| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This volume contains portions of the CRISM Limb Data Record
(LDR) 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 of IR and VNIR data
cubes pointed at Mars' limb, 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' limb have corresponding LDRs. Only those
configurations that affect an LDR 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 LDR.
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 LDR, but the nominal
binning configuration for limb-pointed observations is 10x.
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 LDR.
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 LDR.
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 relative to the surface of Mars and taking out
ground track motion. A limb observation tracks a point near Mars'
limb along the projection of the groundtrack, and superimposes a
scan relative to that point.
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 data pointed at the surface, and LDRs are
created for data pointed at the limb. For DDRs version 0 represents
values based on predicted pointing, and version 1 is based on
actual, reconstructed pointing. For LDRs both version 0 and 1 are
used but both are based on reconstructed pointing.
Each of the planes of an LDR represents some value evaluated at or
above the surface intercept of Mars shape model, or at or above
the intercept with a surface parallel to the areoid. The following
items, some represented in SPICE files or 'kernels,' affect the
properties encoded in an LDR:
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 and LDRs is from the Mars Orbiter Laser altimeter,
or MOLA, gridded at 128 pixels per degree [SMITHETAL1999].
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 or LDR represents the information
corresponding to 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 or LDR 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, intercepts on the MOLA
shape model or at a tangent height above it, and intercepts at a
tangent height above the areoid are calculated for each spatial
pixel (sample at the reference detector row).
The derived image bands include the following:
1. Latitude. Defined in one of two ways:
(a) If the instrument boresight does not intercept the Martian
areoid: Evaluated at the tangent point of the limb observation;
the intercept with the areoid of a vector originating at the
center of the planet, that is normal to the instrument boresight.
(b) If the instrument boresight does intercept the Martian
areoid: same definition as in the DDR. Evaluated at the intercept
of the instrument boresight with the MOLA shape model.
2. Longitude (planetocentric, east positive). Defined in one of
two ways:
(a) If the instrument boresight does not intercept the Martian
areoid: Evaluated at the tangent point of the limb observation;
the intercept with the areoid of a vector originating at the
center of the planet, that is normal to the instrument boresight.
(b) If the instrument boresight does intercept the Martian areoid:
same definition as in the DDR. Evaluated at the intercept of the
instrument boresight with the MOLA shape model.
3. Incidence angle at the latitude and longitude described above.
Defined in one of two ways:
(a) If the instrument boresight does not intercept the Martian
areoid: the angle between a vector originating at the center of
the planet that is normal to the instrument boresight, and a
vector from the center of the sun to the surface intercept of the
first vector.
(b) If the instrument boresight does intercept the Martian areoid:
the angle the normal to the areoid at the latitude and longitude
described above, and a vector from the latitude and longitude
on the areoid to the center of the sun.
4. Emission angle at the latitude and longitude described above.
Defined in one of two ways:
(a) If the instrument boresight does not intercept the Martian
areoid: the angle between a vector originating at the center of
the planet that is normal to the instrument boresight, and the
instrument boresight. By definition, 90 degrees.
(b) If the instrument boresight does intercept the Martian areoid:
the angle between the instrument boresight and the normal to the
areoid at the latitude and longitude described above.
5. Phase angle at the latitude and longitude described above.
The angle between the instrument boresight and a vector from the
sun to Mars.
6. Altitude relative to the areoid. Defined in one of two ways:
(a) If the instrument boresight does not intercept the Martian
areoid: The length of a segment along a vector originating at
the center of Mars that passes through the latitude and longitude
described above. One end of the segment is the intercept of the
vector with the areoid. The other end is the intercept of the
vector with the instrument boresight.
(b) If the instrument boresight does intercept the Martian areoid:
zero
7. Altitude relative to the MOLA shape model. Defined in one of
two ways:
(a) If the instrument boresight does not intercept the Martian
areoid: The length of a segment along a vector originating at
the center of Mars that passes through the latitude and
longitude described above. One end of the segment is the intercept
of the vector with the MOLA shape. The other end is the intercept
of the vector with the instrument boresight.
(b) If the instrument boresight does intercept the Martian areoid:
zero
CRISM data products 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
Limb Data Records or LDRs. There are 15 layers in each LDR,
all represented as 32-bit real numbers arranged in band-sequential
format:
1. Solar incidence angle at the areoid, units degrees
2. Emission angle at the areoid, units degrees
3. Phase angle, units degrees
4. Planetocentric latitude at the tangent point of the line
of sight, units degrees N
5. Longitude at the tangent point of the line of sight, units
degrees E,
6. Solar incidence angle at a surface intercept relative to
a MOLA shape model, units degrees
7. Emission angle at a surface intercept relative to a MOLA
shape model, units degrees
8. Elevation at the tangent point of the line of sight, units
meters relative to areoid
9. Elevation at the tangent point of the line of sight, units
meters relative to the MOLA shape model
10. Local Solar Time, units hours
11. Ephemeris Time of observation, seconds past noon January 1,
2000
12. Sub-solar planetocentric latitude, units degrees N
13. Sub-solar longitude, units degrees E
14. Sub-spacecraft planetocentric latitude, units degrees N
15. Sub-spacecraft longitude, units degrees E
Ancillary Data
==============
Ancillary data are 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 LDRs arise from uncertainties
in instrument pointing knowledge, and from coverage of the MOLA data
set.
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.
LDR Versions
============
Changes in the processing of LDRs are denoted by incrementing the
software version. There is no difference between versions 0 and 1.
Known Issue with LDR Accuracy
=============================
None.
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 LDRs 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 LDR.
Only a subset of instrument configurations represent 'scene' data,
as indicated by the keyword MRO:ACTIVITY_ID. Only scene data
aimed at Mars' limb have corresponding LDRs.
Limitations
===========
None.
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