| DATA_SET_DESCRIPTION |
Data Set Overview
=================
A major activity on MESSENGER has been acquisition of enhanced
observations pointed at special regions of scientific interest,
so-called targeted observations. Mostly these are directed at
features for which higher-resolution morphological or multispectral
imaging or denser spectral sampling could help in geological
characterization or hypothesis testing. Other imaging is acquired
as context for MLA or MASCS measurements, or to characterize
photometric properties of Mercury's surface.
These observations are scattered throughout the CDR data set linked
by the SITE_ID in their PDS label. In the RTM data set, the
images for each SITE_ID are collected, coregistered if they are
multispectral, map projected, and photometrically corrected to
a standard geometry of incidence angle i=30 degrees, emission
angle e=0 degrees, and phase angle g=30 degrees. All map
projections are orthographic and near the native resolution of
the images to preserve spatial resolution.
RTM products are named based on their SITE_ID, OBSERVATION_ID of
the first image, and number of image bands:
MDIS_ppp_cbb_siteid_observationid_v.IMG
where:
ppp = product type = RTM
c = camera (W WAC or N NAC)
bb = bands (01, 03, 08, 11 depending on type of observation)
siteid = a 6-digit integer giving the unique SITE_ID of the
region covered by the product
observationid = image observation ID of the first image (lowest ID)
v = version number
The following is an example file name with a description of
the individual components:
MDIS_RTM_N01_000276_1214047_0.IMG
For this image:
Product type = RTM (RTM)
Camera = NAC (N)
Bands = 1 (01)
SITE_ID = 276 (000276)
OBSERVATION_ID = 1214047
Version = 0
The RTM directory, present in the RTM archive volume, is organized
into subdirectories based on camera/band (cbb from the file name)
followed by year and day of year with the naming format:
MDIS_RTM_CBB/YYYY_DDD
where
C = camera (W WAC or N NAC)
BB = bands (01, 03, 08, 11 depending on type of observation)
YYYY = year of the beginning of acquisition of the first image in
the observation
DDD = day of year of the beginning of acquisition of the first
image in the observation
An RTM:
- Consists of map-projected photometrically normalized I/F CDRs
assembled into a regional mosaic;
- Contains image data in I/F corrected photometrically to i=30
degrees, e=0 degrees, g=30 degrees at a resolution close to
the native resolution of the imaging data;
- Is composed of images acquired by the NAC or through
multiple spectral filters by the WAC;
- Contains images acquired during a targeted observation of one
region of interest denoted by a site ID; and
- NAC mosaics contain 4 backplanes: (a) observation id, unique
to each image, (b) solar incidence angle, (c) emission angle,
and (d) phase angle. WAC color products contain 3 backplanes:
(a) solar incidence angle, (b) emission angle, and (c) phase
angle.
Versions
========
Version numbers of RTMs increment on reprocessing.
- Version 0 is released at PDS release 13. It is built from
version 4 CDRs projected onto a sphere using version 0 DDRs.
It uses a Hapke-form photometric correction, with different
parameters for low- and high-incidence angle products.
- Version 1 is released at PDS release 15 at end of mission. It
uses version 5 CDRs projected onto a digital elevation model
using version 1 DDRs. It uses a Kasseleinen-Shkuratov
photometric correction with a common set of parameters
among all data products.
Parameters
==========
MDIS observing variables pertaining to the RTMs are as follows.
Pixel Binning: Some RTM 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, only where needed to increase image cadence and
along-track overlap. MP binning was used only for WAC photometric
targets to control data volume.
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). Typically, WAC multispectral images
are 12 bits except and NAC image strips 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 spacecraft
main processor (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, especially for WAC
multispectral observations.
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. Most images in RTMs were acquired using
automatic exposure, with an upper limit on exposure time to limit
image smear.
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
targeted observations, the pointing of the pivot varied depending
on the type of target and whether the observation was coordinated
with MASCS or MLA. NAC image strips are mostly pointed at low to
moderate emission angles, and solar incidence angles near 68
degrees. However NAC images taken to provide context for MASCS or
MLA observations are pointed close to the +Z axis coaligned with
the appropriate other instrument. WAC targeted 3- or 11-color
image sequences are typically targeted at low solar incidence
angles with constrained emission angles. WAC 8- or 11-color image
sequences targeted for photometry have whatever pointing is
required in order to meet specified bounds on incidence, emission,
or phase angle for the particular region of interest.
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. RTMs may consist of only NAC images, or WAC
images in 3, 8, or 11 filters. The usage of filters in types of
WAC observations is given below:
Filter Filter Center Bandpass 3- 8- 11-
Number letter wavelength width color color color
in file (nm) (nm)
name
1 A 698.8 5.3 x
2 B 700 600.0
3 C 479.9 10.1 x x
4 D 558.9 5.8 x x
5 E 628.8 5.5 x x
6 F 433.2 18.1 x x x
7 G 748.7 5.1 x x x
8 H 947.0 6.2 x
9 I 996.2 14.3 x x x
10 J 898.8 5.1 x x
11 K 1012.6 33.3 x
12 L 828.4 5.2 x x
Processing
==========
A sequence of processing creates an RTM 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 RTM, 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 RTMs 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.
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,
g = 30 degrees.
(e) Gimbal positions are extracted from the spacecraft housekeeping
and formatted as a gimbal C kernel.
(f) Using the pivot C kernel and other SPICE kernels, DDRs are
created. The surface intercept on a model of Mercury's surface
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 RTMs 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 (if applicable)
Solar Incidence Angle
Emission Angle
Phase Angle
where OBSERVATION_ID is taken from the CDR label, the
ordinal number of the image among MDIS images taken
post-launch. The values for all backplanes are those for the
filter 7 image within the color sequence.
The stacking order ('which image is on top') is that the first
image in time is map projected first, the second image in time
overlays the first, and so on, so that the last image overlays
all others and is 'on top'.
The photometric correction applied to MDIS images
to create version 0 RTMs 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 RTMs, 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. RTMs were acquired at
different photometric geometries depending on the type of
observation. NAC image strips were acquired predominantly at
high solar incidence angles like BDRs/HIEs/HIWs and therefore
all use those parameters; WAC multispectral images were acquired
predominantly at low solar incidence angles like MDRs/MD3s and
therefore use their parameters instead.
VERSION 0 PHOTOMETRIC CORRECTION FOR WAC TARGETED OBSERVATIONS:
The parameters used, those for the MDR/MD3 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 low incidence angle mapping campaigns, plus additional
supplementary observations intended to expand the phase angle
range. The data from each of the regions was combined into a
single data set and a single set of parameters derived for each
filter.
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.
Derivation of the photometric correction involved: (1) calculating the
reflectance at the observed geometry for each pixel in each image
(Ro), (2) calculating the reflectance at i=30 degrees, e=0 degrees,
g=30 degrees (R30), (3) calculating the correction factor
(R30/Ro), and (4) applying the correction factor to each pixel within
each image. The Hapke model parameters used are listed below for each
WAC filter included in 3-, 8- or 11-color RTMs, where w is the single
scattering albedo, theta is the surface roughness parameter,
and b and c are the Heyney-Greenstein single particle scattering
function parameters defined above.
filter, wavelength, w, b, c , theta
F, 433.2, 0.151360115, 0.155099698, 0.126102310, 14.60131926
C, 479.9, 0.169685245, 0.147410229, 0.106108941, 14.74522393
D, 558.9, 0.197384444, 0.136483073, 0.081821900, 14.78008707
E, 628.8, 0.218696307, 0.129164430, 0.070385011, 14.65283436
A, 698.8, 0.237307445, 0.124224188, 0.068452363, 14.44035788
G, 748.7, 0.249052388, 0.122256228, 0.072924183, 14.27068804
L, 828.4, 0.265449469, 0.121966502, 0.090225001, 14.02954160
J, 898.8, 0.277789564, 0.124805212, 0.115960759, 13.90988549
H, 947.0, 0.285371188, 0.128609684, 0.139801777, 13.91310187
I, 996.2, 0.292068663, 0.133854741, 0.167852540, 14.00895011
K, 1012.6, 0.293865419, 0.135640972, 0.176741965, 14.05611393
For all wavelengths, the width of the opposition surge, h, is 0.09
and the strength of the opposition surge, B0, is 3.086.
VERSION 0 PHOTOMETRIC CORRECTION FOR NAC TARGETED OBSERVATIONS:
The parameters used, for the BDR/HIE/HIW photometric correction, were
derived by modeling whole-disk observations of Mercury taken at a
large number of photometric geometries during the Mercury flybys
using different filters in the wide-angle camera.
Those data were modeled using a least squares grid search
routine over the available model 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 Hapke model parameters used for the RTMs containing NAC images
is given in the table below, where w is the single scattering
albedo, theta is the surface roughness parameter, and b and c are
the Henyey-Greenstein single particle scattering function parameters
defined above. 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
The width of the opposition surge, h, is 0.09
and the strength of the opposition surge, B0, is 3.086.
VERSION 1 PHOTOMETRIC CORRECTION FOR ALL TARGETED OBSERVATIONS:
In version 1 RTMs delivered at the end of the mission, a different
photometric correction is used for all filters and all geometries,
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. Their values were fit using
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. Parameter 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
F 430 6.92413210E-02 6.37228563E-01 6.28836906E-01
C 480.4 7.98153978E-02 6.21777913E-01 6.27629117E-01
D 559.2 9.10849913E-02 5.97475375E-01 6.18544492E-01
E 628.7 9.86118777E-02 5.80013748E-01 6.22758382E-01
A 698.8 1.05807514E-01 5.68069278E-01 6.35596439E-01
G 749 1.11116798E-01 5.62741989E-01 6.42377921E-01
L 828.6 1.19413553E-01 5.56997602E-01 6.36801364E-01
J 898.1 1.25034169E-01 5.49548099E-01 6.17408232E-01
H 948 1.26684133E-01 5.38610109E-01 6.09847145E-01
I 996.8 1.24975849E-01 5.19691856E-01 6.30847041E-01
K 1010 1.23758640E-01 5.12689614E-01 6.45356466E-01
Data
====
There is one data type associated with this volume, RTMs consisting
of mosaicked, photometrically corrected WAC 3-, 8-, or 11-color
or NAC CDRs, appended with 4 backplanes describing the component
CDRs and their photometric geometries.
Ancillary Data
==============
There may be two types of ancillary data provided with this
dataset:
1. The EXTRAS directory in the RTM archive contains a list of all
SITE_IDs targeted by MDIS or other instruments, describing their
latitude/longitude coordinates and the motivation for their
targeting. This list is the primary mechanism for tracing the
science rationale for acquisition of the data in the RTM.
2. 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 RTMs, the IAU2000 reference system for
cartographic coordinates and rotational elements is used for
computing latitude and longitude coordinates of planets. However
a Mercury radius of 2440.0 km is used.
In version 1 RTMs, 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 RTM 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) The image was taken as part of a targeted observation directed
at the SITE_ID on which the RTM is based.
The OBSERVATION_ID for a part of an RTM is a pointer back to the
WAC filter 7 or NAC image used for that part of the RTM.
Version 0 RTMs are based on version 4 CDRs which correct a number
of earlier calibration artifacts. Version 1 RTMs are based on
version 5 CDRs. The following issues may affect the component
images.
(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.
(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.
(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 RTMs.
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 RTMs 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 RTMs 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 RTMs are 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 RTMs 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 RTMs greatly reduces residuals between images acquired at
different photometric geometries, implying reduced systematic errors.
Limitations
===========
None
|
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