltage to several volts in 32 small steps.
Digital values from the A/D converter could be dumped to the
spacecraft tape recorder, transmitted to Earth, or transmitted to
an orbiter in real-time (usually done at beginning or end of a
transmission link when the bit error rate was relatively high).
Each camera had two pinlights in the post assembly. Using a
special command, the lights turned on and an image was acquired
while viewing the pinlights. This command, known as a scan
verification, monitored the mechanical scanning operation of the
camera. A third pinlight with four different radiance levels was
located between the dark current shutter and the photosensor
array. This pinlight was used with the shutter closed for
radiometric calibration of the camera. Scan verification and
internal calibration sequences are further discussed in the
CALINFO.TXT file.
Operation of Viking Lander cameras was versatile because
many parameters for the image could be specified. Commands to
acquire an image involved selecting the sampling mode, diode,
start and stop azimuths, center elevation, gain and offset, scan
rate, and specifying whether automatic dark current subtraction,
internal calibration, rescan, or dusting were done. Image start
time and whether the image was transmitted in real-time or sent
to the tape recorder were also specified. The camera had seven
sampling modes: low resolution (0.12 deg) stepping with three
diodes (known as triplet mode); low resolution stepping with one
diode; high resolution (0.04 deg) stepping with one diode; and
four different calibration lamp intensity levels. In triplet
mode, elevation scanning was repeated three times with three
different diodes at every azimuth position. An unusual sampling
mode effect was an elevation shift when the step size did not
match the diode resolution. There was a -5.6 deg elevation shift
when using a low resolution diode with high resolution stepping
and a +5.6 deg shift when using a high resolution diode with low
resolution stepping. These non-nominal modes were mostly used to
generate high resolution color images or to better resolve the
solar disk. For the triplet mode, the specified diode was the
first of the three diodes, e.g., BLUE diode for a color image.
Start and stop azimuths had to be multiples of 2.5 deg. Center
elevation had to be a multiple of 10 deg. The elevation range
was determined by the step size with a range of about 20 deg for
high resolution and about 60 deg for low resolution because there
were always 512 steps in an elevation scan. Rescan was done by
inhibiting the azimuth rotation and repeatedly scanning in
elevation. Rescan could be activated by commanding the camera to
operate for a time longer than it took to scan the azimuth range
of the image.
The original EDR dataset was produced by the Viking Project
and Lander Imaging Team. Levinthal et al. [1977a] explains the
processing done to produce the original EDR images. Initial EDR
processing included selecting and merging the best available data
recorded at one or more DSN stations. After merging several
playbacks, some imaging data were still not extracted from the
telemetry stream because of noise in the data identification
tags. Thus, special purpose software searched the telemetry
stream to identify and extract imaging data with noisy tags. In
some cases, this secondary processing greatly increased the
amount of data recovered from the telemetry stream. Even with
the secondary recovery, the EDR data contained random noise and
missing lines. Random noise was usually due to transmission
errors. Missing scan lines were filled with zero valued pixels.
Missing data appear as black vertical lines in the image because
the primary scan direction was vertical. As noted in the
previous section, the camera voltage signal was digitized on the
spacecraft to a 6-bit integer number (possible values between 0
and 63). However, the maximum brightness value output from the
camera was 62 due to limitations of the system. As part of the
standard EDR processing, the original 6-bit value was converted
to an 8-bit integer number by multiplying the 6-bit values by 4.
These 8-bit values are the data archived in this dataset.
Several steps were taken to insure that the best available
image data and label information were included in this archive.
The primary source of digital image data was the Viking Lander
Imaging Team set of EDR 9-track magnetic tapes at Washington
University. The data from these tapes were transferred to 8-mm
tape and then to CD-WO before this archive was made. Because the
original EDR files did not contain histogram or checksum values,
automated data validation could not be done. Instead, each
digital image was visually inspected and compared to a
photoproduct version of the image. This check insured that the
file contained the correct image data and that there were no
obvious errors, such as missing or corrupted data. In a few
cases errors were detected and a replacement image was obtained
from the EDR set at the Jet Propulsion Laboratory (JPL).
The original EDR image files were stored in a VICAR format with
metadata attached in a VICAR label. These metadata were used to
populate the PDS label. Label parameters were checked for acceptable
values and corrected if necessary. Remaining anomalies were
documented in the ERRATA.TXT file.
Catalogs [Tucker, 1981; Jones et al., 1981; Wall and Ashmore, 1985]
and planning notes produced by the Lander Imaging Team were used to
determine the purpose for acquiring a given image. This information
was added to the PDS label as a NOTE field. PDS image files for this
archive were generated by extracting label information from the VICAR
header, computing the histogram and checksum for the data, and then
writing the label, histogram, and image data in a PDS format. Note
that the index table was also generated by extracting information from
the original VICAR headers, so that the PDS labels and index table
contain the same information.
The volume and directory structure of this CD-ROM conforms to the
ISO-9660 standard, except that files do not contain any Extended
Attribute Records (XAR). The directory content, file format, and
supporting documentation conform to PDS standards version 3.2 [PDS,
1995]. The AAREADME.TXT file in the
top-level directory has an outline of the archive directory structure.
File names within this archive conform to the "8.3" convention.
EDR images, browse images, supplemental files, and
documentation are located in separate directories. Directories
with ancillary files and data are as follows: The CATALOG
directory contains PDS high-level catalog information about the
Viking Mission, Viking Lander spacecraft and cameras, and the EDR
image dataset. The DOCUMENT directory contains documentation for
the EDR dataset and the archive. The INDEX directory contains
index tables about images from both landers. The CALIB directory
contains documentation and data for radiometric calibration of
Viking Lander images. The GEOM directory contains documentation
and figures that describe the geometric aspects of the Viking
landers and cameras. The BROWSE directory contains HTML files
and browse images for use as a simple image browser.
EDR image files are subdivided into directories based on the
image PRODUCT_ID. Image file names are also based on the
PRODUCT_ID. For Viking Lander images, the PRODUCT_ID has the
form LCXSSS-FFF. L is the lander number (1 or 2) and C is the
camera number (1 or 2). The sequence XSSS is an image identifier
where X is a letter and SSS is a number from 000 to 255. The
initial value of this identifier was A001 for both landers. The
number portion of the identifier was incremented by 1 for each
image. However, the number could not exceed 255 because of
spacecraft software. Thus, the letter portion was incremented
and the number was reset to 000 after 255 was reached. For
example, image identifier A255 was followed by B000. The
sequence FFF in the PRODUCT_ID is an abbreviation for the filter
(diode) name. EDR images are grouped in directories by the image
identifier portion such that all images with the same first two
characters of the identifier are in the same directory. For
example, the directory B1XX has files with image identifiers
between B100 and B199. This scheme was selected to have a
reasonable number of files in each directory and to keep the
images in chronological order. Image file names have the form
LCXSSS.FFF where the components are the same as for the
PRODUCT_ID.
Browse images are also subdivided into directories under the
IMAGE subdirectory in the BROWSE directory. The scheme for
grouping the browse images is the same as for the EDR image
files. Names of browse image files have the pattern of
LCXSSSFF.GIF, where the first six characters are the same as in
the image PRODUCT_ID and FF is the first and last character of
the filter name from of the PRODUCT_ID. This file naming scheme
was adopted to: 1) maintain the "8.3" file name standard; 2)
have the ".GIF" extension so that software will recognize the GIF
format; and 3) have unique file names.
The structure of the PDS image file will be briefly described here.
Additional details about the PDS image file can be found in the CDVOLSIS.TXT file. Each PDS image
file in this archive has three components, the PDS label, the image
histogram, and the image data. The file can be considered as having
fixed length records even though there are no record delimiters per se
in the file. The record size is determined by the number of samples
in the image. If the PDS label or the histogram do not completely
fill a record, they are padded with zero valued bytes to maintain the
fixed length records. Thus, each file object starts on a record
boundary.
The PDS label is the first object in the file. It has a
"keyword=value" format [PDS, 1995] with sections that contain the file
size and object locations, metadata about the images, and descriptions
of the histogram and image objects. Each "keyword=value" pair in the
label is delimited by a carriage control (ASCII 13) and line feed
(ASCII 10) character pair. Keyword values are either numbers with
optional units in brackets, standard values, or text strings. Double
quotes are used to enclose text strings for keyword values. The CDVOLSIS.TXT file contains an
example label and definitions for each of the label keywords.
The image histogram object is the second component in the
file. It begins at the record specified by the ^IMAGE_HISTOGRAM
keyword. The histogram consists of 256 numbers, each a 32-bit
signed integer in binary format with most-significant byte first
order. The value of the first item in the histogram is the
number of image pixels with brightness value 0. The second item
is the number of pixels with brightness value 1, and so on until
the last item, which has the number of pixels with brightness
value 255.
The third object in the file is the image data. The image
object starts at the record specified by the ^IMAGE keyword.
Each EDR image contains 512 lines. The number of samples in each
image varies from image to image because the start and stop
azimuth could be commanded independently. Each pixel value is an
8-bit unsigned integer. The value of the CHECKSUM keyword in the
PDS label is the sum of all pixel values in the image.
Browse images were generated from the PDS images and were stored in
GIF format. Images were reduced in size by a factor of four in lines
and samples. Resampling was done by skipping pixels. Browse images
were also contrast enhanced for better viewing of the data. An image
browser is provided with this archive that uses HTML for easy and
rapid image viewing. The file BROWINFO.TXT in the BROWSE directory
has more information about the browser.
Full quantitative use of Viking Lander image data requires
an understanding of the radiometric and geometric properties of
the Viking Lander cameras. EDR pixel values can be converted to
radiance or reflectance at the sensor using the calibration data
provided with this archive. Topographic data can be extracted by
using the stereoscopic capabilities of the two cameras.
Radiometric and geometric properties of Viking Lander cameras are
described in detail in the documents and data found in the CALIB
and GEOM directories. These calibration procedures will be
briefly outlined here.
Radiometric calibration of a Viking Lander image is basically a linear
scaling of pixel values that is dependent on the diode, gain and
offset, and time (i.e., Mars-Sun distance). These scaling parameters
can easily be computed using the equations given in the CALINFO.TXT file and data provided in
the CALIB directory. Additional corrections can be made for
atmospheric effects. The radiometric calibration data in the CALIB
directory are based on pre-flight testing. In-flight calibration
measurements (internal calibration images) are included in this
archive because they are part of the EDR dataset. However, analysis
of internal calibration images is not included in this archive.
Internal calibration images are described in the CALINFO.TXT file.
The GEOM directory contains the GEOMINFO.TXT document that describes
the location and orientation of the landers. The document also
explains camera coordinate systems and how to extract the
three-dimensional location of objects with stereo images.
Three-dimensional measurements can be used to construct topographic
maps [Liebes, 1982]. Such data can also be used to determine
distances between points in the scene; the size, shape, and
orientation of objects [Moore et al., 1987]; and emission and phase
angles for objects. Given image coordinates of the same point seen in
both camera 1 and 2 images (i.e., conjugate images), the equations in
the GEOMINFO.TXT file can be used
to compute the location of the point. These equations can be easily
programmed in a spreadsheet or using one of many programming
languages.
1. Introduction
2. Viking Mission
Orbiter 1
Lander 1
Orbiter 2
Lander 2
Launch
8/20/75
8/20/75
9/9/75
9/9/75
Orbit Insertion
6/19/76
6/19/76
8/7/76
8/7/76
Landing
7/20/76
9/3/76
Transmissions End
8/7/80
11/13/82
7/25/78
4/11/80
3. Viking Lander Spacecraft
Mission Phase
Start
End
Primary Mission
7/20/76
11/15/76
Extended Mission
11/15/76
5/31/78
Continuation Mission
5/25/78
2/26/79
Interim Period
2/26/79
7/19/79
Survey Mission
7/19/79
8/7/80
Completion Mission
8/7/80
11/19/82
4. Lander Camera System
Camera
Serial Number
PSA Serial Number
Viking 1, camera 1
FC-1B
M017
Viking 1, camera 2
FC-2A
M020
Viking 2, camera 1
FC-3A
M015
Viking 2, camera 2
SPARE
M019
5. EDR Image Generation
6. Archive Data Preparation
7. Disk Directory Structure
8. PDS Image and Browse Image File Formats
9. Radiometric and Geometric Calibration