| DATA_SET_DESCRIPTION |
DATA SET OVERVIEW
=================
The Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP)
CODMAC Level 5 Gridded Data Record is a collection of gridded data
products (maps) derived from the far ultraviolet photon detections
obtained by the LAMP instrument, corrected for instrumental effects,
radiometrically calibrated, and correlated with the auxiliary information
needed to locate the observations in space and time. These gridded data
products are presented as standalone maps, formatted as standard PDS
images, with detached PDS labels. The LAMP GDR archive will be of
interest to investigators studying the far ultraviolet properties of the
Moon.
PARAMETERS
==========
The fundamental parameters contained in the LAMP GDR data set are:
1) far-UV brightness
2) far-UV albedo
3) H2O absorption feature depths
4) raw far-UV photon counts
5) calibrated far-UV photon counts
6) exposure times
7) far-UV illumination
8) statistical errors (noise)
9) data quality
PROCESSING
==========
LAMP GDR data products are generated at the LAMP Science Operations Center
(SOC), located at the Southwest Research Institute in Boulder, Colorado.
The map processing proceeds generally as follows. (Please refer to the
LAMP EDR and RDR Data Product SIS documents for more information about the
LAMP EDR and RDR data products, as this discussion assumes the existence
of LAMP RDR products containing radiometrically calibrated and spatially
located far-UV photon detections.) From each RDR product a corresponding
'exposure record' is generated that records the exposure time of each LAMP
spatial pixel in each map cell. Calibrated photon counts (i.e.,
photons/cm2, where the cm2 is related to the wavelength-dependent
instrument effective area) from the RDR products are then paired with
these exposure records, one-to-one, to produce integrated photon count and
exposure maps. These, in turn, are used to generate the brightness maps
themselves. Far-UV illumination models are separately used to estimate
the time-dependent far-UV flux incident on the Moon's night side and
permanently shaded regions (PSRs). These, combined with the brightness
maps yield the albedo maps. Finally, the ratios of the off-band
brightness/albedo to the on-band brightness/albedo yields the H2O
absorption feature depth maps.
DATA
====
As alluded to above, the LAMP Gridded Data Record contains the following
nine types of maps:
1) far-UV brightness - units are photons/cm2/sec/sr
- four wavelength ranges are included:
Lyman-alpha (119.57 nm - 123.57 nm)
full stellar (129.57 nm - 189.57 nm)
on-band (129.57 nm - 155.57 nm)
off-band (155.57 nm - 189.57 nm)
2) far-UV albedo - units are dimensionless albedo
- the above four wavelength ranges are used
3) H2O absorption feature depths (ratios) - units are dimensionless
4) raw photon counts - units are photons
- the above four wavelength ranges are used
5) calibrated photon counts - units are photons/cm2
- the above four wavelength ranges are used
6) exposure times - units are secs
7) far-UV illumination - units are photons/cm2/sec
- the above four wavelength ranges are used
8) statistical errors (noise) - units are photons/cm2
- the above four wavelength ranges are used
9) data quality - units are dimensionless
- the above four wavelength ranges are used
All of these maps are derived from 'night side' data (i.e., when the LAMP
detector FOV was located on the night side of the Moon or in a permanently
shaded region [PSR] and the instrument aperture door was open). In
addition to the cumulative ensemble average maps generated using data
taken during the whole nominal ESMD phase of the LRO mission, monthly maps
are also provided for all of the above products except for the H2O
absorption feature depth maps.
Each map is a polar stereographic map centered on either the north or
south lunar pole and extending 10 degrees in latitude away from the pole
(i.e., to either +80 deg or -80 deg latitude). Each map has a true
resolution at the pole of 240 meters/pixel, thus yielding a map of
2501 x 2501 pixels. Each map element is stored using a 4-byte floating
point value, thus giving a total size of 25 MB per map image.
For each map type (except for the exposure and H2O absorption feature
depth maps) there are 112 maps (4 wavelength ranges x 2 poles x 14
[13 monthly + 1 cumulative]). Since the exposure times are the same for
all wavelengths, there are only 28 exposure maps. Also, there are only
4 H2O absorption feature depth maps (2 [brightness and albedo as ratio
source] x 2 poles). All told, there are 816 total maps in the GDR data
set.
ANCILLARY DATA
==============
Other than the ancillary data used in the generation of the LAMP RDR data
set, the only ancillary data specifically used in the creation of the LAMP
GDR data set is the illumination model (used to generate the illumination
maps, which are in turn used to derive the albedo maps).
The ancillary data used in the generation of the LAMP RDR data set include
those calibration values required to apply instrumental corrections and
calibrations to the science data as well as trajectory and pointing
information for the LRO spacecraft and clock conversion tables required
to enable the geometric calibration of the science data. The source of
the instrumental calibration data are the LAMP engineers and LAMP
scientists at the Southwest Research Institute in San Antonio, Texas
[see GLADSTONEETAL2008 for further details], while the source of the
spacecraft trajectory, pointing, and clock data are SPICE kernels
provided to the LAMP SOC by the LRO MOC, having been ultimately generated
by the MOC itself, the Flight Dynamics Facility (FDF) at GSFC, or the
Navigation and Ancillary Information Facility (NAIF) at JPL.
COORDINATE SYSTEM
=================
The Lunar Reconnaissance Orbiter mission uses a standard coordinate
system to describe all RDR (and hence GDR) data, namely lunar
planetocentric/body-fixed coordinates with east-positive longitude from 0
to 360 degrees. A mean Earth/polar axis (ME) reference system (also
called the mean Earth/rotation system) is used, with the z-axis being the
mean rotational pole and with the prime meridian (zero degrees longitude)
defined by the mean Earth direction.
The ME reference system is used for all LRO archival data. This LRO
standard is documented in 'A Standardized Lunar Coordinate System for
the Lunar Reconnaissance Orbiter, LRO Project White Paper, 451-SCI-000958,
Version 4, May 14, 2008'. Using coordinates in the ME system is
consistent with recommendations from the International Astronomical Union
(IAU)/International Association of Geodesy (IAG) Working Group on
Cartographic Coordinates and Rotational Elements.
A Jet Propulsion Laboratory (JPL) planetary and lunar ephemeris and
corresponding Euler angle set are used to define an ME frame to which the
LRO data are registered. The LRO Data Working Group (LDWG) determines
which ephemeris and Euler angle set should be used. Alternatively, LRO
data can be registered to an existing or new reference frame in the ME
system, via ties to surface points known in the frame (examples include
Lunar Laser Ranging [LLR] retroreflectors, points in images and Digital
Elevation Models).
When a JPL planetary and lunar ephemeris is used, the JPL Navigation and
Ancillary Information Facility (NAIF) provides the necessary lunar
ephemeris file (SPK) and binary lunar orientation file (PCK) in a
Principal Axes (PA) reference frame for use with the SPICE Toolkit. NAIF
also provides the frames kernel (FK) used for accessing the PA orientation
in the PCK and for transforming from the PA frame to the ME frame.
Alternatively, the JPL lunar ephemeris information is available in an
ASCII format not requiring the use of the SPICE Toolkit. This information
is available from a JPL website: http://ssd.jpl.nasa.gov.
SOFTWARE
========
Because the LAMP GDR data product files adhere to the image format
described in version 3 of the PDS standards reference, the PDS-supplied
NASAView software should be able to successfully read these files. There
are likely other such image readers freely available in the public domain
capable of reading the LAMP GDR maps.
MEDIA/FORMAT
============
The LAMP GDR data product files are written in standard PDS image (.img)
format, and the format of each file is described in an accompanying
detached PDS label. The GDR archive itself will be written on hard drive
media and physically delivered from the LAMP SOC to the PDS Imaging Node,
where it will be made generally available via the World Wide Web.
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