CRIS DATA SET INFORMATION - cpshooter/geoML GitHub Wiki
DATA_SET_ID = "MRO-M-CRISM-3-RDR-TARGETED-V1.0"
OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = "MRO CRISM TARGETED REDUCED DATA RECORD V1.0" DATA_SET_COLLECTION_MEMBER_FLG = "N" DATA_OBJECT_TYPE = {"TABLE", "IMAGE"} START_TIME = "N/A" STOP_TIME = "N/A" DATA_SET_RELEASE_DATE = UNK PRODUCER_FULL_NAME = "SCOTT MURCHIE" DETAILED_CATALOG_FLAG = "N" ARCHIVE_STATUS = "IN QUEUE"
CITATION_DESC = "Murchie, S., Mars Reconnaissance
Orbiter Compact Reconnaissance Imaging Spectrometer for Mars
Targeted Reduced Data Record, MRO-M-CRISM-3-RDR-TARGETED-V1.0, NASA
Planetary Data System, 2006."
DATA_SET_TERSE_DESC = "Targeted Reduced Data Records for
IR and VNIR image cubes for CRISM (Compact Reconnaissance Imaging
Spectrometer for Mars)."
ABSTRACT_DESC = "This dataset is intended to include
IR and VNIR data from the CRISM instrument on MRO, processed to
several different levels. The core structure parallels that of an
EDR with a multiband image and a text file containing frame-specific
housekeeping information for each of the concatenated image frames
in the multiband image. However the image data has been converted to
units of radiance using level-4 and level-6 CDRs, and analog
housekeeping items in the text file (voltages, currents, and
temperatures) have been converted into physical units using a
level-6 CDR. Both files share a common label.
A TRDR may also contain separately labeled multiband images in which
radiance has been processed to I/F (radiance divided by (pi * solar
flux at 1 AU * heliocentric distance^2)), Lambert albedo, or a set
of derived spectral parameters (summary products) that provide an
overview of the data set. The summary products include Lambert
albedo at key wavelengths, or key band depths or spectral
reflectance ratios. To create Lambert albedo or most summary
products, estimated corrections for atmospheric and photometric
effects are applied to the I/F data."
DATA_SET_DESC = "
Data Set Overview
=================
This volume contains portions of the CRISM Targeted Reduced Data
Record (TRDR) Archive, a collection of multiband images from the
Compact Reconnaissance Imaging Spectrometer for Mars on the Mars
Reconnaissance Orbiter spacecraft. Images consist of
data calibrated to units of radiance or I/F plus a text file with
housekeeping information, and optionally image data to which
further corrections have been applied. Image data are in sensor
space and non-map-projected. 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 keyword MRO:ACTIVITY_ID. Only scene data that are aimed
at planetary objects are processed to TRDRs. Only those
configurations that affect the contents or dimensionality of a
TRDR 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 are processed to a TRDR.
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 TRDR, but the pixel
binning configuration will affect the dimensionality of the
TRDR.
Row selection: Detector row selection. All detector rows,
sampling different wavelengths, having useful signal can be
saved. Alternatively an arbitrary, commandable subset of rows can
be saved. The number of rows with useful signal is 545, 107 in
the VNIR and 438 in the IR, and these subsets of VNIR and IR
detector rows are used for hyperspectral observing. Prior to
10 Dec 2006 the nominal number of rows for multispectral mode
was 73, 18 in the VNIR and 55 in the IR. On 10 Dec 2006 an
extra channel was added to the VNIR for calibration purposes,
for a total of 19. For each detector, there are four options
of channel selection to choose from rapidly by command:
hyperspectral (545 total channels), multispectral (73 total
channels prior to 10 Dec 2006, 74 total channels on and after
10 Dec 2006), and two sets of expanded multispectral (84 and 92
channels prior to 10 Dec 2006, 85 and 93 channels on and after
10 Dec 2006). New options are set by uploading a data structure
to the DPU. On 12 Jan 2010, the smaller of the two expanded
multispectral channel selections was replaced with the largest
value supportable at a 15 Hz frame rate, all 107 VNIR channels
and 155 IR channels; most of the IR channels are contiguous from
1.8-2.55 ?m to cover key mineral absorptions.
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 be processed to a TRDR.
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 be processed to a TRDR.
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 TRDR.
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 text file is based on the 220 housekeeping items. Five
of the items are composite in that each byte encodes
particular information on gimbal status or control. These separate
items are not broken out, except for the gimbal status at the
beginning, middle, and end of each exposure, from which gimbal
position is broken out (3 additional items). The housekeeping is
pre-pended with 10 additional frame-specific items useful in data
validation, processing, and sorting, for a total of 233 items per
frame. Further information can be found in the data product SIS
in the DOCUMENT directory.
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 data are assembled into EDRs, they are calibrated into TRDRs.
Image data are converted to units of radiance using level-4 and
level-6 CDRs, and analog housekeeping items in the text file
(voltages, currents, and temperatures) have been converted into
physical units using a level-6 CDR. Both files share a common
label. The calibration algorithms are discussed at length in an
Appendix in the CRISM Data Products SIS.
A TRDR may also contain separately labeled multiband images in
which radiance has been processed to one of the following:
I/F (radiance divided by (pi * solar flux at 1 AU * heliocentric
distance^2)),
Lambert albedo, or
a set of derived spectral parameters (summary products) that
provide an overview of the data set. The summary products
include Lambert albedo at key wavelengths, or key band depths
or spectral reflectance ratios.
To create Lambert albedo or most summary products, estimated
corrections for atmospheric, photometric, and thermal effects are
applied to the I/F data using corrections given in ADRs. The
formulations for all of the summary products have been validated
using data from Mars Express/OMEGA.
The sequence of processing that creates a TRDR is as follows:
(a) EDRs are assembled from raw data.
(b) The radiance multiband images in 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.
(c) Gimbal positions are extracted from the EDR housekeeping and
formatted as a gimbal C kernel.
(d) Using the gimbal C kernel and other SPICE kernels, DDRs are
created. 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. 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.
(e) Optionally, radiance is converted to I/F by dividing by (pi *
solar flux at 1 AU * heliocentricdistance^2)). Solar flux is
maintained in a level 4 CDR, and solar distance is written in the
label to the radiance image.
(f) Beginning with version 3 of TRDRs, additional processing is
applied to I/F multiband images to remediate the effect of noise
in the flight data and propagated noise from ground and flight
calibrations. The IR multiband image only is processed using an
iterative kernel filter. A kernel with settable, typical dimensions
of 3x3x5 spatial x spatial x spectral pixels is used to identify
afflicted pixels using a Grubbs test for outliers; an outlying
value is replaced by interpolation linearly in the spatial
direction and using a 2nd degree polynomial in the spectral
direction. Both the VNIR and IR multiband images are then
processed using the ratio shift correction, which searches for
values from detector elements that are systematically high or low
and using a ratio to nearby pixels attempts to remove systematic
error. Testing on a variety of multiband images shows that most
systematic errors propagated from calibration files are removed,
and the spurious data values due to elevated IR detector operating
temperatures are greatly reduced in number and magnitude.
(f) Optionally, I/F is converted to Lambert albedo. Some or all
of the following corrections may be made:
I/F is divided by cosine of the solar incidence angle, the
estimated contribution to and attenuation of the signal by
atmospheric aerosols is normalized or removed, and the
estimated attenuation of the signal by atmospheric gases
is removed.
The thermal emission from longer IR wavelengths is removed.
MRO:ATMO_CORRECTION_FLAG and MRO:THERMAL_CORRECTION_MODE
indicate whether such corrections have been performed.
CRISM data products are described in greater detail in the Data
Products Software Interface Specification and the Data Archive
Software Interface Specification in the DOCUMENT directory.
Details of DN to radiance conversion
====================================
FLIGHT AND GROUND CALIBRATION DATA:
----------------------------------
All calibration matrices are stored in 'calibration data
records' or CDRs, separate from the main algorithm coded in
software. There are two general classes of calibration matrices,
those derived from ground data and updated infrequently,
and those that represent highly time-variable properties of
the instrument. Examples of the former include the constants
needed to uncompress data, correct non-linearity, or correct
bias for effects of detector or focal plane electronics
temperature. Examples of the latter include bias and IR thermal
background, which depend on detector and spectrometer housing
temperature respectively.
There are two formats for storing the values in the matrices,
distinguished by the levels of processing. Level 6 CDRs, or
CDR6s, are tabulated numbers in ASCII format, and level 4 CDRs,
or CDR4s, are images each derived from a collection of flight or
ground calibration measurements.
Calibration matrices that are highly time variable are
measured inflight, and include the following.
For the VNIR detector bias is measured directly, with the
shutter closed and at the same integration time as
accompanying measurements of Mars. For the IR, shutter-closed
measurements also include thermal background. The bias is
therefore measured for the IR detector by taking data at
several integration times and extrapolating to zero exposure.
The step function, a discontinuity in measured bias at some
detector row, is modeled deterministically as a function of
integration time. Currently, the default is to take a VNIR
bias measurement with every observation and IR bias
observations several times daily.
IR thermal background is the response of the IR detector to
'glow' of the inside of the instrument predominantly at >2300
nm. The change in spectrometer housing temperature that perturbs
thermal background by the equivalent to read noise is about
0.02K, whereas the spectrometer housing is predicted to change
by several degrees over the course of an orbit. Therefore
shutter-closed IR measurements are taken interspersed
within all observations, at an interval of approximately once
per 3 minutes. To correct any given frame of scene data,
temporally adjacent shutter-closed measurements are used.
The onboard integrating sphere serves as the radiance reference
against which CRISM's radiometric responsivity as a system is
pegged. Multiple measurements are conducted daily. The signal
from the sphere is adequate at wavelengths greater than 560 nm;
at shorter wavelengths a fixed responsivity derived from ground
calibrations is used.
The integrating sphere provides sufficient signal for a
preliminary 'flat-field' correction of IR data, but not
VNIR data. Pixel-to-pixel variations in VNIR detector
responsivity are measured monthly using bland regions of Mars.
Co-temporal IR measurements of the same bland regions are used
to derive a secondary flat-field correction for IR data; in
practice this is thought mostly to reduce propagated artifacts
from ground and flight calibrations.
RADIOMETRIC CALIBRATION:
Radiometric calibration to units of radiance involves
uncompressing data, correcting instrument artifacts, subtracting
bias and background, dividing by exposure time, and converting
of the result of these steps to units of radiance by comparing
against a radiometric reference. This approach explicitly uses
measurements of the internal integrating sphere. A simplified
form of the equation to reduce measurements to units of radiance,
using ground and flight calibration measurements, is:
RD(x,lambda) = M(x,lambda,Hz)( ( K(x,lambda,Hz)( D14lambda(
DN(x,lambda,TV,TW,TI,TJ,T2,Hz,t) ) -
BiaT(x,lambda,TV,TW,TI,TJ,Hz,t) ) / t - Bkgd(x,lambda,TI,T2,Hz) -
Scat(x,lambda,TV,TI,T2,Hz) ) / RST(x,lambda,TV,TI,T2,T3,S) )
Subscripts define the variables on which calibration coefficients
depend, and include the following:
x is spatial position in a row on the focal plane, in detector
elements.
lambda is position in the spectral direction on the focal plane,
in detector elements.
Hz is frame rate, and implicitly includes with it compression
configuration including wavelength table and binning mode
TI, is IR detector temperature in degrees K.
TV is VNIR detector temperature in degrees K.
T2, is spectrometer housing temperature in degrees K.
T3 is temperature of the integrating sphere in degrees K.
TJ is IR focal plane board temperature in degrees K.
TW VNIR focal plane board temperature in degrees K.
t is integration time in seconds.
s is choice of sphere bulb, side 1 (controlled by IR focal
plane electronics) or side 2 (controlled by VNIR focal plane
electronics).
Discussion of the various instrument effects being corrected is
including in the 'Confidence Level Note' below.
All of the input temperatures come from instrument housekeeping,
and are monitored by the focal plane electronics. Temperatures are
corrected for electronics noise by substituting for temperatures,
currents, and voltages in the image headers the corresponding
values at the same spacecraft time from the low-speed telemetry
stored in 'ST' CDR6s. This step is performed because the low-speed
telemetry maintains a fixed timing relative to instrument current
variations on 1-second cycles, whereas the image headers do not;
this makes the electronics noise more easily calibrated in the
low-speed telemetry. The raw digital values are corrected for
effects of frame rate and variable current loads, including lamps
and coolers, using additive and multiplicative coefficients
maintained in the 'HD' CDR6, and then scaled to physical units
using other coefficients maintained in the 'HK' CDR6.
The terms in the equation and their sequential application are as
follows:
Data decompression. D14lambda converts from raw 8- or 12-bit DNs
to 14-bit DNs. This is accomplished by inverting the 12-to-8-bit
LUT using the 'LI' CDR6, then dividing by the gain and adding
the offset used onboard, whose values are contained in the 'PP'
CDR6.
Bias subtraction. BiaT(x,lambda,TV,TW,TI,TJ,Hz,t) is detector
bias derived as described above from flight measurements, and
stored as a 'BI' CDR4. For the VNIR, it is just a decompressed
shutter-closed measurement. For the IR, it is the zero-exposure
intercept of the pixel-by-pixel fit of 14-bit DN to exposure
time in the bias measurements, added to the bias step function
stored in the 'BS' CDR6.
Bias is corrected for changes in focal plane electronics and
detector temperature since the time of bias measurement, using
telemetered detector and electronics temperature and the 'DB'
and 'EB' CDR6's respectively.
Electronics artifacts correction. K(x,lambda,Hz) applies
detector ghost and detector nonlinearity corrections.
This is actually a composite of distinct steps.
The correction for detector ghosts subtracts the scaled, bias-
removed DN from each quadrant from every other quadrant of the
detector. Scaling coefficients are stored in the 'GH' CDR6.
The nonlinearity correction scales bias- and ghost-removed DN
to account for nonlinearity in detector response. Detector-
averaged scaling coefficients are stored in the 'LI' CDR6.
Pixel-dependent nonlinearity at higher frame rates can be
corrected using images of bland regions of Mars taken at the
same frame rate. A non-uniformity matrix, or flat-field,
the 'NU' CDR4, is constructed from several thousand frames of
different scenes along-track that are averaged to remove
non-uniform illumination of the surface due to topography,
and the spatial image at each wavelength is normalized by its
mean value.
Background subtraction. Bkgd(x,lambda,TI,T2,Hz) is a 'BK' CDR4
constructed by applying the D14, BiaT, and K corrections to a
shutter-closed IR measurement interspersed with Mars
measurements. The actual background subtracted from a scene
measurement is a time-weighted average of preceding and
subsequent 'BK' shutter-closed measurements, to allow for the
continuous variation of IR thermal background as spectrometer
housing temperature changes.
Scattered light subtraction. Scat(x,lambda,TV,TI,T2,Hz) is the
stray light subtraction, and includes two components. The first
component is glare from the gratings, which produces a low level
of light at a distance of tens or more of pixels from a source.
For the VNIR detector, this component of scattered light is
measured directly at each row of the detector as the mean level
in the scattered light columns at a given row. The locations of
the scattered light and scene pixels are stored in the 'DM' CDR4.
It is then extrapolated across the detector using a function
based on signal at the shortest wavelengths (UV), which is
dominated by scatter. The shape of he function varies as a
function of wavelength. For the IR detector, transient bad
pixels render this correction noisy so instead this correction
was derived using a bland, dusty scene on Mars, applying the
correction both to the scene and to the accompanying sphere
measurement. The extrapolation across the field of view uses a
function resembling that at the longest VNIR wavelengths. The
correction was then median filtered and multiplied into the
sphere radiometric model. Given the derivation of the IR
scattered light correction, it is most accurate for uniformly
illuminated scenes.
The second component of scattered light is the second-order
light leaked through zone 3 of the IR order sorting filter.
This is removed by scaling and subtracting the measured signal
at second order wavelengths from the measured first-order signal
in zone 3. For each detector row (wavelength), which second-
order rows to use and their weightings are stored in the 'LL'
CDR4.
Responsivity correction using sphere data.
RST(x,lambda,TV,TI,T2,T3,S) is spectral responsivity derived
from onboard sphere calibration images. It is calculated by
processing a sphere measurement through the aforementioned
steps with two exceptions, and dividing by exposure time to
create an 'SP' CDR4. The exception is that the background image
is taken looking into the unilluminated sphere instead of with
the shutter closed, in order to subtract out the blackbody
radiation of the sphere's structure. The 'SP' product is
corrected for non-reproducibility in shutter position by
measuring the 'peak' in sphere DN/ms near VNIR detector row
232, and scaling a multiplicative correction stored in the
'SH' CDR4 by the magnitude of that peak.
The SP CDR4 is divided by the sphere spectral radiance model
stored in the 'SS' CDR4 to derive a snapshot of instrument
responsivity. The model uses as an input the choice of sphere
bulb. Normally the bulb controlled by the IR focal plane
electronics is used.
M(x,lambda,Hz) applies the detector mask in the 'DM' CDR4,
flagging non-scene data (e.g. scattered light and masked pixels)
with a value of 65535. This is a standard value for missing or
'bad' (saturated) data.
Converting radiance to I/F. RD(x,lambda) is the observed
spectral radiance in W/m2/steradian/um at the instrument
aperture, and is the output of the preceding steps for a scene
measurement. That radiance may be converted to I/F by dividing
by squared solar distance (stored in EDR and TRDR labels) and
the solar irradiance model stored in the 'SF' CDR4. That model
is itself derived convolving a predicted solar spectrum with
the measured center wavelength (stored in the 'WA' CDR4) and
spectral bandpass (stored in the 'SB' CDR4) of every detector
element.
Data
====
DATA DESCRIPTION:
There is only one data type associated with this volume, the
Targeted Reduced Data Records or TRDRs.
The TRDR consists of the output of one of the constituent macros
associated with a target ID that contains scene data (Mars or
other). Not all EDRs are processed to TRDR level; those
containing bias, background, sphere, or focal plane lamp data
are processed instead to CDRs. Only scene EDRs are processed to
the TRDR level.
The TRDR contains one or more multiple-band images (suffix *.IMG).
One matches the dimensions of the multiple-band image of raw DN in
an EDR, except that the data are in units of radiance. The size of
the multiple-band image varies according to the observation mode
but is deterministic given the ID of the command macro used to
acquire the data. Appended to the multiple-band image is a binary
table of the detector rows that were used, as selected by the
wavelength filter. This is a one-column table, with each row
containing one detector row number expressed as a 16-bit unsigned
integer values, most significant bit first.
Other multiple-band images may contain I/F, Lambert albedo, or
derived summary products. The I/F and Lambert albedo images, if
present, parallel the structure of the radiance image except lack
the list file. The summary products image has the same spatial
dimensions, but a different dimension in the spectral direction
and it lacks that table of row numbers. Each of these three
multiple-band images has its own label.
There are 45 summary parameters, as follows:
SURFACE PARAMETERS: from Lambert albedo
NAME: R770
PARAMETER: 0.77 micron reflectance
FORMULATION *: R770
RATIONALE: rock/dust ratio
NAME: RBR
PARAMETER: red/blue ratio
FORMULATION *: R770 / R440
RATIONALE: rock/dust ratio
NAME: BD530
PARAMETER: 0.53 micron band depth
FORMULATION *: 1 - (R530/(a*R709+b*R440))
RATIONALE: crystalline ferric minerals
NAME: SH600
PARAMETER: 0.60 micron shoulder height
FORMULATION *: R600/(a*R530+b*R709)
RATIONALE: select ferric minerals
NAME: BD640
PARAMETER: 0.64 micron band depth
FORMULATION *: 1 - (R648/(a*R600+b*R709))
RATIONALE: select ferric minerals, especially maghemite
NAME: BD860
PARAMETER: 0.86 micron band depth
FORMULATION *: 1 - (R860/(a*R800+b*R984))
RATIONALE: select ferric minerals
NAME: BD920
PARAMETER: 0.92 micron band depth
FORMULATION *: 1 - ( R920 / (a*R800+b*R984) )
RATIONALE: select ferric minerals
NAME: RPEAK1
PARAMETER: reflectance peak 1
FORMULATION *: wavelength where 1st derivative=0 of 5th order
polynomial fit to R600, R648, R680, R710, R740, R770, R800, R830
RATIONALE: Fe mineralogy
NAME: BDI1000VIS
PARAMETER: 1 micron integrated band depth; VIS wavelengths
FORMULATION *: divide R830, R860, R890, R915 by RPEAK1 then
integrate over (1 - normalized radiances)
RATIONALE: crystalline Fe+2 or Fe+3 minerals
NAME: BDI1000IR
PARAMETER: 1 micron integrated band depth; IR wavelengths
FORMULATION *: divide R1030, R1050, R1080, R1150
by linear fit from peak R between 1.3 - 1.87 microns to R2530
extrapolated backwards, then integrate over (1 - normalized
radiances)
RATIONALE: crystalline Fe+2 minerals; corrected for overlying
aerosol induced slope
NAME: IRA
PARAMETER: 1.3 micron reflectance
FORMULATION *: R1330
RATIONALE: IR albedo
NAME: OLINDEX (prior to TRDR version 3)
PARAMETER: olivine index
FORMULATION *: (R1695 / (0.1*R1050 + 0.1*R1210 + 0.4*R1330 +
0.4*R1470)) - 1
RATIONALE: olivine will be strongly +; based on fayalite
NAME: OLINDEX2 (beginning with TRDR version 3)
PARAMETER: olivine index with less sensitivity to illumination
FORMULATION *: (((RC1054 ? R1054)/RC1054) * 0.1)
+ (((RC1211 ? R1211)/(RC1211) * 0.1)
+ (((RC1329 ? R1329)/RC1329) * 0.4)
+ (((RC1474 ? R1474)/RC1474) * 0.4)
RATIONALE: olivine will be strongly positive
NAME: LCPINDEX
PARAMETER: pyroxene index
FORMULATION *: ((R1330-R1050) / (R1330+R1050)) *
((R1330-R1815) / (R1330+R1815)
RATIONALE: pyroxene is strongly +; favors low-Ca pyroxene
NAME: HCPXINDEX
PARAMETER: pyroxene index
FORMULATION *: ((R1470-R1050) / (R1470+R1050)) *
((R1470-R2067) / (R1470+R2067)
RATIONALE: pyroxene is strongly +; favors high-Ca pyroxene
NAME: VAR
PARAMETER: spectral variance
FORMULATION *: find variance from a line fit from 1 - 2.3 micron
by summing in quadrature over the intervening wavelengths
RATIONALE: Ol & Px will have high values; Type 2 areas will have
low values
NAME: ISLOPE1
PARAMETER: -1 * spectral slope1
FORMULATION *: (R1815-R2530) / (2530-1815)
RATIONALE: ferric coating on dark rock
NAME: BD1435
PARAMETER: 1.435 micron band depth
FORMULATION *: 1 - ( R1430 / (a*R1370+b*R1470) )
RATIONALE: CO2 surface ice
NAME: BD1500
PARAMETER: 1.5 micron band depth
FORMULATION *: 1 - ( ((R1510+R1558)*0.5) / (a*R1808+b*R1367)
RATIONALE: H2O surface ice
NAME: ICER1
PARAMETER: 1.5 micron and 1.43 micron band ratio
FORMULATION *: R1510 / R1430
RATIONALE: CO2, H20 ice mixtures
NAME: BD1750
PARAMETER: 1.75 micron band depth
FORMULATION *: 1 - ( R1750 / (a*R1660+b*R1815) )
RATIONALE: gypsum
NAME: BD1900
PARAMETER: 1.9 micron band depth
FORMULATION *: 1 - ( ((R1930+R1985)*0.5) / (a*R1857+b*R2067) )
RATIONALE: H2O, chemically bound or adsorbed
NAME: BDI2000
PARAMETER: 2 micron integrated band depth
FORMULATION *: divide R1660, R1815, R2140, R2210, R2250, R2290,
R2330, R2350, R2390, R2430, R2460 by linear fit from peak R
between 1.3 - 1.87 microns to R2530, then integrate over
(1 - normalized radiances)
RATIONALE: pyroxene abundance and particle size
NAME: BD2100
PARAMETER: 2.1 micron band depth
FORMULATION *: 1 - ( ((R2120+R2140)*0.5) / (a*R1930+b*R2250) )
RATIONALE: monohydrated minerals
NAME: BD2210
PARAMETER: 2.21 micron band depth
FORMULATION *: 1 - ( R2210 / (a*R2140+b*R2250) )
RATIONALE: Al-OH minerals
NAME: BD2290
PARAMETER: 2.29 micron band depth
FORMULATION *: 1 - ( R2290 / (a*R2250+b*R2350) )
RATIONALE: Mg,Fe-OH minerals (at 2.3); also CO2 ice
(at 2.292 microns)
NAME: D2300
PARAMETER: 2.3 micron drop
FORMULATION *: 1 - ( (CR2290+CR2320+CR2330) /
(CR2140+CR2170+CR2210) ) (CR values are observed R values
divided by values fit along the slope as determined between 1.8
and 2.53 microns - essentially continuum corrected))
RATIONALE: hydrated minerals; particularly clays
NAME: SINDEX
PARAMETER: Convexity at 2.29 microns due to absorptions at
1.9/2.1 microns and 2.4 microns
FORMULATION *: 1 - (R2100 + R2400) / (2 * R2290) CR
values are observed R values divided by values fit along the
slope as determined between 1.8 - 2.53 microns (essentially
continuum corrected))
RATIONALE: hydrated minerals; particularly sulfates
NAME: ICER2
PARAMETER: gauge 2.7 micron band
FORMULATION *: R2530 / R2600
RATIONALE: CO2 ice will be >>1, H2O ice and soil will be about 1
NAME: BDCARB
PARAMETER: overtone band depth
FORMULATION *: 1 - ( sqrt [ ( R2330 / (a*R2230+b*R2390) ) *
( R2530/(c*R2390+d*R2600) ) ] )
RATIONALE: carbonate overtones
NAME: BD3000
PARAMETER: 3 micron band depth
FORMULATION *: 1 - ( R3000 / (R2530*(R2530/R2210)) )
RATIONALE: H2O, chemically bound or adsorbed
NAME: BD3100
PARAMETER: 3.1 micron band depth
FORMULATION *: 1 - ( R3120 / (a*R3000+b*R3250) )
RATIONALE: H2O ice
NAME: BD3200
PARAMETER: 3.2 micron band depth
FORMULATION *: 1 - ( R3320 / (a*R3250+b*R3390) )
RATIONALE: CO2 ice
NAME: BD3400
PARAMETER: 3.4 micron band depth
FORMULATION *: 1 - ( (a*R3390+b*R3500) / (c*R3250+d*R3630) )
RATIONALE: carbonates; organics
NAME: CINDEX
PARAMETER: gauge 3.9 micron band
FORMULATION *: ( R3750 + (R3750-R3630) / (3750-3630) *
(3920-3750) ) / R3920 - 1
RATIONALE: carbonates
ATMOSPHERIC PARAMETERS: from I/F
NAME: R440
PARAMETER: 0.44 micron reflectance
FORMULATION *: R440
RATIONALE: clouds/hazes
NAME: IRR1
PARAMETER: IR ratio 1
FORMULATION *: R800 / R1020
RATIONALE: Aphelion ice clouds vs. seasonal or dust
NAME: BD1270O2
PARAMETER: 1.265 micron band
FORMULATION *: 1 - ( (a*R1261+b*R1268) / (c*R1250+d*R1280) )
RATIONALE: O2 emission; inversely correlated with high altitude
water; signature of ozone
NAME: BD1400H2O
PARAMETER: 1.4 micron band depth
FORMULATION *: 1 - ( (a*R1370+b*R1400) / (c*R1330+d*R1510) )
RATIONALE: H2O vapor
NAME: BD2000CO2
PARAMETER: 2 micron band
FORMULATION *: 1 - ( R2010 / (a*R1815+b*R2170) )
RATIONALE: atmospheric CO2
NAME: BD2350
PARAMETER: 2.35 micron band depth
FORMULATION *: 1 - ( (a*R2320+b*R2330+c*R2350) / (d*R2290+e*R2430) )
RATIONALE: CO
NAME: IRR2
PARAMETER: IR ratio 2
FORMULATION *: R2530 / R2210
RATIONALE: aphelion ice clouds vs. seasonal or dust
NAME: BD2600
PARAMETER: 2.6 micron band depth
FORMULATION *: 1 - ( R2600 / (a*R2530+ b*R2630) )
RATIONALE: H2O vapor
NAME: R2700
PARAMETER: 2.70 micron reflectance
FORMULATION *: R2700
RATIONALE: high aerosols
NAME: BD2700
PARAMETER: 2.70 micron band depth
FORMULATION *: 1 - ( R2700 / (R2530*(R2530/R2350)) )
RATIONALE: CO2; atmospheric structure (accounts for spectral slope)
NAME: IRR3
PARAMETER: IR ratio 3
FORMULATION *: R3750 / R3500
RATIONALE: aphelion ice clouds vs. seasonal or dust
Note *: 'a', 'b', 'c', 'd', 'e' in band depth formulations
represent fractional distances between wavelengths wavelength;
for example, given BD(c), a band depth at a central wavelength
'c' with nearby continuum points defined at shorter and longer
wavelengths 's' and 'l': BD(c) = 1 - R(c) / (a*R(s) + b*R(l)),
where a = 1 - b and b = (lambda(c) - lambda(s)) / (lambda(l) -
lambda(s))
DATA DIMENSIONALITY:
The size of the multiple-band image varies according to the
observation mode but is deterministic given the ID of the
onboard macro the generated the data. 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=640/binning, where binning is 1, 2, 5, or 10, and dark is the
number of masked and scattered light pixels
YY=the number of frames of data taken by the macro, and
ZZ=the number of rows (wavelengths) that are retained by
the instrument.
The data in a TRDR do not have optical distortions removed. In
one column, the projection onto Mars' surface may vary by as much
as +/-0.4 not-binned detector elements in the XX dimension
depending on position in the FOV (distortions are worst at the
edges of the VNIR and IR FOVs). For a single wavelength, its
location in the ZZ direction may vary by as much as +/-1
not-binned detector elements depending on wavelength and position
in the XX direction (distortions are worst at the short- and long-
wavelength ends of the IR detector).
To correct for optical distortions, multiband images may be
resampled in the spectral or spatial direction. Three types of
resampling may have occurred: (a) resampling in the wavelength
direction, as coded in the PS CDR; (b) resampling in the spatial
direction, to remove differences in spatial scale with wavelength
or band, using the CM CDR; and (c) VNIR data may be rescaled to
match the slightly different magnification of the IR spectrometer,
also the CM CDR. A resampled TRDR is distinguished by values of
the keywords MRO:SPATIAL_RESAMPLING_FLAG,
MRO:SPATIAL_RESCALING_FLAG, and MRO:SPECTRAL_RESAMPLING_FLAG.
Ancillary Data
==============
There are various types of 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.
2. The MTRDR BROWSE directory contains browse images in PNG format,
and HTML documents that support a web browser interface,
that correspond with TRDRs in this directory.
See BROWINFO.TXT in the MTRDR archive 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
=========================
There is a number of sources of uncertainty in the interpretation
of TRDRs including:
(A) Stochastic noise
Random noise in the data due to statistical uncertainties in
counting photons. This is manifested as noisy calibrated data.
Noise is most significant in darker areas. Typically, the signal
to noise ratio at <2500 nm is 400 in bright areas and 200 in dark
areas, in the constituent observations, before pixel binning.
(B) Optical distortions
Optical distortion can affect spectra of small-scale features.
In one column, the projection onto Mars' surface may vary by as
much as +/-0.4 not-binned detector elements in the XX dimension
depending on position in the FOV (distortions are worst at the
edges of the VNIR and IR FOVs). For a single wavelength, its
location in the ZZ direction may vary by as much as +/-1
not-binned detector elements depending on wavelength and position
in the XX direction (distortions are worst at the short- and
long-wavelength ends of the IR detector). In other words,
different wavelengths include slightly different combinations
of signal from spatially adjacent pixels, so that compositional
interpretations of features near the scale of a pixel are weakly
wavelength-dependent. Also, wavelength drifts across the field
of view. Compositional interpretations based on exact
wavelengths of absorptions may thus be weakly spatially dependent.
The resampling approach outlined above can remove much of this
uncertainty.
(C) Variable spectral resolution
In order to distinguish spectrally similar minerals that
have different geological implications for their environments
of formation, adequate spectral resolution is necessary. This
requires sufficiently high density spectral sampling, as well
as a sufficiently narrow full width half maximum (FWHM) of the
instrument response in the spectral direction. This 'slit
function,' the effective bandpass for a single detector
element, represents the convolution of spectral sampling and
the point-spread function in the spectral direction. CRISM's
benchmark is distinguishing the minerals montmorillonite and
kaolinite, which form in hydrothermal environments under
different temperature regimes [SWAYZEETAL2003]. The requirements
for this are (a) <20 nm FWHM and (b) sampling of the spectrum
at this or smaller increments. CRISM's spectral sampling
requirement is <10 nm/channel to provide oversampling, and the
actual performance is better at 6.55 nm/channel. FWHM is 8 nm
in the VNIR across the FOV. In the IR it increases from 10 nm
at short wavelengths to 15 nm at the longest wavelengths at the
center of the FOV, and broadens by about 2 nm at 0.8 degrees
from the center of the field of view. Outside +/-0.9 degrees
from the center of the field of view the telescope is slightly
vignetted, so further degradation is expected at extreme field
angles. Although the spectral sampling and resolution meet
requirements, their variation across the field-of-view must be
accounted for when comparing with rock and mineral analog
spectra.
(D) Calibration artifacts
There are several instrument artifacts that are corrected in
the calibration pipeline. Residual errors in the corrections will
introduce systematic errors into the data.
Optical Effects:
(1) The boundary of zones 1 and 2 of the VNIR order sorting
filter is a joint between two distinct glasses with different
indices of refraction. When illuminated during detector-level
tests, it was found to cause significant (>10%)
scattered light at shorter wavelengths (<670 nm). This was
correcting by replacing the VNIR focal plane assembly with the
flight spare, onto which a narrow black stripe was painted to
shadow the joint. The black stripe attenuates the light from
610-710 nm and causes a dip in response at those wavelengths.
In the processed data, the are two major effects: signal to
noise ratio is decreased, and non-reproducibility of the
exact position of the shutter when observing the sphere causes
shifting of the shadow in the wavelength direction. Measurement
and correction of this effect are discussed below.
(2) The spectrometer slit - which defines the mapping of
wavelengths to detector rows as well as the spatial FOV - is
mounted on a curved surface whose axis of curvature is parallel
to the wavelength direction. The slit assembly is fixed with
pins through holes whose diameters are oversized to provide
margin for fastening the assembly. During instrument-level
vibration testing, the slit assembly shifted in the wavelength
direction by the tolerance in the hole diameters, shifting
wavelength calibration by about 15 nm in both the VNIR and IR.
Additional shifting of the slit assembly during and after
launch is thought to have occurred. Wavelength calibration is
observed to shift with major thermal excursions of the
optomechanical assembly, at a magnitude of up to +/- 1 nm.
This shift has been quantified and can be calibrated out using
measured positions of Martian atmospheric gas absorptions,
as recorded in the 'WS' CDR.
(3) Shutter position irreproducibility - To illuminate the
spectrometer slit's full 2.12 degree field of view, CRISM's
telescope illuminates a circular region of slightly larger
diameter surrounding the slit. The base of the shutter, on the
hinge end, just protrudes into the illuminated area. At
position 0, originally intended as the 'open' position, the
reflective rear surface of the shutter provides the detectors
an unbaffled view of the scene approximately 1 degree from the
center of the field of view in a cross-slit direction, creating
an out-of-focus 'ghost' image of that location. Moving the
shutter through successive steps redirects the angle from which
the ghost image originates to further from the center of the FOV.
At position 3, the angle from which the ghost image originates is
baffled by the telescope, and the ghost disappears. To remediate
the ghost image, the open position of the shutter is defined in
software to position 3.
There is a small (about 0.1 degree) non-reproducibility in the
angle at which the sphere is viewed and the fact that, unlike the
external scene, the spectrometer's view of the sphere is vignetted
by the sphere's aperture. With a slight shift in shutter position,
the cone of sphere light entering spectrometer optics shifts. The
filling of the dual zone gratings changes slightly, decreasing
responsivity at long VNIR wavelengths and short IR wavelengths.
Also, the shadow of the black strip on the VNIR order-sorting
filter zone boundary shifts, creating a distinctive trough and
peak pattern at detector rows 222-235 (approximately 605-690 nm).
Because this effect is characteristic as a function of
wavelength, it is correctable. Ratios of different sphere
observations during ground calibration are used to create a
multiplicative correction to a sphere image as a function of
wavelength, that is maintained in a level 4 CDR. In flight data
to be corrected, the magnitude of the peak near VNIR row 235 is
measured. The correction is scaled by the magnitude of the peak,
and it is multiplied by the data. The VNIR row 235 peak is used
to scale the corrections for both the VNIR and IR. Errors in
this correction would lead to high or low values especially
at 600-700 nm, with the error being systematic within
a group of observations processed using a single sphere
observation, but random between such groups of observations.
Put differently, the systematic errors would change every few
orbits.
To the limits of measurement error, the small irreproducibility
of shutter position at the 'open' position has no measurable
effect on external scene data.
(4) IR 2nd order leakage: Zone 3 of the IR order sorting filter
admits up to 3% of the 2nd order light from the grating, at
wavelengths 1400-1950 nm, that falls at detector rows whose
nominal wavelengths are 2800-3900 nm. The leakage peaks at a
nominal wavelength of 3400 nm. Due to the falloff of both the
solar spectrum and the Martian reflectance spectrum with
increasing wavelength, the relative magnitude of the leakage
to the signal in zone 3 is enhanced so that it becomes tens
of percent of the total signal in that wavelength range.
Ground testing provided sufficient data for an empirical
correction for this effect, in which scaled values of signal at
second-order wavelengths are subtracted from first-order
(nominal) wavelengths. The correction is maintained in a level 4
CDR. Errors in this correction would be manifested in processed
data as a negative positive additive component to the values
from 2760-3920 nm, centered and strongest at 3400 nm.
Electronics Ghost:
Both detectors, but especially the VNIR
detector, are subject to a weak ghost image of any illuminated
spot into its corresponding location in every other of the four
160-column quadrants of the of the 640-column detector. This is
a small effect at the <1% level, and is removed by scaling the
image of each quadrant by an empirically determined value that
is nonlinearly related to signal level, and then subtracting
the scaled quadrant image from that of every other quadrant. The
scale factors are maintained in a level 6 CDR. To the
uncertainties in measurement, each of the four quadrants in a
detector behaves only slightly differently. There is a minimal
effect of frame rate, but ghost magnitude is apparently
unaffected by detector temperature. Errors in this correction
could be manifested as anomalously dark or bright spots exactly
one-fourth of the detector width away (160 samples, 80 2x-binned
samples, 32 5x-binned samples, or 16 10x-binned samples.
IR 'Bad Pixels':
The IR detector is operated at cryogenic temperature
to minimize dark current and bias level of the detector. As the
MRO mission has progressed, the setpoint for the IR detector has
been raised to lessen the wear on the mechanical coolers that
maintain IR detector temperature.
With increasing detector temperature, not all pixels accrue an
elevated bias level or dark current - the latter of which adds
noise due to its electron counting statistics - at the same rate.
The most susceptible pixels, within which effective SNR or
available dynamic range are adversely impacted, are
'bad pixels.' Beginning with version 3 TRDRs, and improved
approach to filtering is applied to I/F cubes to interpolate
over bad pixels are interpolated over.
VNIR Calibration Versions
=========================
Version 0 was the first version of VNIR radiometric calibration
applied to flight data. Five major sources of inaccuracy and image
artifacts were identified.
The first arose from the method by which scattered light from the
grating was extrapolated from scattered light columns across the
scene. A simple linear interpolation was applied, and this
underestimated the total scattered light within the scene. When
this procedure was applied to ground calibration data to derive
a model for sphere radiance, results include an unrealistically red
spectral slope. When applied to flight measurements of Mars and the
integrating sphere, results include residual cross-track
(along-slit) color variations.
The second problem arose from the choice of which sphere bulb to
use as the primary calibration source. Originally it was the bulb
controlled by the VNIR focal plane electronics. However,
for unknown reasons, this bulb yields much greater scatter from the
grating than does the bulb controlled by the IR focal plane
electronics. Residuals within the shadow of zones 1 and 2 of the
wavelengths order sorting filter led to a large negative artifact at
600-700 nm.
The third problem arose from low light levels in the sphere at
<560 nm. That is, measured sphere signal levels are low enough that
they introduced systematic noise at short wavelengths into
observations calibrated using them.
The fourth problem arose from propagated statistical errors in
sphere measurements used to calibrate the data. That is, corrections
for all of the artifacts outlined above require a series of
algebraic operations each of which propagates small errors or
effects of noise. The sphere data require more corrections, and at
lower-signal wavelengths the residual errors at each detector
element are manifested as a spuriously high or low value at the
corresponding wavelength and spatial position. This can appear as
wavelength-dependent striping in the along-track direction.
Fifth, it was discovered that the bias varies image to image,
leading to striping in the cross-track direction in processed data.
Version 1 used an arbitrary scaling across of scattered light from
the grating across the field of view, in an attempt to remediate the
first two effects. It proved unsuccessful, and was abandoned after
validation of the first observations to which it was applied.
Version 2 addresses and largely corrects each of the five major
problems with version 0.
First, there is a more sophisticated extrapolation from the scattered
columns. At short wavelengths, the distribution is measured from
measured signal at UV wavelengths. The detector is nearly unresponsive
to UV signal from Mars, so in most regions the nominally UV
wavelengths instead provide a measure of the scattered light at the
corresponding position along the slit. This gradually transitions
with longer wavelength to a linear extrapolation between the scattered
light columns, as with version 0. This correction is applied both to
ground calibration data used to derive the sphere radiance model,
and to flight scene and sphere data.
Second, the sphere bulb controlled by the IR focal plane electronics
is used as the primary radiometric reference, because the lower
scattered light from it is more easily corrected and leaves lesser
artifacts at low-signal wavelengths.
Third, calibration of the data is handled differently at <560 nm
and at >560 nm. At longer wavelengths, sphere measurements and the
sphere radiance model traceable to ground measurements are used to
determine a snapshot of detector responsivity, and apply that to scene
data to derive radiance. This is appropriate because of temperature
dependence of optical throughput, especially the beamsplitter, at
>560 nm. At shorter wavelengths, responsivity is derived directly from
ground calibration measurements, and low-signal sphere data are not
used. The approach has consistently yielding nearly identical,
continuous results at the wavelengths at which the two approaches
overlap, about 530-600 nm.
Fourth, to eliminate propagation of statistical errors in processing
of sphere data, the sphere-derived responsivity at each wavelength
is averaged. Correction for spatial nonuniformity occurs using a Mars
flat-field calibration and the NU CDR derived from it.
Fifth, to remove frame-to-frame variations in bias, an additive
correction is applied to each frame while still in units of DN,
to make the physically masked columns at the edge of the detector
zero after dark subtraction.
There are two alternate versions of VNIR version 2 specifically
for special types of observations. The bland Mars scenes used to
measure non-uniformity are processed to version 9 TRDRs. The
version 9 processing is the same as version 2 except it doesn't
include the non-uniformity correction. Deimos, Phobos, and any
other pointlike or compact targets like stars are processed to
version 8 TRDRs. The major difference in the processing for such
scenes is that within-scene scattered light from the grating is
much less, so the correction for intra-scene scattered light is
skipped. The VNIR version 8 processing uses special version 8
'SS' and 'NU' CDR4s.
Version 3 is the current version of the VNIR calibration, released
in late 2010. The most substantive change from version 2 is an
improved correction for shutter mirror irreproducibility. Version 7
replaces version 9 for bland Mars scenes. Version 6 replaces version
8 for Phobos and Deimos.
For I/F cubes - but not radiance cubes -
the ratio shift correction was introduced to further reduce the
magnitude of propagated artifacts of ground calibration. (The
radiance cube is a 'control' in case the ratio shift correction
introduces its own artifacts.)
Known Issues with VNIR Radiometric Calibration
==============================================
Version 3 has been validated using observations of the MER Spirit and
Opportunity landing sites, with PANCAM measurements that were modeled
at the top of the atmosphere using CRISM viewing geometry and solar
longitude. Four artifacts or data quality concerns remain.
(a) Radiance at <410 nm is typically low, because there is not much
signal at those wavelengths and they are most susceptible to artifacts
from scattered light subtraction.
(b) Some artifact at the wavelengths of the filter zone boundary. In
some parts of the field of view at sharp brightness contrasts it
extends to 644-684 nm.
(c) Scenes with large coverage by ice, especially near either edge of
the field of view, have more significant artifacts from the scattered
light correction because, unlike typical Martian soils, ice has
significant UV reflectance and that decreases accuracy of the
correction. Typical effects may include degradation at wavelengths
below 480 nm and above 1010 nm.
(d) Due to spectral smile, mineralogic absorptions located where
instrument response varies steeply with wavelength exhibit variation
with field angle. This is most prominent with Fe mineral
absorptions near 900 nm.
IR Calibration Versions
=======================
Version 0 was the first version of IR radiometric calibration
applied to flight data. Five major sources of inaccuracy and image
artifacts in version 0 are known.
The first originated from inadequacy of the bad pixel correction.
On ground, three types of bad pixels were observed: hot with high
dark current, dead with no response, and noisy. Based on that the
bad pixel correction was planned to use dark measurements to
identify bad pixels. Inflight, a new type of bad pixel was
identified, whereby a detector element abruptly develops high dark
current, then abruptly returns to normal. At least 2 percent of
pixels, and probably more, display this behavior at some time.
This was manifested as sharp, wavelength-dependent striping in the
along-track direction in image data.
The second problem originated from incomplete correction of bad
pixels in ground calibration measurements used to derive the sphere
radiance model. This was manifested as diffuse, wavelength-dependent
striping in the along-track direction in image data.
The third problem was spuriously high radiances near the boundaries
of the order sorting filters, especially at 1630-1680 and 2690-2770.
A lesser spuriously high radiance occurred at 1800 nm. Spuriously
high values also occurred at <1050 nm.
The fourth problem was 'bumps' in radiances at 1370 and 1850 nm,
water vapor in the beam and adsorbed water on ground calibration
sources was under-corrected.
The fifth problem was that the correction for leaked second order
light at >2700 nm was not implemented in version 0.
Version 1 partially corrected problems with version 0 but did not
close out known IR calibration issues.
First, within-scene bad pixels were identified were interpolated
over. For each scene, the entire scene was collapsed to one median
frame matching the layout of the detector. Then a median filter was
run in the spatial direction. The difference between the median image
and the filtered median image was calculated, and detector elements
having more than a 1.4 standard deviation difference were identified
as bad pixels. These were replaced throughout the scene by the average
of the adjacent non-bad pixels at the same wavelength.
Second, artifacts in the sphere radiance model were removed using a
Mars flat-field calibration and the NU CDR derived from it. However
this is not done within strong atmospheric absorptions near 1400,
1600, 2000, and 2700 nm because spectral smile would introduce new
artifacts.
Third, systematically high radiances at the filter zone boundaries
were corrected by smoothing the sphere radiance model.
Fourth, the 'bumps' in radiances at 1370 and 1850 were corrected by
performing an approximate atmosphere removal on bland, dusty terrain
at the summit of Olympus Mons. The magnitude of the 'bumps' was
estimated, and the appropriate multiplicative correction to the sphere
model to remove them was applied.
Version 2 added six further corrections for outstanding issues
identified in version 1.
First, in version 1, incomplete correction for water vapor in the
spectra of ground calibration sources had introduces a 'bump' in
radiance at 2550-2650 nm. This wavelength is on the edge of a strong
3-micron absorption in Mars; spectrum, so Olympus Mons observations
could not be used to estimate a correction. Instead this artifact
was corrected using observations of Deimos, which lacks a 3-micron
band. The magnitude of the 'bump' was estimated, and the appropriate
multiplicative correction to the sphere model to remove them was
applied.
Second, smoothing of the sphere radiance model that had been used
in version 1 was performed in an attempt to correct anomalously
high derived Mars radiances in the 2 channels closest in wavelength
to 2700 nm.
Third, in version 2 leaked second order light at >2700 nm was
corrected. This was performed by subtracting scaled radiances from
one-half the wavelength using the 'LL' CDR4s.
Fourth, in version 1, the bad pixel correction had been set to
too sensitive a threshold, so that abrupt brightness boundaries
often triggered the bad pixel correction, leading to aliasing at
those boundaries. The threshold for identifying a bad pixel in the
median-scene image was increased from 1.4 to 2.4 standard
deviations in version 2.
Fifth, slight misalignment of the instrument aperture with the ground
calibration source introduced systematic error into the sphere
radiance model. This preferentially affected wavelengths below
1500 nm, and in version 1 scene radiances it had introduced a broad,
sigmoidal-shaped bump centered near 1400 nm. For version 2, this
effect was modeled using grating theory, and a one-time correction
for it was applied to the sphere radiance model.
Sixth, in version 1, no attempt had been made to remove scattered
light from the grating at IR wavelengths, either in scene data or
in observations of the internal integrating sphere. The effect was
overestimation of IR scene radiance at <1900 nm. In version 2, the
correction was introduced. However, unlike the VNIR correction for
grating scatter, for the IR detector transient bad pixels render a
correction of this form noisy. Instead this correction was derived
using a bland, dusty scene on Mars, applying the correction both to
the scene and to the accompanying sphere measurement. The
extrapolation across the field of view uses a function resembling
that at the longest VNIR wavelengths. The correction was then
median filtered and multiplied into the sphere radiometric model.
Given the derivation of the IR scattered light correction, it is
most accurate for uniformly illuminated scenes.
There are two alternate versions of IR version 2 specifically
for special types of observations. The bland Mars scenes used to
measure non-uniformity are processed to version 9 TRDRs. The
version 9 processing is the same as version 2 except it doesn't
include the non-uniformity correction. Deimos, Phobos, and any
other pointlike or compact targets like stars are processed to
version 8 TRDRs. The major difference in the processing for such
scenes is that within-scene scattered light from the grating is
much less, so the correction for intra-scene scattered light is
skipped. The IR version 8 processing uses special version 8 SS
CDR4s.
Version 3 is the current version of IR radiometric calibration,
released in late 2010. There are several improvements over version
2.
First, the effects of shutter mirror position irreproducibility
were found to vary with each sphere measurement, that was in
turn applied to a group of scene measurements. That group was
in many cases systematically too high or low in radiance at
1000-1700 nm. Instead, each sphere measurement is corrected
using as a measurement of the shutter mirror effect the shape of
the VNIR zone boundary artifact. This approach was found to yield
greatly improved reproducibility of different TRDRs covering the
same scene.
Second, the estimation of leaked 2nd order light at IR wavelengths
near 3100-3300 nm was improved. The algorithm used in version 2 had
propagated an artifact at the zone 1 - zone 2 filter boundary near
1630 nm, where the leakage was being estimated, to 3180 nm, where
the leakage was being removed. Interpolation of the leakage correction
over a wider band at the zone 1 - zone 2 boundary was found to
greatly reduce the artifact near 3180 nm.
Third, the correction for atmospheric water vapor in ground
calibration measurements was modified to further reduce spectral
artifacts near 1850 and 2550 microns.
Fourth, the width of the band near 2000 nm within which flat-fielding
is not applied was increased, to remove distortions in Mars'
atmospheric CO2 absorption near 1830 nm.
For I/F cubes - but not radiance cubes - the ratio shift correction
and iterative kernel filter were introduced to further reduce the
magnitude of propagated artifacts of ground calibration and noise
in flight data. (The radiance cube is a 'control' in case the
ratio shift correction and iterative kernel filter
introduce their own artifacts.)
Known Issues with IR Radiometric Calibration
============================================
Due to spectral smile, mineralogic absorptions located where
instrument response varies steeply with wavelength exhibit variation
with field angle. This is most prominent at the edge of atmospheric
gas absorptions near 1400 and 2000 nm.
The responsivity correction at IR wavelengths 3000-3920 is suspected
to contain low wavelength frequency errors, perhaps leading to a
broad 'bump' centered near 3400 nm. This is currently under
investigation and may be corrected in a future version of the IR
radiometric calibration.
Summary of VNIR and IR 'bad channels'
=====================================
The following channels can be routinely excluded:
VNIR: wavelengths less than 410, 644-684, greater than 1023 nm
IR: wavelengths less than 1021 nm, 2694 and 2701 nm, and greater
than 3924 nm
The following channels may be 'degraded' and their quality is
observation-dependent. Caution is recommended but the data
may be valid.
VNIR:
Wavelengths less than 442 nm (due to artifacts from scattered
light correction in very contrasty scenes)
Wavelengths greater than or equal to 970 nm (radiances are
observed to misalign with IR radiances; the reason is uncertain
but may be related to uncorrected effects of beamsplitter
temperature)
IR:
Wavelengths less than 1047 nm radiances are
observed to misalign with IR radiances; the reason is uncertain
but may be related to uncorrected effects of beamsplitter
temperature)
Wavelengths 2660-2800 nm (the reason is uncertain but may be due
to problems with correction of water vapor in measurements of the
ground calibration sources)
The shape of the spectrum at 3100-3800 nm is suspect and there
may be a broad, low 'bump'.
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 TRDR 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 TRDR.
Only a subset of instrument configurations represent 'scene' data,
as indicated by the keyword MRO:ACTIVITY_ID. Only scene data
aimed at planetary targets have corresponding TRDRs.