International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012
XXXIX-B4, 2012 XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
e NAC images are sampled at 12 bits and companded (e.g. 3. METHODOLOGY
LROC NAC TE AC E. bits. Each camera has a field of view of 2.869,
which are mounted to overlap by approximately 135 pixels in To produce DEMs of key regions of interest (see workflow in
the cross-track direction and are offset from each other by ~185 Figure 4), Integrated Software for Image and Spectrometers
pixels in the down-track direction (Robinson et al. 2010). (ISIS; http:// isis.astrogeology.usgs.gov/index.html; Anderson, et
conter ‘and tlic al., 2004) routines ingest the images, perform a radiometric
Stereo images are collected by acquiring images on two correction, and export the image data in SOCET SET format.
different orbits so that the convergence angle is between 10? The NAC files imported into SOCET SET are radiometrically
LZ 85287 and 45°. The overlap between the two NAC-L and NAC-R corrected images and contain a list of keywords of relevant
5m images provides three or four stereo models from which to parameters regarding the spacecraft position and pointing.
: : collect elevation data. The number of models depends on the :
- 86001 ~(mrosick, orientation of the images and how they intersect one another. ~~ SOCET SET includes a push broom sensor model that has been
The amount of overlap and the actual footprint are affected by adapted to handle many spacecraft Cameras (including LROC
the topography and acquisition parameters such as center NAC) that is used to relate the Image space to ground
latitude, center longitude, and slew angle. coordinates. A bundle adjustment is performed on the images to
correct for offsets in camera pointing using a multi-sensor
g, Processing 22 LOLA triangulation (MST) algorithm. MST is used to update camera
pointing, improve registration between areas of stereo coverage,
and ground truth using tie-points, sensor position, and camera
pointing. Tie points relate a point in the overlapping area of two
or more images, while ground points register a point or
identifiable object in the image to a point on the ground.
Selected parameters, such as the position, velocity, and pointing
angles of the cameras are adjusted so that the root mean square
(RMS) errors for all the tie point measurements are minimized.
Stereo pairs can be mosaicked together by placing tie points in
areas of stereo coverage (Figure 3). Mosaicking stereo pairs
tends to increase the overall RMS error when applying a bundle
adjustment to the sets of images, revealing the presence of
unmodeled systematic errors. For example, in images with
minimal change in slew angle during image acquisition, the
offset between image sets is less than 1 meter. When spacecraft
maneuvers do not maintain a constant slew angle during image
acquisition (which can be quite irregular and may not be well
modeled by the adjusted parameters), the offsets can vary
between 3 m to 15 m.
observations with the
iis work describes the
1 of USGS integrated
tate University (ASU).
> than 130 stereo pairs
sampling is typically 2
v of the lunar surface
addition to exploration
nposition and geologic
ES
Figure 2. LOLA spot pattern (Smith et al. 2010).
Altimetry obtained from the Lunar Orbiter Laser Altimeter
(LOLA) is used to increase the absolute accuracy of NAC
DEMs. LOLA was designed to measure the shape of the Moon
by precisely measuring the range from the spacecraft to the
surface, and incorporating precision orbit determination of LRO
by referencing surface ranges to the Moon's center of mass. Its
primary objective is to produce a global geodetic grid for the
Moon to which all other observations can be precisely
referenced (Smith et al. 2010). LOLA is a pulse detection
time-of-flight altimeter that incorporates a five-spot pattern to
measure the precise distance to the surface at 5 spots
simultaneously, thus providing 5 profiles for each orbit. LOLA
; iy fires at a fixed, 28 Hz rate, so that for a nominal 1600 m/s
vo image acquisition ground track velocity, there is one shot approximately every 57
meters. At a nominal 50 km altitude, each spot within the five-
Spot pattern has a diameter of 5 meters while each detector field Figure 3. Color shaded relief terrain model derived from a NAC
cameras built using the of view has diameter of 20 meters. The spots are 25 meters DEM mosaic of 13 stereo pairs of Lichtenberg Crater. The
rray is a 5064 element apart and form a cross pattern canted by 26° counter-clockwise enlargements display color shaded terrain, slope map, and
icron pixels. The two to provide five adjacent profiles (Figure 2) (Smith et al. 2010, orthophoto derived from the DEM mosaic.
t (NAC-L) and NAC- Zuber etal. 2010). The LOLA instrument boresight is aligned ;
| on the spacecraft such With the LROC NAC cameras to enable altimetry collection in To improve absolute accuracy between the images, LOLA
e spacecraft’s X-axis. the overlap region between NAC-L and NAC-R. By using altimetric profiles are used to define the geodetic reference
0.5 meter pixel scale Earth-based laser ranging tracking and crossover analysis the frame for the DEMs. The images are shifted in relation to their
swath cross-track, for a accuracy of the LOLA dataset is improved. original latitude, longitude, scale, elevation, and horizontal and
tude of 50 km. Each vertical rotation, using a script in MATLAB, in order to better
allowing for an image fit the LOLA data.
it the native resolution.
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