| DATA_SET_DESCRIPTION |
Data Set Overview
=================
A major imaging campaign for MDIS in MESSENGER's primary mission
was acquisition of a global data set at low emission angles for
cartographic purposes, and moderate to high incidence angles that
highlight topography. Those images were mosaicked into basemap
data records (BDRs). The HIE data set is complementary in that it
highlights low-relief topography that is less evident. The
illumination from the east favors asymmetric topography more
steeply sloped to the east than to the west. The companion
HIW data set favors asymmetric topography more steeply sloped
to that west than to the east.
Map tiles are named based on the quadrant of the Mercury chart they
span:
MDIS_ppp_rrrPPD_Hxxddv.IMG
where:
ppp = product type = HIE
rrr = resolution in pixels/degree (PPD)
Hxx = Mercury chart designation
dd = quadrant within Mercury chart (NW, NE, SW, or SE), or a
polar chart (NP, SP)e limits
v = version number
The following is an example file name with a description of the
individual components:
MDIS_HIE_256PPD_H03NE0.IMG
For this image:
Product type = HIE (HIE)
Resolution = 256 pixels/degree (256PPD)
Mercury chart = Shakespeare (H03)
Quadrant = Northeast (NE)
Version = 0
The HIE directory, present in the HIE archive volume, contains
MDIS Map Projected High Incidence Angle Basemap Illuminated from
the East Reduced Data Records (HIEs). The HIEs are organized
into subdirectories based on the Mercury Chart containing the
HIE. Latitude and longitude limits of Mercury Charts,
as named at the end of mission delivery, are:
Quadrangle Subdirectory Lat. (degrees) Long. (deg. east)
H-1 Borealis H01 65 to 90 0 to 360
H-2 Victoria H02 22.5 to 65 270 to 360
H-3 Shakespeare H03 22.5 to 65 180 to 270
H-4 Raditladi H04 22.5 to 65 90 to 180
H-5 Hokusai H05 22.5 to 65 0 to 90
H-6 Kuiper H06 -22.5 to 22.5 288 to 360
H-7 Beethoven H07 -22.5 to 22.5 216 to 288
H-8 Tolstoj H08 -22.5 to 22.5 144 to 216
H-9 Eminescu H09 -22.5 to 22.5 72 to 144
H-10 Derain H10 -22.5 to 22.5 0 to 72
H-11 Discovery H11 -65 to -22.5 270 to 360
H-12 Michelangelo H12 -65 to -22.5 180 to 270
H-13 Neruda H13 -65 to -22.5 90 to 180
H-14 Debussey H14 -65 to -22.5 0 to 90
H-15 Bach H15 -90 to -65 0 to 360
An HIE:
- Consists of map-projected photometrically normalized I/F CDRs
mosaicked into a basemap map tile;
- Contains image data in I/F corrected photometrically to i=30
degrees, e=0 at a resolution of 256 pixels per degree (~166 m/pixel
at the equator);
- Represents one latitude-longitude bin in a global map;
- Is composed of images acquired with by the NAC or by the WAC in
filter 7, both centered near 750 nm;
- Contains images acquired as part of the high-incidence angle
basemap campaign for which SUBSOLAR_LONGITUDE is located east
of the images's CENTER_LONGITUDE; and
- Contains 5 backplanes:
(a) observation id, (b) BDR metric, modified for the optimal
incidence angle to be 78 degrees, (c) solar incidence angle,
(d) emission angle, and (e) phase angle.
Versions
========
Version numbers of HIEs increment on reprocessing or addition of
new data. Polar tiles are in polar stereographic projections, other
tiles in equirectangular projection.
Version 0 is uncontrolled, projected onto an ellipsoidal model of
Mercury, and photometrically corrected using a Hapke photometric
model with parameters optimized to higher solar incidence angles
(and different from parameters used in map products containing
lower-incidence angle data). Version 1, released at end of mission,
is compiled using NAC or WAC 750-nm images from any campaign that
best fit the intended illumination geometry, i.e., low emission angle
and incidence angle near 78 degrees. It is controlled and projected
onto a global digital elevation model. It uses a Kasseleinin-Shkuratov
photometric correction, whose parameters are the same for any given
wavelength band across all MESSENGER end-of-mission map data products.
Parameters
==========
MDIS observing variable pertaining to the HIEs are as follows.
Pixel Binning: Some HIE images are unbinned and 1024x1024 pixels.
Some images are 2x2 pixel binned in the focal plane hardware
(also known as 'on-chip' binning), resulting in 512 x 512
images. The WAC was used to acquire HIE images from lower
altitudes, and the NAC was used at higher altitudes. Within
the altitude range for either camera, on-chip binning was used
within the lower portion of the range, to control data volume and
to manage data flow on the spacecraft. No further binning by the
spacecraft main processor (MP) was used.
12-8 bit compression: Images are read off the detector in 12-bit
format. 12 bit images may converted to 8 bit images using one of
eight lookup tables (LUTs). All images collected as part of
the HIE basemap have been converted to 8 bits.
FAST/DPCM compression: All images are compressed losslessly using
FAST/DCPM compression as they are read out of the DPU, to conserve
recorder space. Once the data are written to the recorder, they can
be uncompressed and recompressed more aggressively in the MP.
Wavelet compression: Images may be integer wavelet transform-
compressed in the MP, typically at 3:1 for color data and 4:1 for
monochrome data, but any value from 1 to 32 can be used. The initial
configuration in Mercury orbit was to perform 12 to 8 bit conversion
using LUT0 for the WAC and LUT2 for the NAC, with a wavelet
compression ratio of 8:1 for monochrome imaging and 4:1 for color
imaging. Initial images exhibited unexpectedly visible compression
artifacts. Beginning 19 April 2011, LUT0 and LUT2 were replaced
with LUT1 which better preserves image dynamic range, and
compression ratios were decreased to 4:1 or less for monochrome
data and 3:1 or less for color data where possible. Lossless
compression was used when downlink allowed.
Exposure Control: The exposure time of MDIS images can be set
manually by command, or automatically by the software. In manual
mode, exposure times from 1-989, 1001-1989, ..., to 9001-9989 ms
are available. In autoexposure mode the exposure time of the next
image is computed by the DPU software, and cannot exceed 989 ms
in duration. If the time of the next image occurs before the
calculation can be completed, and pixel binning or filter
position change, then the algorithm compensates for predicted
changes in scene brightness and filter transmission using an
onboard data structure. All images in HIEs were acquired using
automatic exposure.
Pointing: The MDIS imagers are mounted on a pivot platform, which
is itself mounted to the MESSENGER spacecraft deck. The pivot
platform is controlled by a stepper motor, which is controlled by
the Data Processing Unit (DPU). The pivot platform can move in
either direction. The total range of motion is 240 degrees, limited
by mechanical 'hard' stops, and is further constrained by 'soft'
stops applied by the software. The nominal pointing position for
MDIS is defined as 0 degrees, aligned with the spacecraft +Z axis
and the boresight for several other instruments. The range of the
soft stops is set to 40 degrees in the spacecraft -Y direction
(toward the MESSENGER sunshade) and +50 degrees in the +Y direction
(away from the sunshade). The pivot position can be commanded in
intervals of 0.01 degrees within this range. During acquisition of
the HIE basemap, the pivot was used to point the WAC or NAC
to low emission angles on the surface, at times when the solar
incidence angle was close as possible to 78 degrees.
Filter selection: The WAC imager contains a 12 position filter
wheel to provide spectral imaging over the spectral range of the
CCD detector. WAC filter 7 (750 BP 5) was chosen to complement the
NAC because its bandpass within that of the NAC lessens any
discontinuities that might result from regional variations in
spectral slope.
Processing
==========
A sequence of processing creates an HIE from CDRs and DDRs. A Derived
Data Record (DDR) consists of multiband images whose line and sample
dimensions and coordinates correspond one-for-one with those of a
CDR. It has 5 bands of data used to help create an HIE, including for
every image pixel: (a) latitude, (b) longitude, (c) incidence angle,
(d) emission angle, and (e) phase angle. The DDRs are an intermediate
product used to create HIEs and other map products, are defined as a
distinct data product in the MDIS CDR/RDR Software Interface
Specification, and are delivered to the PDS beginning with
delivery 11.
The sequence of processing is as follows:
(a) Experiment Data Records (EDRs) are assembled from raw data.
(b) Radiance images are created from the EDRs and calibration files.
(c) Radiance is converted to I/F CDRs by dividing by (pi * solar flux
at 1 AU / heliocentricdistance_in_AU^2).
(d) I/F is photometrically corrected to i = 30 degrees, e = 0 degrees.
(e) Gimbal positions are extracted from the spacecraft housekeeping
and formatted as a gimbal C kernel.
(f) Using the gimbal C kernel and other SPICE kernels, DDRs are
created. The surface intercept on a sphere of Mercury's radius
is calculated for each spatial pixel. 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.
(g) I/F corrected to i = 30 degrees, e = 0 degrees is map projected
into HIEs using the latitude and longitude information
in the DDRs. The same procedure is used on DDRs to assemble the
backplanes with derived information. They are appended to the
image band in the following order:
OBSERVATION_ID
BDR metric
Solar Incidence Angle
Emission Angle
Phase Angle
where OBSERVATION_ID is taken from the CDR label, the
ordinal number of the image among all MDIS images taken
post-launch, and the photometric angles are taken from the DDR.
The BDR metric or stacking order ('which image is on top') was first
defined for BDRs. For HIEs, the objective is to have 'on top' those
images with high spatial resolution, low emission angle, and a solar
incidence angle as close as possible to 78 degrees. This incidence
angle was chosen to highlight subtle topographic shading.
For version 0 HIEs, any image taken as part of the high
incidence angle campaign is a candidate to include. For
version 1 HIEs, images from any campaign with suitable
illumination can be included within the following criteria:
- Controlled images are primarily used, but non-controlled images are
used as needed to minimize gaps in coverage
- Incidence angle at the center of the images is < 90 degrees
- The north polar tile (H01) trims pixels with incidence angle
> 88 degrees
- The south polar tile (H15) trims pixels with incidence angle
> 89.5 degrees
- The image pixel scale > 100 m/pixel
The stacking order is determined at the camera boresight using a
metric that represents spatial resolution and image geometry;
lowest values represent the 'best' image. The 'worst' complete, map-
projected image with the highest value for the metric is laid into
the HIE first; then the complete image with the second-highest value
is laid in second, overwriting the first image where the coverage
coincides, and so on until the complete 'best' image with the lowest
value for the metric is on top. There are 3 expressions for the
modified BDR metric depending on latitude greater or less than
65 degrees and solar incidence angle greater or less than 78 degrees.
(a) Where abs(lat) <= 65 degrees and i => 78 degrees, the metric is:
PIXEL_SCALE / (cos e * ( cos ( flatten_factor * i) /
cos ( flatten_factor * 78 ) ) )
where i is solar incidence angle, e is emission angle, lat is
planetocentric latitude, and flatten_factor is set to 0.85 to
de-emphasize low solar incidence angles.
(b) Where abs(lat) <= 65 degrees and i < 78 degrees, the metric is:
PIXEL_SCALE / (cos e * (cos 78 / cos i))
(c) Where abs(lat) > 65 degrees, the metric is:
PIXEL_SCALE / (cos i * cos e )
where i is solar incidence angle, e is emission angle.
In each case, the value of PIXEL_SCALE is limited not to be below
approximately 166 meters so that unfavorably illuminated images with
high spatial resolutions not captured at the resolution of an HIE
do not overwrite more favorably illuminated images.
In version 0 HIEs, the photometric correction applied to MDIS WAC
G filter and NAC images to create HIEs is based on bi-directional
reflectance equations formulated by [HAPKE1993]. The general
equation for I/F is given by:
I/F=(w/4)[mu_not'/(mu' +mu_not')]{[1+B(g)]P(g)-
1+H(mu_not')H(mu')}S(i,e,g,theta)
where w is single scattering albedo, i is incidence angle, e is
emission angle, g is phase angle, p(g) is the single particle
scattering function, theta is a parameter representing
macroscopic roughness, and mu_not' and mu' are modified versions
of the cosines of the incidence and emission angle that take
into account effects of theta. H(mu_not') and H(mu') describe
approximations to the Chandrasehkar H-functions. The surface
roughness function, S(i,e,g,theta), modifies the radiative
transfer equation to account for surface roughness.
In addition, a Henyey-Greenstein function is used to describe
the single particle scattering function p(g). The form of the
Henyey-Greenstein function used corresponds to the form
utilized in the USGS ISIS software, and is given by:
p(g)=c(1-b^2)(1-2b cos(g)+b^2)^(-3/2) + (1-c)(1-b^2)(1+2b
cos(g)+b^2)^(-3/2),
where g is the phase angle, b is the scattering amplitude
parameter, and c is the partition parameter between forward and
backward scattering.
In version 0 HIEs, no single set of Hapke parameters was found that
yields close matches for corrected I/F across boundaries of images
taken at different photometric geometries, for both the 3- and 8-color
maps taken predominantly at low solar incidence angles (average,
about 45 degrees) included in MDRs and MD3s, and monochrome maps
taken predominantly at high solar incidence angles (BDRs, HIEs, HIWs).
Therefore map products emphasizing low or high incidence angles
initially used different sets of photometric parameters optimized
for each to minimize seams between images.
The parameters for the version 0 HIE Hapke photometric correction were
derived by modeling data acquired from multiple regions between
24 degrees and 46 degrees south latitude and 330 degrees and 353
degrees east longitude. These regions sample incidence angle (i),
emission angle(e), and phase angle (g) coverage commensurate
with global mapping campaigns. In addition whole-disk
Mercury images taken at a large number of geometries during
the Mercury flybys expand the phase angle range.
The photometric measurements were modeled using a least squares
grid search routine over the available parameter space. The model
parameter values were individually plotted as a function of wavelength
over the MDIS filter central wavelength values. A polynomial trend was
fit to each parameter as a function of wavelength. The polynomial
trend value at each filter central wavelength was then used as the
model parameter values for determining the photometric correction.
The parameter values applicable to LOIs are given in the
table below. Photometric behavior of Mercury in NAC images is assumed
to be equivalent to that in the WAC G filter.
WAC
filter, wavelength, w, b, c , theta
G, 749 , 0.278080114, 0.227774899, 0.714203968, 17.76662946
In version 1 HIEs delivered at the end of the mission, a different
photometric correction is used, a Kasseleinen-Shkuratov function
described by [DOMINGUEETAL2016]. The form of the function is
given as:
I/F = AN*exp[-(g*mu)]{c_sub_l[2cos i/(cos i + cos e)]+[1-c_sub_l]cos i}
where AN is normal albedo at a given wavelength, and mu and c_sub_l
are wavelength-dependent parameters whose values were fit in the
same manner as the parameters for the Hapke model. The values are
given in the table below:
WAC
filter, wavelength, AN, mu, c_sub_l
G, 749, 0.1111, 0.5628, 0.6424
Data
====
There is one data type associated with this volume, HIEs consisting
of mosaicked, photometrically corrected WAC filter 7 (G filter)
CDRs and NAC CDRs, appended with 5 backplanes describing the
component CDRs and their photometric geometries as recorded in DDRs.
Ancillary Data
==============
There is one type of ancillary data provided with this
dataset:
1. There may be a BROWSE directory containing browse images in
PNG and/or GeoTIFF format. See BROWINFO.TXT in that directory
for more details.
Coordinate System
=================
The cartographic coordinate system used for the MDIS data products
conforms to the J2000 celestial reference frame for star imaging,
and the IAU planetocentric system with East longitudes being
positive for planetary surfaces.
In version 0 HIEs, the IAU2000 reference system for cartographic
coordinates and rotational elements was used for computing latitude
and longitude coordinates of planets. However a Mercury radius of
2440.0 km is used.
In version 1 HIEs, the value for Mercury radius is updated to
2439.4 km.
Media/Format
============
The MDIS archive is organized and stored in the directory
structure described in the Mercury Dual Imaging System (MDIS)
Calibrated Data Record (CDR) and Reduced Data Record (RDR)
Software Interface Specification (SIS). The contents of the
archive, along with fiduciary checksums, are compressed into
a single 'zip archive' file for transmittal to the PDS Imaging
node. The zip archive preserves the directory structure
internally so that when it is decompressed the original
directory structure is recreated at the PDS Imaging node.
The zip archive is transmitted to the PDS Imaging node via
FTP to the URL specified by the node for receiving it.
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
Known issues of concern are described below.
Review
======
This archival data set was examined by a peer review panel
prior to its acceptance by the Planetary Data System (PDS). The
peer review was conducted in accordance with PDS procedures.
Data Coverage and Quality
=========================
Only a subset of raw EDR data are calibrated to CDRs and then
incorporated into HIE products. Briefly, the following criteria
are met:
(a) The data represent a scene and not the instrument test
pattern, as indicated by data quality index (DQI) byte 0.
(b) The exposure time is greater than zero (zero exposures
occur in some images due to software features), as indicated
by DQI byte 1.
(c) Less than 20 percent of the image is saturated (empirically
this is a threshold dividing wholly corrupted images from
everything else).
(d) The target of the image is MERCURY.
(e) In version 0 HIEs, the image was taken as part of the
monochrome basemap campaign. In version 1 HIEs, the image may be
taken from a different campaign if the illumination geometry more
closely approaches that desired for HIE data products.
Version 0 HIEs are based on version 4 CDRs which correct a number
of earlier calibration artifacts, and on version 0 DDRs. Version 1
HIEs are based on version 5 CDRs and version 1 DDRs. For version 1
HIEs, some component images may contain residuals from
the following issues.
(1) COMPRESSION ARTIFACTS. Images may be integer wavelet transform-
compressed in the MP, typically at 3:1 for color data and 4:1 for
monochrome data, but any value from 1 to 32 can be used. The initial
configuration in Mercury orbit was to perform 12 to 8 bit conversion
using LUT0 for the WAC and LUT2 for the NAC, with a wavelet
compression ratio of 8:1 for monochrome imaging and 4:1 for color
imaging. Initial images exhibited unexpectedly visible compression
artifacts. Beginning 19 April 2011, LUT0 and LUT2 were replaced
with LUT1 which better preserves image dynamic range, and
compression ratios were decreased to 4:1 or less for monochrome
data and 3:1 or less for color data where possible. Lossless
compression was used when downlink allowed. Images that are
part of HIE products typically were compressed 4:1 or losslessly.
(2) RADIOMETRIC ACCURACY. The radiometric calibration of the
WAC was updated several times over the mission to iteratively
reduce residuals from 3 sources of error: (a) time-variable
responsivity of the detector, (b) residuals in the flat-field
correction, and (c) residuals in the correction to
responsivity for detector temperature. For multispectral
products, the residuals from time-variable responsivity
initially led to distinct seams; correction of this artifact
is treated in more detail below.
In version 4 CDRs, an additional update to responsivity
improved temperature dependence over the full operating range.
The correction was derived empirically by fitting as a function
of CCD temperature the median values of images acquired from
Mercury orbit at a wide range of temperatures but a narrow
range of photometric geometries. The flat field was updated
empirically using the median of hundreds of photometrically
corrected images of relatively bland field-filling images
of Mercury.
In version 5 CDRs, a new, final temperature correction used all
Mercury images satisfying the illumination criteria. A new, final
flat-field correction was derived similarly to the updated
correction used in version 4, except using more images and
a Kasseleinen-Shkuratov photometric correction
(3) SCATTERED LIGHT. In the NAC, scattered light from out-of-field
sources is an issue. The geometry contributing most of the scatter
is 1-2 fields-of-view sunward of the NAC boresight. For a very
large, evenly illuminated source that overfills the field-of-view
by a factor of several, ray-trace studies supported by testing
during Venus flyby 2 suggest that 2-7% of the radiance measured in
the field-of-view will have come from out-of-field sources. The
spatial pattern of the scatter is variable, due to diffuse
reflections off the internal instrument housing.
The WAC is subject to scattered light
originating from within the field-of-view or just outside it. One
source is multiple reflections off of 13 optical surfaces (2
sides of each of 4 lenses, the spectral filter, and the CCD cover
glass, as well as the CCD surface itself). The scatter becomes
worse at longer wavelengths. Just off the limb of a large
extended source near 1 field-of-view in size, like Venus or
Mercury, measured radiance increases with wavelength from 2% to 7%
of the value measured on the extended source. The value decreases
with distance off the target more quickly at longer than at
shorter wavelengths, but remains at 1% hundreds of pixels from
the source. Conversely, light must be scattering from bright
parts of an image to dark parts of an image. Averaged over
sources tens of pixels in area, and away from abrupt brightness
contrasts, scattered light affects shapes of spectra measured
from WAC data at least at the 1-2% level, worse near brightness
boundaries or for small, bright crater ejecta. The expected effect
is enhanced brightness at >650 nm in dark areas, and decreased
brightness at >650 nm in small bright areas.
In the end-of-mission delivery 15, a forward model of the
expected WAC scatter from a given scene was derived using
optical design software modeling CCD structure and hardware,
with magnitudes of scatter calibrated against flight measurements.
The ray trace analysis reveal an in-scene component from light
diffracted by the CCD and reflected by the CCD cover glass, and
an out-of-scene component from light reflected off metallic
surfaces alongside the CCD and back off the cover glass.
These analyses suggest that scattered light is present in
monochrome map products but in general is not an issue in
morphologic interpretations. However caution is urged in using
quantitative photometric analysis in high-contrast or shadowed
terrain in these products.
(4) TIME-VARIABLE WAC RESPONSIVITY. During Mercury orbit it was
recognized that filter-dependent changes in WAC responsivity on
the order of +/- 15% occurred over timescales as short as several
days. Because those variations were not consistent from filter to
filter, they led to spurious spectral features, which were
particularly conspicuous near 750 nm. The cause(s) of these
variations in responsivity are not known, but they could include
transient radiation effects on the detector or electronics, aging
of filters, periodic deposition and burn-off of contaminants on
filters, or incorrect recording of exposure time. An initial
empirical correction for images acquired in the first year of
operations was developed and utilized in version 4 WAC CDRs used
to create version 0 HIEs.
For version 5 WAC CDRs in delivery 15 at end of mission, an
updated correction covers the full duration of the orbital
phase. Overlaps between color image sets in color mapping campaigns
were used to derive a multiplicative correction factor for each
filter and for each Earth day (2-3 orbits). Version 1 HIEs use
CDRs with this updated correction. An analysis of overlap among
individual images shows that residual differences (which include
errors from calibration, scattered light, and possible incomplete
correction of photometric variation) average <2% for the majority
of the planet.
(5) UNCONTROLLED MOSAIC PROJECTED ONTO A SPHERE. Version 0 HIEs were
constructed by uncontrolled mosaicking, projecting the image data
onto a sphere. Systematic errors in spacecraft position and in
knowledge of spacecraft and MDIS attitude, systematic errors in
range to the surface due to ignoring topography, and systematic
errors in latitude and longitude due projecting onto a sphere instead
of a shape model will all contributed to mosaicking errors. In
general these are expected to be under 1 km but locally might
exceed 4 km.
Version 1 HIEs were constructed from images controlled using
c-smithed kernels and a global digital elevation model (DEM), both
derived using a least-squares bundle adjustment of common
features, measured as tie point coordinates in overlapping NAC and
WAC-G filter images of Mercury at favorable solar incidence and
emission angles. Empirically, misregistration errors between images
decreased generally to the pixel scale of the map, (0.2 km) in
most locations. Derivation of smithed kernels and the DEM for
end of mission data products is described by [BECKERETAL2016].
(6) INACCURACY IN THE PHOTOMETRIC CORRECTION. The Hapke correction
applied to version 0 HIEs required the use of illumination-dependent
parameters implying the possibility of systematic inaccuracy. As shown
by [DOMGINUEETAL2016] the Kasseleinen-Shkuratov correction used in
version 1 HIEs greatly reduced residuals between images acquired at
different photometric geometries, implying reduced systematic errors.
Limitations
===========
None
|
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