Narrow Angle Camera Research Articles

Morphological analysis and mapping of complex craters of Copernican age: Crookes, Ohm and Stevinus

The aim of this study is to analyze and compare morphological characters of three complex lunar impact craters- Crookes (10.3° S, 164.5° W), Ohm (18.4° N, 113.5° W) and Stevinus (32.5° S, 54.2° E), where Crookes and Ohm are located on the far side of the Moon and Stevinus is located on the near side of the Moon. High resolution data of Narrow Angle Camera (NAC) and Wide Angle Camera (WAC), onboard Lunar Reconnaissance Orbiter (LRO) of NASA is used for the morphological observations of all three craters. In this study we identify and compare various geomorphic features of three craters including central peak, central mounds, isolated mounds, diverse floor units: Hummocky, Intermediate and Smooth, melt ponds and cooling cracks. Using ArcGIS software, morphological maps of all three craters are generated where special attention is given to the crater floors, with NAC and WAC data as base-maps. Lunar Orbiter Laser Altimeter (LOLA) Digital Elevation Model (118 ​m) is used to generate elevation graphs for all three craters along four different topographic sections using which, relative elevation measurements of central peak, central mounds and raised crater rims and thickness of wall terraces are obtained. Using the ENvironment for Visualising Images (ENVI) software, 3-dimensional surface view models were generated. These 3D models give enhanced view of ‘highs’ and ‘lows’ of impact craters along with ponded surfaces trapped in between wall-terraces. All three craters are oblique impact craters with varying degrees of impact angles. Crookes exhibits later stage modification characters with an impact direction NNE and an impact angle between 15° and 45°. Ohm exhibits elongated crater planform with zone of avoidance in NNE direction. The impact angle is between 30° and 45°. Stevinus represents an impact angle higher than 15° and lower than 40°. The study reveals that there are striking similarities in the morphological features of these three craters of the same age along with few distinct characters of their own.

An integrated photogrammetric and photoclinometric approach for illumination-invariant pixel-resolution 3D mapping of the lunar surface

High-resolution 3D information on lunar and planetary surfaces is crucial for planetary exploration missions and science. Photogrammetry is a state-of-the-art technology for generating 3D topographic models of surfaces such as digital elevation models (DEMs). The performance of the photogrammetry and the resulting DEMs is affected by image matching. However, most matching algorithms fail when the images have large differences in illumination and subtle textures. This problem can be addressed by integrating photoclinometry into the photogrammetric process. This paper presents an integrated photogrammetric and photoclinometric approach that is able to generate pixel-resolution DEMs of the lunar surface and is illumination invariant. The incorporation of photoclinometry into photogrammetry involves two main steps. First, a photoclinometry assisted image matching (PAM) approach is developed by integrating photometric stereo analysis in the image matching to create pixel-wise matches, even for images with large illumination differences. Second, the DEM derived from photogrammetry using the matching results is further refined to pixel-wise resolution using photoclinometry with a shadow constraint. The proposed approach has been used for high-resolution topographic mapping at the Chang’E-4 and Chang’E-5 landing sites using Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images acquired under different illumination conditions. The results indicate that the proposed approach is robust to severe inconsistencies in illumination and subtle textures in cases where the conventional approaches fail. The approach is able to achieve geometric accuracies comparable to photogrammetry but more small-scale topographic details. The proposed approach can also be used for high-resolution topographic mapping of other planetary bodies such as Mercury or asteroids, and provides a useful reference for similar topographic mapping on Earth.

Boulder Distributions Around Young, Small Lunar Impact Craters and Implications for Regolith Production Rates and Landing Site Safety

AbstractWe use Lunar Reconnaissance Orbiter Camera Narrow Angle Camera images to characterize boulder populations around six small (<1 km), young (<200 Ma) impact craters near spacecraft landing sites. The Narrow Angle Camera boulder counts are used to analyze how boulder distributions vary around craters of different sizes and ages. These comparisons inform how various properties affect the distance to which boulders are ejected and the size and density of boulders produced by an impact event. The counts show that boulder population densities decrease with crater age, with few boulders remaining at craters older than a few hundred million years, consistent with results of other studies of boulder degradation rates on the Moon. Variations in boulder distributions around younger craters may provide information regarding impact conditions; South Ray crater has a larger population of small boulders than the larger North Ray crater, which could be explained by variations in impact velocity. Large craters generally excavate more boulders than smaller craters, and the size of the largest boulder ejected is related to crater size by a power‐law function. Larger boulders occur closer to the crater rim (within 2–4 crater radii), whereas smaller boulders occur at all distances. The density of boulders is greater near the crater rim and decreases with increasing radial distance; this data can aid in establishing safe landing zones for future missions. Analyzing boulder distributions across craters of varying ages allows us to test models of boulder breakdown rates, with implications for understanding the Moon's regolith production rate.

Surface evolution of the Anhur region on comet 67P/Churyumov-Gerasimenko from high-resolution OSIRIS images

Context. The southern hemisphere of comet 67P/Churyumov-Gerasimenko (67P) became observable by the Rosetta mission in March 2015, a few months before cometary southern vernal equinox. The Anhur region in the southern part of the comet’s larger lobe was found to be highly eroded, enriched in volatiles, and highly active. Aims. We analyze high-resolution images of the Anhur region pre- and post-perihelion acquired by the OSIRIS imaging system on board the Rosetta mission. The Narrow Angle Camera is particularly useful for studying the evolution in Anhur in terms of morphological changes and color variations. Methods. Radiance factor images processed by the OSIRIS pipeline were coregistered, reprojected onto the 3D shape model of the comet, and corrected for the illumination conditions. Results. We find a number of morphological changes in the Anhur region that are related to formation of new scarps; removal of dust coatings; localized resurfacing in some areas, including boulders displacements; and vanishing structures, which implies localized mass loss that we estimate to be higher than 50 million kg. The strongest changes took place in and nearby the Anhur canyon-like structure, where significant dust cover was removed, an entire structure vanished, and many boulders were rearranged. All such changes are potentially associated with one of the most intense outbursts registered by Rosetta during its observations, which occurred one day before perihelion passage. Moreover, in the niche at the foot of a new observed scarp, we also see evidence of water ice exposure that persisted for at least six months. The abundance of water ice, evaluated from a linear mixing model, is relatively high (>20%). Our results confirm that the Anhur region is volatile-rich and probably is the area on 67P with the most pristine exposures near perihelion.

Spectrophotometric variegation of the layering in comet 67P/Churyumov-Gerasimenko as seen by OSIRIS

Context.Between August 2014 and September 2016, the ESA space mission Rosetta escorted comet 67P/Churyumov-Gerasimenko (67P) during its perihelion passage. The onboard OSIRIS Narrow Angle Camera (NAC) acquired high-resolution multispectral images of the cometary surface. These datasets allowed a characterization of the spectrophotometric variegation of the layering of the large lobe, correlated with the layer structural elevation.Aims.We perform a spectrophotometric characterization of the outcropping stratification of the small lobe of 67P as a function of its structural elevation, and consequently, a spectrophotometric comparison of the layered outcrops of the two lobes.Methods.We selected two sequences of post-perihelion OSIRIS NAC images (~2.4 au outbound tothe Sun), from which we built up two multispectral images, framing an extended geological section of the onion-like stratification of the small lobe. Then we classified the consolidated areas of the outcropping and the relative coarse deposits that were identified in the multispectral data with a two-class maximum likelihood method. For this, we defined the classes as a function of the structural elevation of the surface.Results.As a result, we identified a brightness variegation of the surface reflectance that is correlated with the structural elevation. The outer class, which is located at higher elevations, appears darker than the inner class. This fits previously obtained results for the large lobe. The reflectance values of the nucleus of 67P tend to decrease with increasing structural elevation.Conclusions.The observed spectrophotometric variegation can be due to a different texture as well as to a different content of volatiles and refractories. We suggest that the outer outcrops appear darker because they have been exposed longer, and the inner outcrops appear brighter because the surface has been more effectively rejuvenated. We interpret this variegation as the result of an evolutionary process.

Quantitative analysis of isolated boulder fields on comet 67P/Churyumov-Gerasimenko

Aims.We provide a detailed quantitative analysis of isolated boulder fields situated in three different regions of comet 67P/Churyumov-Gerasimenko: Imhotep, Hapi, and Hatmehit. This is done to supply a useful method for analyzing the morphology of the boulders and to characterize the regions themselves.Methods.We used OSIRIS Narrow Angle Camera images with a spatial scale smaller than 2 m px−1and analyzed the size-frequency distribution and the cumulative fractional area per boulder population. In addition, we correlated shape parameters, such as circularity and solidity, with both the spatial and the size-frequency distribution of the three populations.Results.We identified 11 811 boulders in the Imhotep, Hapi, and Hatmehit regions. We found that the Hatmehit and Imhotep areas show power indices in the range of −2.3/−2.7. These values could represent a transition between gravitational events caused by thermal weathering and sublimation, and material formed during collapses that has undergone sublimation. The Hapi area is characterized by a lower power index (−1.2/−1.7), suggesting that those boulders have a different origin. They can be the result of material formed during gravitational events and collapses that has undergone continuous fragmentation. We calculated the cumulative fractional area (CFA) in order to investigate how the area is covered by boulders as a function of their sizes. The Hatmehit and Imhotep regions show a CFA that is well fit by a power law. In contrast, the Hapi area does not show the same trend. We analyzed the fractal distributions, finding that the populations seem to be fractal at all dimensions, except for the Hapi distribution, which shows a possible fractal behavior for small dimensions only. Finally, the average values of the shape parameters reveal solid and roundish boulders in all populations we studied.

High-Resolution Large-Area Digital Orthophoto Map Generation Using LROC NAC Images

The Chang'e-5 mission of China is planned to be launched in 2019 to the landing area near Mons Rümker located in Oceanus Procellarum. Aiming to generate a high-resolution and high-quality digital orthophoto map (DOM) of the planned landing area for supporting the mission and various scientific analyses, this study developed a systematic and effective method for large-area seamless DOM production. The mapping results of the Chang'e-5 landing area using over 700 Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images are presented. The resultant seamless DOM has a resolution of 1.5 m, covers a large area of 20° in longitude and 4° in latitude, and is tied to SLDEM2015. The results demonstrate that the proposed method can reduce the geometric inconsistencies among the LROC NAC images to the subpixel level and the positional errors with respect to the reference digital elevation model to about one grid cell size.

A GENERIC RIGOROUS SENSOR MODEL FOR PHOTOGRAMMETRIC PROCESSING OF PUSHBROOM PLANETARY IMAGES

Abstract. Currently, each planetary exploration mission team always develops its own software modules to support the photogrammetric processing of planetary images, and as a result of that the main drawbacks are lacking software reusability and the high cost of software development and maintenance. This is mainly due to that there is lack of a highly universal sensor model in the planetary mapping community. This paper presents a generic rigorous sensor model (RSM) for the photogrammetric processing of pushbroom planetary images. The main contributions of this paper include: (1) the implementation details of the generic RSM; (2) the optimized coordinates transformation methods between 3D ground points and 2D image points for linear pushbroom images; (3) a pipeline to acquire exterior orientation (EO) parameters for each planetary image. The generic RSM is developed based on the methodology used in airborne linear scanners ADS40. Specifically, the generic RSM comprises of a camera file and an orientation data file for each image. The camera file stores each detector’s calibrated image coordinates and the orientation data file contains each scan line’s EO parameters, such that the RSM can perform coordinates transformation among pixel coordinates, focal plane coordinates and ground coordinates. Furthermore, the generic RSM supports varying exposure time, summing mode and image distortions, which are typical problems that need to be solved in planetary mapping. We tested the generic RSM with Lunar Reconnaissance Orbiter (LRO) Narrow Angle Camera (NAC), Chandrayaan-1 Moon Mineralogy Mapper (M3) and Mars Express (MEX) High Resolution Stereo Camera (HRSC) images. The geometric accuracy and computational efficiency of the developed generic RSM were compared with the famous planetary mapping software, namely Integrated System for Imagers and Spectrometers (ISIS). The experimental results demonstrate that the generic RSM has the merits of processing various types of pushbroom planetary images with a unified way and decreasing the software development and maintenance burden. Moreover, the developed generic RSM significantly improves the computational efficiency of orthophoto generation and tie points extraction for pushbroom planetary images.

BUNDLE ADJUSTMENT OF SPACEBORNE DOUBLE-CAMERA PUSH-BROOM IMAGERS AND ITS APPLICATION TO LROC NAC IMAGERY

Abstract. The TU Berlin group of the Lunar Reconnaissance Orbiter Camera (LROC) team has implemented a Bundle Adjustment (BA) for spaceborne multi-lenses line scan imagers, by rigorously modeling the geometric properties of the image acquisition. The BA was applied to stereo image sets of the LROC Narrow Angle Camera (NAC) and first results show, that the overall geometry of the stereo models were significantly improved. Ray intersection accuracies of initially up to several meters were homogenized within the integrated stereo models and improved to 0.14 m on average. The mean point error of the adjusted 3D object points was estimated by the BA to be 0.95 m. The inclusion of available Lunar Orbiter Laser Altimeter (LOLA) shots as 3D ground control to the BA, accurately tied to the image space by an aforegoing co-registration, allowed to register the final adjusted NAC DTM to the currently most accurate global lunar reference frame. The BA also provides accuracy assessments of the individual LOLA tracks used for georeference during the adjustment, which will be useful to further assess LOLA derived products.

TOPOGRAPHIC ANALYSIS OF CHANG’E-4 LANDING SITE USING ORBITAL, DESCENT AND GROUND DATA

Abstract. The Chang’e-4 (CE-4) probe, which includes a lander and a rover, was successfully landed in Von Kármán crater inside the South Pole-Aitken basin on January 3, 2019. Landing site mapping and topographic analyses have been performed at different scales using orbital, descent and ground data to support mission operations and various scientific investigations. We produced a 0.9 m-resolution seamless digital orthophoto map (DOM) mosaic of the CE-4 landing site region covering 51 km × 30 km using 100 Lunar Reconnaissance Orbiter Camera (LROC) Narrow-Angle Camera (NAC) images. Within an area of 3.2 km × 3.0 km of the the DOM around the lander, more than 11,000 craters with diameter greater than 5 m have been identified and measured. A 0.03 m- resolution DOM, which covers an area of 211 m × 187 m, was generated using descent images, and has been used as one of the base maps for overall rover traverse planning. During surface operations, local DEMs with 0.02 m resolution are routinely produced and topographic analysis have been performed at each waypoint using Navcam images and at some locations using Pancam images, to support waypoint-to-waypoint path planning, science target selection and scientific investigations.

Automated Detection of Lunar Rockfalls Using a Convolutional Neural Network

This paper implements a novel approach to automatically detect and classify rockfalls in Lunar Reconnaissance Orbiter narrow angle camera (NAC) images using a single-stage dense object detector (RetinaNet). The convolutional neural network has been trained with a data set of 2932 original rockfall images. In order to avoid overfitting, the initial training data set has been augmented during training using random image rotation, scaling, and flipping. Testing images have been labelled by human operators and have been used for RetinaNet performance evaluation. Testing shows that RetinaNet is capable to reach recall values between 0.98 and 0.39, precision values between 1 and 0.25, and average precisions ranging from 0.89 to 0.69, depending on the used confidence threshold and intersection-over-union values. Mean processing time of a single NAC image in RetinaNet is around 10 s using a GeForce GTX 1080 Ti and GeForce Titan Xp, which is in orders of magnitudes faster than a human operator. The processing speed allows to efficiently exploit the currently available NAC data stack with more than 1 million images in a reasonable timeframe. The combination of speed and detection performance can be used to produce lunar rockfall distribution maps on large spatial scales for utilization by the scientific and engineering community.

Analysis of Lunar Boulder Tracks: Implications for Trafficability of Pyroclastic Deposits

AbstractIn a new era of lunar exploration, pyroclastic deposits have been identified as valuable targets for resource utilization and scientific inquiry. Little is understood about the geomechanical properties and the trafficability of the surface material in these areas, which is essential for successful mission planning and execution. Past incidents with rovers highlight the importance of reliable information about surface properties for future, particularly robotic, lunar mission concepts. Characteristics of 149 boulder tracks are measured in Lunar Reconnaissance Orbiter Narrow Angle Camera images and used to derive the bearing capacity of pyroclastic deposits and, for comparison, mare and highland regions from the surface down to ~5‐m depth, as a measure of trafficability. Results are compared and complemented with bearing capacity values calculated from physical property data collected in situ during Apollo, Surveyor, and Lunokhod missions. Qualitative observations of tracks show no region‐dependent differences, further suggesting similar geomechanical properties in the regions. Generally, bearing capacity increases with depth and decreases with higher slope gradients, independent of the type of region. At depths of 0.19 to 5 m, pyroclastic materials have bearing capacities equal or higher than those of mare and highland material and, thus, may be equally trafficable at surface level. Calculated bearing capacities based on orbital observations are consistent with values derived using in situ data. Bearing capacity values are used to estimate wheel sinkage of rover concepts in pyroclastic deposits. This study's findings can be used in the context of traverse planning, rover design, and in situ extraction of lunar resources.

Young wrinkle ridges in Mare Imbrium: Evidence for very recent compressional tectonism

Wrinkle ridges are common landforms widely distributed in the lunar maria. Many young wrinkle ridges were found inside Mare Imbrium using Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) high-resolution images. They represent very recent tectonic activity on the lunar surface. Determining their formation times and morphology is important in understanding the Moon's late evolution. The young ridges were mapped, and they have a winding and arched morphology. Based on their small scale and crisp appearance, crosscutting relations with very young small craters whose youth was concluded from spatial boulder densities within their ejecta blankets, these young wrinkle ridges were estimated to be as young as 10 or a few 10s of Ma, and may still be active today. The young ridges were mainly distributed in the inner region of Imbrium basin, which may suggest that the thermal and structural evolution of the inside area of Imbrium basin was protracted.

High‐Resolution Terrain Analysis for Lander Safety Landing and Rover Path Planning Based on Lunar Reconnaissance Orbiter Narrow Angle Camera Images: A Case Study of China's Chang'e‐4 Probe

AbstractChina's Chang'e‐4 (CE‐4) probe will explore the South Pole‐Aitken basin in late 2018. Its preselected landing area is located on the southeastern floor of the Von Kármán crater. Landing experience of China's Chang'e‐3 probe may not be applied to CE‐4 mission directly because the topography of lunar farside is more rugged. Moreover, due to scale dependence and smooth effects, the previous topographic studies derived from digital elevation model data with lower resolution cannot represent the meter‐scale topographic features. Because of low time and space complexities in lunar images, image segmentation algorithm is especially suitable for recognizing lunar features. We divided the narrow angle camera (NAC) mosaic image into three parts: dark area, bright area, and flat area, based on a double‐threshold Otsu method. The first two parts corresponded to the undulating areas (positive and negative terrains). And then we calculated the flat area percentage (Fap) of the previous successful lunar landing missions (including Chang'e‐3, Apollo, Surveyor, and Luna series) and obtained the Fap threshold (>0.6) for lunar safe landing. According to Fap map (~1.5 m per pixel) of CE‐4 preselected landing area, the divided square grids with a size of 0.01° can be classified into safe grids (Fap > 0.6) and unsafe grids (Fap ≤ 0.6). The CE‐4 preselected landing areas can be greatly reduced to five potential landing areas. In the last, we proposed a route planning method, which took both the distance and security into account, for CE‐4 rover to dynamically generate a safe and short route between current grid and target grid.

Coordinates and Maps of the Apollo 17 Landing Site

AbstractWe carried out an extensive cartographic analysis of the Apollo 17 landing site and determined and mapped positions of the astronauts, their equipment, and lunar landmarks with accuracies of better than ±1 m in most cases. To determine coordinates in a lunar body‐fixed coordinate frame, we applied least squares (2‐D) network adjustments to angular measurements made in astronaut imagery (Hasselblad frames). The measured angular networks were accurately tied to lunar landmarks provided by a 0.5 m/pixel, controlled Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) orthomosaic of the entire Taurus‐Littrow Valley. Furthermore, by applying triangulation on measurements made in Hasselblad frames providing stereo views, we were able to relate individual instruments of the Apollo Lunar Surface Experiment Package (ALSEP) to specific features captured in LROC imagery and, also, to determine coordinates of astronaut equipment or other surface features not captured in the orbital images, for example, the deployed geophones and Explosive Packages (EPs) of the Lunar Seismic Profiling Experiment (LSPE) or the Lunar Roving Vehicle (LRV) at major sampling stops. Our results were integrated into a new LROC NAC‐based Apollo 17 Traverse Map and also used to generate a series of large‐scale maps of all nine traverse stations and of the ALSEP area. In addition, we provide crater measurements, profiles of the navigated traverse paths, and improved ranges of the sources and receivers of the active seismic experiment LSPE.

Astrometric reduction of Cassini ISS images of Helene

Helene is a small irregular Saturnian satellite. It is difficult to observe Helene by ground-based telescopes because of its weak lightness (18.5 mag) and proximity to Saturns rings. The number of historical observations of the satellite is small, but a large number of images of Helene were obtained between 2004 and 2017 using the Imaging Science Subsystem (ISS) in Cassini Spacecraft. In this paper, astrometry reduction was performed based on these observations. Specifically, we first corrected the initial pointing of the Narrow Angle Camera in ISS using the referencing Gaia DR2, and then measured the Helenes center position in image coordinates using the two-dimensional Gaussian fitting method. Finally the position was transformed to the Right Ascension ($\alpha$) and Declination ($\delta$) in International Celestial Reference Frame (ICRF) centered at Cassini. The result was compared with JPL SAT393 ephemerides, and the position residual was calculated. Totally 890 images were effectively measured. The results showed that compared with SAT393, the mean values of all residuals in $\alpha$$*$cos$(\delta)$ and $\delta$ directions were $0.23$ and $-0.63$ km, respectively, and the standard deviations were $3.48$ and $2.23$ km, respectively. This precision was much higher than that of ground observations, indicating the advantage of space-based observations.

Analysis of layering-related linear features on comet 67P/Churyumov–Gerasimenko

We analysed layering-related linear features on the surface of comet 67P/Churyumov-Gerasimenko (67P) to determine the internal configuration of the layerings within the nucleus. We used high-resolution images from the OSIRIS Narrow Angle Camera onboard the Rosetta spacecraft, projected onto the SHAP7 shape model of the nucleus, to map 171 layering-related linear features which we believe to represent terrace margins and strata heads. From these curved lineaments, extending laterally to up to 1925 m, we extrapolated the subsurface layering planes and their normals. We furthermore fitted the lineaments with concentric ellipsoidal shells, which we compared to the established shell model based on planar terrace features. Our analysis confirms that the layerings on the comet's two lobes are independent from each other. Our data is not compatible with 67P's lobes representing fragments of a much larger layered body. The geometry we determined for the layerings on both lobes supports a concentrically layered, `onion-shell' inner structure of the nucleus. For the big lobe, our results are in close agreement with the established model of a largely undisturbed, regular, concentric inner structure following a generally ellipsoidal configuration. For the small lobe, the parameters of our ellipsoidal shells differ significantly from the established model, suggesting that the internal structure of the small lobe cannot be unambiguously modelled by regular, concentric ellipsoids and could have suffered deformational or evolutional influences. A more complex model is required to represent the actual geometry of the layerings in the small lobe.

High-precision co-registration of orbiter imagery and digital elevation model constrained by both geometric and photometric information

The alignment of images to a reference digital elevation model (DEM) has many applications, especially in planetary exploration. In this paper, we propose a novel high-precision co-registration method for pixel-based matching between an image and a reference DEM to rectify the ever-increasing number of orbiter images to DEMs in a unified reference frame automatically. First, the DEM is converted to a simulated image using a hillshading technique based on a photometric model and the illumination conditions of the image. Then, an initial matching between the simulated and input images is performed based on affine scale-invariant feature transform (ASIFT). The rational function model (RFM) of the image is established and used as a geometric constraint. Then, with tie points (TPs) generated by the initial ASIFT matching, the RFM geometric model of the image is refined, and the gross errors in the TP set are iteratively eliminated. Finally, a high-precision co-registration by pixel-based Least-Squares (LS) image matching is performed using the refined geometric model. Lunar Reconnaissance Orbiter (LRO) Narrow Angle Camera (NAC) and SLDEM2015 (a combined product of Lunar Reconnaissance Orbiter Laser Altimeter (LOLA) and a DEM generated by the Japanese Selenological and Engineering Explorer (SELENE) terrain camera) images are used in our experiment. Results using 17 NAC images at different latitudes demonstrate that the proposed co-registration method is effective and can attain an accuracy of 18.3 pixels in the image space and 25.17 m (0.5 pixel of the reference DEM) in the object space. The method offers an automatic and high-precision matching of LRO NAC images to SLDEM2015. It is also applicable to the co-registration of other orbital images to a reference DEM.

Analysis of Rock Abundance on Lunar Surface From Orbital and Descent Images Using Automatic Rock Detection

AbstractInformation on the distribution and abundance of rocks on the lunar surface is essential for understanding the characteristics of lunar regolith and for selecting suitable landing sites for lunar exploration missions. This paper first presents a novel automatic rock detection approach based on the gradient differences in illumination direction. Multiple‐source images covering different regions of the lunar surface are then used for rock detection and rock abundance analysis, including the Chang'E‐3 descent images (0.02–0.17 m/pixel) around the landing area, the Lunar Reconnaissance Orbiter (LRO) Narrow Angle Camera images (0.3–1.1 m/pixel) at the landing sites of the Chang'E‐3, Luna 17, and Luna 23, and the LRO Narrow Angle Camera image mosaic (1.5 m/pixel) in the Oceanus Procellarum area. A rock abundance model using an exponential form is derived from the results of the abovementioned analysis to describe the overall cumulative fractional area of rocks versus their diameters. The rock detection results and the derived rock abundance estimated by the proposed model are compared with the ground measurements obtained using the rover images of the Chang'E‐3 landing site and the LRO Diviner Radiometer‐derived rock abundance. A comparison analysis indicates that the derived rock abundance model can feasibly represent the rock abundance in various situations on local and large scales. In general, a remarkable consistency is observed between our results and the results obtained using the ground measurements at the Chang'E‐3 landing site and the orbit measurements from the Diviner radiometer, while our rock detection results and rock abundance model exhibit better performance in presenting detailed information in local areas. Further analyses on rock abundance and the crater morphology and other crater characteristics in the Oceanus Procellarum area indicate that it is feasible to use rock abundance information to estimate the surface maturity.

AN INTEGRATED PHOTOGRAMMETRIC AND PHOTOCLINOMETRIC APPROACH FOR PIXEL-RESOLUTION 3D MODELLING OF LUNAR SURFACE

Abstract. High-resolution 3D modelling of lunar surface is important for lunar scientific research and exploration missions. Photogrammetry is known for 3D mapping and modelling from a pair of stereo images based on dense image matching. However dense matching may fail in poorly textured areas and in situations when the image pair has large illumination differences. As a result, the actual achievable spatial resolution of the 3D model from photogrammetry is limited by the performance of dense image matching. On the other hand, photoclinometry (i.e., shape from shading) is characterised by its ability to recover pixel-wise surface shapes based on image intensity and imaging conditions such as illumination and viewing directions. More robust shape reconstruction through photoclinometry can be achieved by incorporating images acquired under different illumination conditions (i.e., photometric stereo). Introducing photoclinometry into photogrammetric processing can therefore effectively increase the achievable resolution of the mapping result while maintaining its overall accuracy. This research presents an integrated photogrammetric and photoclinometric approach for pixel-resolution 3D modelling of the lunar surface. First, photoclinometry is interacted with stereo image matching to create robust and spatially well distributed dense conjugate points. Then, based on the 3D point cloud derived from photogrammetric processing of the dense conjugate points, photoclinometry is further introduced to derive the 3D positions of the unmatched points and to refine the final point cloud. The approach is able to produce one 3D point for each image pixel within the overlapping area of the stereo pair so that to obtain pixel-resolution 3D models. Experiments using the Lunar Reconnaissance Orbiter Camera - Narrow Angle Camera (LROC NAC) images show the superior performances of the approach compared with traditional photogrammetric technique. The results and findings from this research contribute to optimal exploitation of image information for high-resolution 3D modelling of the lunar surface, which is of significance for the advancement of lunar and planetary mapping.