| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This volume contains the Level 1B (L1B) Archive, a collection of
radiometrically-calibrated and pixel-located data products acquired
during from November 2008 through August 2009 by the
Moon Mineralogy Mapper (M3) instrument during the Chandrayaan-1 mission
to the Moon.The data were acquired while the instrument was operated
in global or targeted mode. The documentation in the DOCUMENT
directory provides detailed information about the data files and their
format and content.
A M3 Level 1B Data Product consists of pixel-located, resampled,
calibrated data in units of spectral radiance that make up one
observation tagged by a unique file name. The data in one Level 1B
Product represent a consistent instrument configuration (frame rate,
pixel binning). There is a single multiple-band image
(BIN; suffix *_RDN.IMG) stored in one file with a detached PDS label
(ASCII; suffix *_L1B.LBL), and a detatched header file (ASCII;
suffix *_RDN.HDR), plus several files containing data related to
pixel location (BIN; suffix *_LOC.IMG), observation geometry (BIN;
suffix *_OBS.IMG), and UTC timing for each image line (ASCII;
suffix *_TIM.TAB).
As of Septemer 2011, we have developed a detector-temperature-driven
empirical gain adjustment to the M3 Level 1B data. We refer to this
correction as the SSC_ADJ correction and it is based on a family of
spectral gain curves that are applied to each Level 1B radiance cube
after its nominal radiometric calibration. Based on the detector
temperature for each scene, a single curve is chosen from the family
of temperature-based curves and each spectrum in the radiance cube is
multiplied by this correction factor. This update should replace
versions 1.0 and 2.0 of the M3 Level 1B data products.
Processing
==========
Level 1B processing involves the following operations:
- converts the decompressed, uncalibrated image cube data into
resampled, scaled, calibrated spectral radiance image cubes
- calculates the surface location of all pixel centers
- calculates the observation geometry and illumination on a
pixel-by-pixel basis
- calculates the UTC time for the middle of the integration period
for each frame of the image data
Data
====
Under the Data directory there is a separate subdirectory based on
the start and end times of a 13-week imaging period. Each imaging
period subdirectory is further divided into subdirectories for each
month of an Optical Period (OP). Each OP month subdirectory is further
divided based on the type of data product (e.g. Level 0 or Level 1B).
YYYYMMDD_YYYYMMDD/YYYYMM/L1B directory:
----------------------------------
This directory contains the following files:
*RDN.IMG
--------
The L1B multiple-band spectral radiance image cube has
dimensions of sample, line, and wavelength. The size and format of the
M3 spectral radiance image cube depend on the observation mode
(global/target).
M3 calibration data such as wavelength centers,
full-width half-max values and radiometric gain factors can be found in
the CALIB directory of the archive volume.
In target mode, the spectral radiance image cube has the following
characteristics:
32-bit floating point
Little endian
256 spectral channels
608 spatial samples
N image lines
Band interleaved by line
In global mode, the spectral radiance image cube has the following
characteristics:
32-bit floating point
Little endian
85 spectral channels
304 spatial samples
N image lines
Band interleaved by line
During the transmission and encoding/decoding of the
data, some data elements may be lost. Data lost to poor compression or
complete packet loss are noted in the *.LOG files located in the EXTRAS
directory.
All M3 L1B spectral radiance image cubes are standardized to remove
the different effects of the four possible orbit limb and flight yaw
mode combinations: descending/forward; descending/reverse;
ascending/forward and ascending/reverse. In ascending limb data the
lines/times are reversed, so all L1B spectral radiance image cubes
have the northernmost image line first. In descending/reverse and
ascending/forward modes the samples are reversed, so the first sample
is on the west side of the image and do not appear left-right mirrored.
In descending/forward no changes in lines or samples are performed;
this is the only case that matches the Level 0 data. Refer to the
ORBIT_LIMB_DIRECTION and SPACECRAFT_YAW_DIRECTION keywords in the PDS
label (*_L1B_LBL) to reconcile a specific Level 1B spectral radiance
image cube with the associated Level 0 data.
Due to the successive loss of the two star trackers the data become
progressively more difficult to model. Hence our models increase in
complexity and parameterization.
For M3 we have been forced into development of three different
spacecraft attitude models that describe the orientation of the
Chandrayaan-1 spacecraft during an orbit. These models were developed
sequentially as we processed data collected during the mission.
A detailed description of each model and the keywords associated with
each model are provided in the M3 Data Product SIS.
*RDN.IMG.HDR
------------
Each L1B spectral radiance image cube file will be accompanied by a
detached ASCII header file. A detached header provides compatibility
with ENVI software.
*L1B.LBL
--------
A spectral radiance image cube label (*_L1B.LBL) is detached and points
to the following L1B data products:
- the single multi-band image (*_RDN.IMG) and its respective detached
header file (*_RDN.HDR),
- the pixel location data (*_LOC.IMG) and its respective detached header
file (*_LOC.HDR),
- the observation geometry data (*_obs.img) and its respective detached
header file (*_OBS.HDR),
- the UTC timing data (*_TIM.TAB)
*LOC.IMG
--------
The pixel location data for each image are stored in a three-band, band-
interleaved-by-line, binary file of double precision 8-byte values, in
little-endian byte order. The three bands of the file, in order, are as
follows:
1) longitude (reported in decimal degrees)
2) planetocentric latitude (reported in decimal degrees)
3) radius (reported in meters from the Moon center)
There are no embedded headers or other data in the file. Each
location file will be accompanied by a detached header file.
A detached header provides compatibility with ENVI software. The
location file is, in essence, a three-band set of 'detached
backplanes' that match the sample and line spatial extent of the
spectral radiance image cube data. No map correction or resampling is
applied to the radiance image cube; the file only reports the
surface locations of the unadjusted pixel centers.
*LOC.HDR
--------
Each location data file will be accompanied by a detached ASCII
header file. A detached header provides compatibility with ENVI
software.
*OBS.IMG
--------
The observation geometry data for each image are provided in a ten-band,
band-interleaved-by-line, binary file of single precision 4-byte values,
in little-endian byte order. The ten bands of the file, in order, are as
follows:
1) to-sun azimuth angle (decimal degrees, clockwise from local north)
2) to-sun zenith angle (decimal degrees, zero at zenith)
3) to-sensor azimuth angle (decimal degrees, clockwise from local north)
4) to-sensor zenith angle (decimal degrees, zero at zenith)
5) observation phase angle (decimal degrees, in plane of to-sun and
to-sensor rays)
6) to-sun path length (decimal au with scene mean subtracted and noted
in PDS label)
7) to-sensor path length (decimal meters)
8) surface slope from DEM (decimal degrees, zero at horizontal)
9) surface aspect from DEM (decimal degrees, clockwise from local north)
10) local cosine i (unitless, cosine of angle between to-sun and local
DEM facet normal vectors)
Similar to the location data, these files are, in essence,
ten-band set of 'detached backplanes' that match the sample and line
spatial extent of the spectral radiance image cube data. No map
correction or resampling is applied to the radiance image cube; the
file only reports the observation parameters of the unadjusted pixel
centers.
*OBS.HDR
--------
Each observation geometry data file will be accompanied by a detached
header file. A detached header provides compatibility with ENVI software.
*TIM.TAB
--------
The timing file (*TIM.TAB) is an ASCII file with four columns of data.
The first column lists the line number of the multiple-band spectral
radiance image cube (*RDN.IMG). The second column lists the corresponding
UTC time for the middle of the integration period for each spectral
radiance image cube line or major frame of the data and is expressed as:
YYYY-MM-DDTHH:MM:SS.SSSSSS.
The third column lists Year reference of Decimal Day of Year (DDOY) as
extracted from the earliest time of each spectral radiance image cube
line expressed as: YYYY.
The fourth column lists DDOY which represents the number of days elapsed
since 00:00 UTC of January 1 of the year associated with the time stamp
of the first image line. The DDOY format is as follows: DDD.dddddddddddd
where DDD represents the integer number of days and dddddddddddd
represent the fractional part of the day of year value.
Note that the M3 Level 0 data products have two representations of image
frame times captured in the 1280-byte image frame header:
1) a copy of the raw bytes of CH-1 and M3 binary clock tick data
and 2) a nominal conversion of those raw clock ticks to a UTC time in an
ASCII format. This nominal conversion is built on the assumed
operation of the clocks in a wholly stable manner with fixed tick
rates and no drift or unexpected rate issues. To achieve optimized
timing for subsequent processing, the Level 1B code develops
individual clock conversion models for each image of sufficient
length. This per-image modeling accommodates any clock drift or rate
changes and provides the best possible frame times for the Level 1B
data products. As such, the ASCII UTC times in the Level 0 data products
may differ from those in the Level 1B data products. The Level 0 data
product times should be considered deprecated and are only approximate
UTC frame times. The per-image optimized conversion used in the Level 1B
processing yields the most accurate image frame times and these are the
times reported in the timing file.
File Naming Convention
======================
The file naming convention for L1B data products is as follows.
M3GYYYYMMDDTHHMMSS_VNN_PT.EXT
Or
M3TYYYYMMDDTHHMMSS_VNN_PT.EXT
M3: The instrument.
G or T: The imaging mode; G for global mode and T for target mode.
YYYY: The year of the time stamp from the first image frame of the
image cube.
MM: The month of the time stamp from the first image frame of the
image cube.
DD: The day of the time stamp from the first frame of the image
cube.
T: A single character string that precedes the UTC time of the
time stamp from the first frame of the image cube.
HH: The hour in UTC of the time stamp from the first frame of the
image cube.
MM: The minute within the hour in UTC of the time stamp from the
first frame of the image cube.
SS: The second within the minute in UTC of the time stamp from the
first frame of the image cube.
VNN: The version number of the product.
PT: The type of data product:
L0 = Level 0
L1B = Level 1B
RDN = Spectral Radiance data
LOC = Pixel Location data
OBS = Observation geometry data
TIM = Observation timing data
EXT: The file name extension:
IMG = Image object
HDR = Detached header file
LBL = Detached label file
TAB = ASCII text file
All fields must occupy the allotted number of characters. Thus, if fewer
digits are required to express a number than are allotted, the
convention fills the unneeded spaces with leading zeroes.
Coordinate Systems
==================
The coordinate system used is the new 'Standardized Lunar Coordinate
System for the Lunar Reconnaissance Orbiter' (LRO Working Group, 2007).
This new lunar coordinate system is being adopted as an international
standard and will greatly facilitate the direct integration of data
from multiple missions and among international partners. The
coordinate system is based on planetocentric coordinates in the Mean
Earth/Polar Axis (ME) reference frame. The z-axis is the mean axis
of rotation with the positive direction pointing north. The x-axis is
the intersection of the Equator and Prime Meridian, as defined by the
mean Earth direction. The y-axis completes the frame in a right-handed
sense and points in the direction of +90 degrees longitude. Latitude
ranges from +90 to -90 form the North Pole to the South Pole.
Longitude will be reported as 0 to 360 degrees increasing to the East.
Software
========
The M3 team uses the commercial software packages ENVI and IDL to display
and analyze M3 data products. ENVI and IDL are distributed by ITT Visual
Information Solutions and are available at http://www.ittvis.com/.
In addition, PDS' NASAView Image Display Software can also be used for
basic image viewing of M3 L1B data products:
http://pds.nasa.gov/tools/nasa-view.shtml.
Nevertheless, the data are in no way in any proprietary format. Instead
they are arranged as simply and as openly as possible. The provision of
both ENVI and PDS labels will guarantee the data will be readily
accessible to the widest possible audience.
Media/Format
============
The M3 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
=========================
Confidence level information will be provided after more thorough
analysis of the data.
Review
======
Prior to creation of the final version of this archival data set,
its keys elements including samples of data products, PDS labels
and catalog files, and the Software Interface Specifications, were
examined during a peer review held by PDS Imaging Node in January 2010.
Data Coverage and Quality
=========================
The M3 image acquisition time will be divided into peak periods or
Optical Periods (OP) when lighting is optimal for observation. The
Optical Periods occur twice a year and are understood to have two
central months of optimal illumination (solar beta angles -30 deg to
+30 deg) with two optional two-week wing periods (solar beta angles
+/-30 deg to +/-45 deg) on either side of the optimal 2 months (thus,
one Optical Period equals 13 weeks). Each 13 week optical period is
followed by a 13-week hiatus. The original instrument operations plan
included the acquisition of the entire surface of the Moon in
low-resolution Global Mode during the first Optical Period (OP1)
while OP2, OP3, OP4 were reserved for high resolution Target Mode
data acquisition.
However, the mission was cut short, just before the halfway point, in
August, 2009 when the spacecraft ceased operations. Despite the
abbreviated mission and numerous technical and scientific challenges
during the flight, M3 was able to cover more than 95% of the Moon in
Global Mode. Only minimal high-resolution Target Mode images were
acquired, as these were to be the focus of the second half of the
mission. The technical challenges encountered during the mission
have complicated the data processing and calibration. These challenges
include thermal issues, loss of the spacecraft star trackers and a
raising of the orbit from 100 km to 200 km on May 19, 2009. Details
of these challenges are currently being documented and will be
referenced and/or included in the delivery of the M3 PDS Archive
Volume. Nonetheless, the data products released in the M3 PDS Archive
Volume will contain optimal calibration and characterization.
(NOTE: If these docs are included in the archive, put them in
DOCUMENT directory, not EXTRAS.)
M3 operations were sustained for two Optical Periods. (For more
detailed information regarding the spacecraft operation schedule,
please see the MISSION.CAT.) Please see the M3 Data Product SIS for
more detail/information.
Limitations
===========
None.
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