| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This volume contains portions of the CRISM Derived Data Record
(DDR) Archive, a collection of multiband images from the Compact
Reconnaissance Imaging Spectrometer for Mars on the Mars
Reconnaissance Orbiter spacecraft. Images consist of
information on observation conditions and surface physical
properties at the Mars surface projections of IR and VNIR data
cubes, mapped to the sensor space of non-map-projected data. The
data are stored with PDS labels.
This volume also contains an index file ('imgindx.tab') that
tabulates the contents of the volume, ancillary data files, and
documentation files.
For more information on the contents and organization of the
volume set refer to the aareadme.txt file located in the root
directory of the data volumes.
Parameters
==========
CRISM observing scenarios are constructed using a set of key
variables ('configurations') which include the following. All
are selectable separately for the VNIR and IR detectors. Only a
subset of the configurations represent 'scene' data, as
indicated by the EDR keyword MRO:ACTIVITY_ID. Only scene data that
are aimed at Mars have corresponding DDRs. Only those
configurations that affect a DDR are discussed below:
Image source: Image data may be generated using digitized output
from the detector, or using one of up to seven test patterns.
Only data from the detector may have a corresponding DDR.
Pixel binning: Pixels can be saved unbinned or binned 2x, 5x, or
10x in the spatial direction. No pixel binning in the spectral
direction is supported. Data with any pixel binning
configuration may have a corresponding DDR, but the pixel
binning configuration will affect the dimensionality of the DDR.
Calibration lamps: 4095 levels are commandable in each of two
lamps at each focal plane, and in two lamps in the integrating
sphere. All lamps can be commanded open-loop, meaning that
current is commanded directly. For the integrating sphere only,
closed loop control is available at 4095 settings. For closed
loop control, the setting refers to output from a photodiode
viewing the interior of the integrating sphere; current is
adjusted dynamically to attain the commanded photodiode output.
Note: lamps reach maximum current at open- or closed-loop
settings <4095. Only data for which the calibration lamps are
off may have an accompanying DDR.
Shutter position: Open, closed, or viewing the integrating
sphere. The shutter is actually commandable directly to position 0
through 32. In software, open=3, sphere=17, closed=32. NOTE:
during integration and testing, it was discovered that at
positions <3 the hinge end of the shutter is directly illuminated
and creates scattered light. Position 3 does not cause this
effect, but the other end of the shutter slightly vignettes
incoming light. Only data in which the shutter is open, and at
position 3, may have an accompanying DDR.
Pointing: CRISM has two basic gimbal pointing configurations and
two basic superimposed scan patterns. Pointing can be (1) fixed
(nadir-pointed in the primary science orbit) or (2) dynamic,
tracking a target point on the surface of Mars and taking out
ground track motion. Two types of superimposed scans are
supported: (1) a short, 4-second duration fixed-rate ('EPF-type')
scan which superimposes a constant angular velocity scan on either
of the basic pointing profiles, or (2) a long, minutes-duration
fixed-rate ('target swath-type') scan. Pointing configuration
affects the contents but not the dimensionality of a DDR.
Processing
==========
The CRISM data stream downlinked by the spacecraft unpacks into a
succession of compressed image frames with binary headers
containing housekeeping. In each image, one direction is spatial
and one is spectral. There is one image for the VNIR focal plane
and one image for the IR focal plane. The image from each focal
plane has a header with 220 housekeeping items that contain full
status of the instrument hardware, including data configuration,
lamp and shutter status, gimbal position, a time stamp, and the
target ID and macro within which the frame of data was taken. These
parameters are stored as part of an Experiment Data Record (EDR),
which consists of raw data, or a Targeted Reduced Data Record, or
TRDR, the 'calibrated' equivalent of an EDR.
The data in one EDR or TRDR represent a series of image frames
acquired with a consistent instrument configuration (shutter
position, frame rate, pixel binning, compression, exposure time,
on/off status and setting of different lamps). Each frame has
dimensions of detector columns (spatial samples) and detector
rows (wavelengths, or bands). The multiple image frames are
concatenated, and are formatted into a single multiple-band
image (suffix *.IMG) in one file, plus a detached list file in
which each record has housekeeping information specific to one
frame of the multiple-band image (suffix *.TAB). The multiple-
band image has dimensions of sample, line, and wavelength. The
size of the multiple-band image varies according to the
observation mode but is deterministic given the macro ID. A
typical multiple-band image might have XX pixels in the sample
(cross-track) dimension, YY pixels in the line (along-track)
dimension, and ZZ pixels in the wavelength dimension, where:
XX (samples) = 640/binning, where 640 is the number of columns
read off the detector, and binning is 1, 2, 5, or 10;
YY (lines) = the number of image frames with a consistent
instrument configuration; and
ZZ (bands) = the number of detector rows (wavelengths) whose
read-out values are retained by the instrument.
Once image data are assembled into EDRs and calibrated into TRDRs,
DDRs are created for the data. A version 0 DDR represents values
based on predicted pointing, and is generated to provide
quick-look information. Version 1 and subsequent versions of a
DDR are based on actual, reconstructed pointing.
Each of the planes of a DDR represents some value evaluated at
the surface intercept of Mars shape model, or at the intercept
with a surface parallel to the areoid but having a distance from
the planetary center equal to that of the intercept with the
shape model. The following items, some represented in SPICE
files or 'kernels,' affect the locations of the surface physical
properties encoded in a DDR:
Position of Mars: This is encoded in the planetary ephemeris
kernel.
Position of MRO: This is encoded in the spacecraft ephemeris
kernel.
Orientation of MRO: This is encoded in the spacecraft C kernel.
Orientation of CRISM's gimbal in the MRO reference frame: This
is encoded in the CRISM part of the MRO frames kernel.
Orientation of CRISM's VNIR and IR fields of view relative to
the gimbal. This also is encoded in the CRISM part of the MRO
frames kernel.
Position of CRISM's gimbal within its plane: This is encoded in
a CRISM C kernel. CRISM's C kernel is derived from gimbal
positions at the beginning, middle, and end of the integration
of each line of an EDR or TRDR images file, which is given in
the table of instrument housekeeping that accompanies every EDR
and TRDR.
Position of each spatial pixel in a CRISM image relative to the
center of the field of view: This is encoded in the instrument
kernel.
Mars shape model and areoid: The shape model and areoid used to
construct DDRs is from the Mars Orbiter Laser altimeter, or MOLA,
gridded at 128 pixels per degree [SMITHETAL1999].
TES bolometric albedo and thermal inertia: These are gridded at 8
pixels per degree [MELLONETAL2000].
CRISM has optical distortions such that each wavelength, or
band, has a very slightly different surface projection onto
Mars. Each band corresponds to a row on the VNIR or IR detector.
To avoid ambiguity, a DDR represents the physical information
corresponding to the surface intercepts at a reference detector
row near 610 nm (row number 223) for the VNIR, or near 2300 nm
(row number 257) for the IR. The relationship between different
bands is included in the instrument kernel.
The sequence of processing that creates a DDR is as follows.
EDRs are assembled from raw data. TRDRs are created from the
EDRs and Calibration Data Records, or CDRs, using a calibration
algorithm discussed at length in an Appendix in the CRISM Data
Products SIS. Gimbal positions are extracted from the EDR
housekeeping and formatted as a gimbal C kernel. Using that and
other SPICE kernels discussed above, the surface intercept on
the MOLA shape model is calculated for each spatial pixel
(sample at the reference detector row). The angles of this pixel
relative to the equatorial plane and reference longitude
constitute the latitude and longitude of the pixel. For that
latitude and longitude, solar incidence, emission, and phase
angles are determined at a surface parallel to the areoid but
having a radius from planetary center equivalent to that of the
surface intercept of the shape model. Solar incidence and
emission are also determined relative to the shape model itself.
Version 0 of the DDR is generated using predicted spacecraft
orientation and ephemeris as soon a the CRISM gimbal C kernel is
ready. Version 1 is generated once reconstructed spacecraft
orientation and ephemeris are available.
Other values in the DDR are retrieved from the MOLA and TES data
sets. Using the latitude and longitude of the surface intercept
of each spatial pixel, TES bolometric albedo and thermal inertia
are retrieved from global map products, and resampled into CRISM
sensor space using nearest neighbor resampling. The same
procedure is used to retrieve MOLA elevation, and the local
slope magnitude and slope azimuth of the MOLA elevation model.
CRISM standard data products and the supplementary browse products
are defined and described in greater detail in the Data Products
Software Interface Specification and the Data Archive Software
Interface Specification in the DOCUMENT directory.
Data
====
There is only one data type associated with this volume, the
Derived Data Records or DDRs. There are 14 layers in each DDR,
all represented as 32-bit real numbers arranged in band-sequential
format:
Solar incidence angle relative to areoid, at the same planetary
radius as surface projection of pixel, units degrees.
Emission angle relative to areoid, at the same planetary radius as
surface projection of pixel, units degrees.
Solar phase angle, units degrees.
Areocentric latitude, units degrees N.
Areocentric longitude, units degrees E.
Solar incidence angle relative to planetary surface as estimated
using MOLA shape model, units degrees.
Emission angle relative to planetary surface as estimated using
MOLA shape model, units degrees.
Slope magnitude, using MOLA shape model and reference ellipsoid,
units degrees.
Slope azimuth, using MOLA shape model and reference ellipsoid,
units degrees clockwise from N.
Elevation relative to MOLA datum, units kilometers.
TES thermal inertia, units J m^-2 K^-1 s^-0.5.
TES bolometric albedo, unitless.
Spare.
Spare.
Ancillary Data
==============
There are ancillary data provided with this dataset:
1. SPICE kernels, used to contruct observational geometry, are
available in the GEOMETRY directory. See GEOMINFO.TXT for more
details.
Coordinate System
=================
The cartographic coordinate system used for the CRISM data
products conforms to the IAU planetocentric system with East
longitudes being positive. The IAU2000 reference system for Mars
cartographic coordinates and rotational elements was used for
computing latitude and longitude coordinates.
Media/Format
============
The CRISM archive will be made available online via Web and FTP
servers. This will be the primary means of distribution.
Therefore the archive will be organized as a set of virtual
volumes, with each data set stored online as a single volume. As
new data products are released they will be added to the volume's
data directory, and the volume's index table will be updated
accordingly. The size of the volume will not be limited by the
capacity of the physical media on which it is stored; hence the
term virtual volume. When it is necessary to transfer all or part
of a data set to other media such as DVD for distribution or for
offline storage, the virtual volume's contents will be written to
the other media according to PDS policy, possibly dividing the
contents among several physical volumes.
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
The major sources of uncertainty in DDRs arise from uncertainties
in instrument pointing knowledge, from coverage of the MOLA data
set, and from the scale of the TES data.
The formal pointing uncertainty for the CRISM gimbal plane is 1
mrad each in the spacecraft yaw(z), roll(x), or pitch/gimbal(y)
axes. The formal uncertainty in reconstructed spacecraft
attitude is similar. Uncertainty in CRISM's gimbal attitude is
negligible, about 0.006 mrad. The formal error in projection
onto a surface location depends on the angle of the
gimbal and typically is of order several hundreds of meters.
Experience during operations suggests that the actual errors
are smaller than expected formal errors, so that typical error
in surface location is about 200 meters.
Latitude and longitude are described by the intersection of
CRISM field of view with the MOLA shape model. Given the
uncertainties in location of a point on the surface, expected
uncertainty near the equator is of the order of 0.005 degrees.
Uncertainties in incidence, emission, and phase angles relative
to the areoid are similar.
Errors in incidence and emission angle relative to the MOLA
shape model are dominated by the lower sampling density of the
shape model. MOLA points are typically a few hundred meters
apart. This compares to CRISM's sampling scale of 15 to 200
meters per spatial pixel, depending on instrument configuration.
In areas of smooth topography the errors are small, but in areas
with topography that is rough at scales less than a few hundred
meters, uncertainty is several degrees. The same uncertainties
apply to slope magnitude.
Errors in bolometric albedo and thermal inertia will have a
large contribution from the different scales of the CRISM and
TES data sets. The TES data from which these values are
retrieved are sampled at 8 pixels per degree, yet depending on
instrument configuration, the native spatial sampling of the
CRSIM data set is 256-4096 pixels per degree. Thus in areas with
heterogeneous surface properties, large errors in bolometric
albedo and thermal inertia may occur.
DDR Versions
============
Changes in the processing of DDRs are denoted by incrementing the
software version, to preserve the significance of version 0 and
version 1 DDRs representing predicted and reconstructed pointing.
Software version 1.8 was initially used and had the following known
problems:
(1) Latitude, longitude, and elevation were calculated inaccurately
south of 85 degrees south latitude.
(2) Slope magnitude and slope azimuth were calculated inaccurately.
(3) The layers purporting to be incidence and emission angles
relative to the MOLA shape model were identical to
incidence and emission angles relative to the areoid, being used
as a placeholder.
(4) Local solar time was calculated incorrectly.
(5) The TES bolometric albedo and thermal inertia layers were
unpopulated.
Software version 1.9 addressed each of the issues above as follows:
(1) Latitude, longitude, and elevation calculations south of 85
degrees south latitude were fixed by a code change to fix an
incorrect data format to which MOLA polar products were being
assigned.
(2) Slope magnitude and slope azimuth were calculations were
repaired by fixing an arithmetic error.
(3) Incidence and emission angles relative to the MOLA shape model
were derived and correctly populated in the appropriate layers.
(4) Local solar time was calculated correctly by repairing
mathematical errors.
(5) The TES bolometric albedo and thermal inertia layers remain
unpopulated. This will be remedied on upgrade to software version
1.10.
Known Issue with DDR Accuracy
=============================
The only known major problem with DDRs generated using software
version 1.9 is that TES bolometric albedo and thermal inertia
layers remain unpopulated.
Review
======
This archival data set will be examined by a peer review panel
prior to its acceptance by the Planetary Data System (PDS). The
peer review will be conducted in accordance with PDS procedures.
Data Coverage and Quality
=========================
For each observation, every EDR is compared against frame-by-frame
predictions of commanded instrument state. The results of the
comparison are written as a data validation report that
accompanies the EDRs for that observation.
In the case of a hardware or configuration discrepancy (shutter
position, lamp status or level, pixel binning, frame rate, channel
selection, power status of detectors), processing of the image
data to RDR level does not occur in order to avoid introducing
invalid results, and DDRs are not created. Also, missing frames or
portions of frames are replaced with a value of 65535 (this cannot
be a valid data value). That portion of the EDR is not further
processed, and it also is propagated to a value of 65535 in all
layers of the DDR.
Only a subset of instrument configurations represent 'scene' data,
as indicated by the EDR keyword MRO:ACTIVITY_ID. Only scene data
aimed at Mars' surface have corresponding DDRs.
Limitations
===========
None.
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