Chang'E-1 Research Articles

GEOPOSITIONING PRECISION ANALYSIS OF MULTIPLE IMAGE TRIANGULATION USING LRO NAC LUNAR IMAGES

This paper presents an empirical analysis of the geopositioning precision of multiple image triangulation using Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images at the Chang’e-3(CE-3) landing site. Nine LROC NAC images are selected for comparative analysis of geopositioning precision. Rigorous sensor models of the images are established based on collinearity equations with interior and exterior orientation elements retrieved from the corresponding SPICE kernels. Rational polynomial coefficients (RPCs) of each image are derived by least squares fitting using vast number of virtual control points generated according to rigorous sensor models. Experiments of different combinations of images are performed for comparisons. The results demonstrate that the plane coordinates can achieve a precision of 0.54 m to 2.54 m, with a height precision of 0.71 m to 8.16 m when only two images are used for three-dimensional triangulation. There is a general trend that the geopositioning precision, especially the height precision, is improved with the convergent angle of the two images increasing from several degrees to about 50°. However, the image matching precision should also be taken into consideration when choosing image pairs for triangulation. The precisions of using all the 9 images are 0.60 m, 0.50 m, 1.23 m in along-track, cross-track, and height directions, which are better than most combinations of two or more images. However, triangulation with selected fewer images could produce better precision than that using all the images.

Qualitative Verification of CE-2's Microwave Measurement: Relative Calibration Based on Brightness Temperature Model and Data Fusion

The calibration quality of the Chang'E-2 (CE-2) microwave radiometer (MRM) and the accuracy of brightness temperature (TB) data should be adequately addressed. The quality of CE-2 MRM data can be demonstrated by the analysis and simulation of TB data with the microwave transfer model and by the relative comparison between the TB and other data sets. To calibrate the TB data in a relative way, we calculated the variation of TB with respect to the latitude over certain typical regions theoretically. Three types of areas in the Moon, including the old crater, the fresh crater, and the Apollo region, are chosen as examples. A detailed method incorporating the topographic effect is employed to compute the TB. The Lunar Reconnaissance Orbiter's Diviner infrared measurement is used to ensure the accuracy of the simulated physical temperature. The consistency of the TB undulation between simulations and observations along the profile of these regions verifies the reasonability of CE-2's TB data partly. Our simulation also confirms the topographic effect of sloping walls at the crater rims on the solar heat flux received per unit surface area and reproduces the TB undulation observed by CE-2. Rock abundance and topography data are used for qualitative comparison with the CE-2 TB data. General similarity between the diurnal TB difference and the altitude profile over most craters and the similarity between the diurnal TB difference and the rock abundance profile over fresh craters can prove the relative qualification of CE-2 MRM data to a certain degree.

Determination of the Earth's plasmapause location from the CE‐3 EUVC images

AbstractThe Moon‐based Extreme Ultraviolet Camera (EUVC) aboard China's Chang'e‐3 (CE‐3) mission has successfully imaged the entire Earth's plasmasphere for the first time from the side views on lunar surface. An EUVC image on 21 April 2014 is used in this study to demonstrate the characteristics and configurations of the Moon‐based EUV imaging and to illustrate the determination algorithm of the plasmapause locations on the magnetic equator. The plasmapause locations determined from all the available EUVC images with the Minimum L Algorithm are quantitatively compared with those extracted from in situ observations (Defense Meteorological Satellite Program, Time History of Events and Macroscale Interactions during Substorms, and Radiation Belt Storm Probes). Excellent agreement between the determined plasmapauses seen by EUVC and the extracted ones from other satellites indicates the reliability of the Moon‐based EUVC images as well as the determination algorithm. This preliminary study provides an important basis for future investigation of the dynamics of the plasmasphere with the Moon‐based EUVC imaging.

Correlated compositional and mineralogical investigations at the Chang′e-3 landing site

The chemical compositions of relatively young mare lava flows have implications for the late volcanism on the Moon. Here we report the composition of soil along the rim of a 450-m diameter fresh crater at the Chang′e-3 (CE-3) landing site, investigated by the Yutu rover with in situ APXS (Active Particle-induced X-ray Spectrometer) and VNIS (Visible and Near-infrared Imaging Spectrometer) measurements. Results indicate that this region's composition differs from other mare sample-return sites and is a new type of mare basalt not previously sampled, but consistent with remote sensing. The CE-3 regolith derived from olivine-normative basaltic rocks with high FeO/(FeO+MgO). Deconvolution of the VNIS data indicates abundant high-Ca ferropyroxene (augite and pigeonite) plus Fe-rich olivine. We infer from the regolith composition that the basaltic source rocks formed during late-stage magma-ocean differentiation when dense ferropyroxene-ilmenite cumulates sank and mixed with deeper, relatively ferroan olivine and orthopyroxene in a hybridized mantle source.

Photometric correction and reflectance calculation for lunar images from the Chang'E-1 CCD stereo camera.

Photometric correction and reflectance calculation are two important processes in the scientific analysis and application of Chang'E-1 (CE-1) charge-coupled device (CCD) stereo camera data. In this paper, the methods of photometric correction and reflectance calculation were developed. On the one hand, in considering the specificity of datasets acquired by the CE-1 CCD stereo camera, photometric correction was conducted based on the digital number value directly using the revised Lommel-Seeliger factor. On the other hand, on the basis of laboratory-measured bidirectional reflectances, the relative reflectance was then calculated using the empirical linear model. The presented approach can be used to identify landing sites, obtain global images, and produce topographic maps of the lunar surface.

The Processing and Analysis of Lunar Penetrating Radar Channel-1 Data from Chang'E-3

Lunar Penetrating Radar (LPR), which is one of the most important science payloads onboard the Chang'E-3 (CE-3) rover, is used to obtain electromagnetic image less than 100 m beneath the lunar surface. This paper describes the system composition and working mechanism of the LPR and presents a detailed analysis of its data. We investigated special signal-processing methods and present the result of channel-1 data. The result shows that the effective echo occurs at depths greater than 100 m. Moreover, an unusual reflection exists at depth of 40 m, which may be the boundary of two geological units beneath the lunar surface.

Orbit optimization of Chang’E-2 by global adjustment using images of the moon

The orbit accuracy of the Chang’E-2 (CE-2) lunar probe is one of the most critical factors for a seamless mosaic of the global lunar topographic map. During the production of the CE-2 global lunar topographic map, a maximum deviation of kilometers magnitude existed in the horizontal direction of homologous points between neighboring images, while the maximum height deviation of these points is up to several hundred meters. This phenomenon indicates that current orbit determination results of CE-2 cannot truly reflect the relative position relationship between probe and lunar surface features. Against this background, global adjustment using images of the moon should be carried out to solve this problem. In this paper, the influence of CE-2 current orbit accuracy on the production of a global lunar topographic map will be analyzed based on the introduction of CE-2 observation data, including images and orbit data. Additionally, key technologies and technical processes of global adjustment using CE-2 images with high-resolution and large amounts of data will be researched. Finally, orbit optimization of CE-2 after global adjustment is analyzed, as well as the accuracy of the CE-2 global lunar topographic map for validation verification.

Estimation of lunar major elemental abundances in Chang’E-3 landing site based on Active Particle-induced X-ray Spectrometer (APXS)

Elemental abundance provides an effective vehicle to understand lunar petrologic characteristics and evolutional history. The APXS mounted on the Yutu rover provides a valuable opportunity to determine the major elemental abundances in lunar soil within a short distance. In this study, we processed the APXS spectra including energy calibration, dead time correction and nonlinear least-squares fitting, and determined the abundances of the lunar major elements using the fundamental parameter method. In the calculation of X-ray fluorescence yield, a finite element method (FEM) was employed to improve the accuracy. The major elemental abundances derived from Chang’E-3 (CE-3) APXS possess a good consistency with the result of LP-GRS (Lunar Prospector gamma-ray spectrometer) data in the landing region. Compared with the chemical composition of the returned lunar rock samples, we draw the conclusion that the lunar soils in CE-3 landing site are fragments of mare basalts. Our conclusion is supported by the geological map of Mare Imbrium.

Contribution of simulated space VLBI to the Chang'E-1 orbit determination and EOPs estimation

Space very long baseline interferometry (SVLBI) is an extension of ground based VLBI to space, which has advantages, such as improving the precision and geometry structure of time delay observables with interconnecting multiple spatial coordinate systems directly. In this paper, a mathematical model of relativistic simulated SVLBI observables for estimating Chang'E-1 (CE-1) transfer orbit and Earth orientation Parameters (EOPs) is derived and discussed. A comparison of parameter estimation precision between ground ΔVLBI measurements and simulated SVLBI observables is carried out to verify the contribution of SVLBI. The optimal observation condition of CE-1 for SVLBI simulated observables is determined based on the analysis of parameter estimation results under CE-1 current observation condition. By using simulated SVLBI time delay observables under the optimal observation condition, the estimation precision of CE-1 orbit can achieve a level of 2 m. On the other side, the precision of some EOPs components can be improved when compared with their predicted values by fixed remaining components as known values. The method discussed in this paper provides a new attempt of deep space probe orbit determination and EOPs estimation.

Setting Diverging Colors for a Large-Scale Hypsometric Lunar Map Based on Entropy

A hypsometric map is a type of map used to represent topographic characteristics by filling different map areas with diverging colors. The setting of appropriate diverging colors is essential for the map to reveal topographic details. When lunar real environmental exploration programs are performed, large-scale hypsometric maps with a high resolution and greater topographic detail are helpful. Compared to the situation on Earth, fewer lunar exploration objects are available, and the topographic waviness is smaller at a large scale, indicating that presenting the topographic details using traditional hypsometric map-making methods may be difficult. To solve this problem, we employed the Chang’E2 (CE2) topographic and imagery data with a resolution of 7 m and developed a new hypsometric map-making method by setting the diverging colors based on information entropy. The resulting map showed that this method is suitable for presenting the topographic details and might be useful for developing a better understanding of the environment of the lunar surface.

Injected transfer orbit determination of lunar probe Chang’E 5T1 using short-arc rocket GPS measurements

The injected transfer orbit of lunar probe Chang’E 5T1 (CE-5T1) is determined immediately after the probe separates from its launcher. As the first orbit in the lunar flight, the CE-5T1 injected transfer orbit is crucial to the consequence of rocket vehicle launch mission and the probe’s subsequent midway orbital manoeuvre. In this paper, we discuss the problem of using rocket GPS measurements to determine the probe velocity increment due to mechanical separation, and subsequently the injected transfer orbit determination of CE-5T1. Motivated by the post-mission analysis of lunar probe Chang’E 3 (CE-3), we give theoretical evidence to explain the physical phenomenon of semi-major axis sudden change at the probe separation instant through the derivation of the Vis-Viva equation. In succession, we focus on the description of the procedure used for the orbit determination performed on separated arcs of rocket GPS measurements through the use of momentum conservation to determine the probe separation velocity. Finally, actual flight data of the CE-3 and CE-5T1 missions are used for the validation.

Difference of brightness temperatures between 19.35 GHz and 37.0 GHz in CHANG’E-1 MRM: implications for the burial of shallow bedrock at lunar low latitude

Indications of buried lunar bedrock may help us to understand the tectonic evolution of the Moon and provide some clues for formation of lunar regolith. So far, the information on distribution and burial depth of lunar bedrock is far from sufficient. Due to good penetration ability, microwave radiation can be a potential tool to ameliorate this problem. Here, a novel method to estimate the burial depth of lunar bedrock is presented using microwave data from Chang’E-1 (CE-1) lunar satellite. The method is based on the spatial variation of differences in brightness temperatures between 19.35 GHz and 37.0 GHz (ΔTB). Large differences are found in some regions, such as the southwest edge of Oceanus Procellarum, the area between Mare Tranquillitatis and Mare Nectaris, and the highland east of Mare Smythii. Interestingly, a large change of elevation is found in the corresponding region, which might imply a shallow burial depth of lunar bedrock. To verify this deduction, a theoretical model is derived to calculate the ΔTB. Results show that ΔTB varies from 12.7 K to 15 K when the burial depth of bedrock changes from 1 m to 0.5 m in the equatorial region. Based on the available data at low lunar latitude (30°N-30°S), it is thus inferred that the southwest edge of Oceanus Procellarum, the area between Mare Tranquillitatis and Mare Nectaris, the highland located east of Mare Smythii, the edge of Pasteur and Chaplygin are the areas with shallow bedrock, the burial depth is estimated between 0.5 m and 1 m.

The Chang’e 3 Mission Overview

The Chang’e 3 (CE-3) mission was implemented as the first lander/rover mission of the Chinese Lunar Exploration Program (CLEP). After its successful launch at 01:30 local time on December 2, 2013, CE-3 was inserted into an eccentric polar lunar orbit on December 6, and landed to the east of a 430 m crater in northwestern Mare Imbrium (19.51°W, 44.12°N) at 21:11 on December 14, 2013. The Yutu rover separated from the lander at 04:35, December 15, and traversed for a total of 0.114 km. Acquisition of science data began during the descent of the lander and will continue for 12 months during the nominal mission. The CE-3 lander and rover each carry four science instruments. Instruments on the lander are: Landing Camera (LCAM), Terrain Camera (TCAM), Extreme Ultraviolet Camera (EUVC), and Moon-based Ultraviolet Telescope (MUVT). The four instruments on the rover are: Panoramic Camera (PCAM), VIS-NIR Imaging Spectrometer (VNIS), Active Particle induced X-ray Spectrometer (APXS), and Lunar Penetrating Radar (LPR). The science objectives of the CE-3 mission include: (1) investigation of the morphological features and geological structures of and near the landing area; (2) integrated in-situ analysis of mineral and chemical composition of and near the landing area; and (3) exploration of the terrestrial-lunar space environment and lunar-based astronomical observations. This paper describes the CE-3 objectives and measurements that address the science objectives outlined by the Comprehensive Demonstration Report of Phase II of CLEP. The CE-3 team has archived the initial science data, and we describe data accessibility by the science community.

The Chang’e-3 lander and rover mission to the Moon

The Chang’e-3 (CE-3) lander and rover mission to the Moon was an intermediate step in China’s lunar exploration program, which will be followed by a sample return mission. The lander was equipped with a number of remote-sensing instruments including a pair of cameras (Landing Camera and Terrain Camera) for recording the landing process and surveying terrain, an extreme ultraviolet camera for monitoring activities in the Earth’s plasmasphere, and a first-ever Moon-based ultraviolet telescope for astronomical observations. The Yutu rover successfully carried out close-up observations with the Panoramic Camera, mineralogical investigations with the VIS-NIR Imaging Spectrometer, study of elemental abundances with the Active Particle induced X-ray Spectrometer, and pioneering measurements of the lunar subsurface with Lunar Penetrating Radar. This special issue provides a collection of key information on the instrumental designs, calibration methods and data processing procedures used by these experiments with a perspective of facilitating further analyses of scientific data from CE-3 in preparation for future missions.

Reconstructing the landing trajectory of the CE-3 lunar probe by using images from the landing camera

An accurate determination of the landing trajectory of Chang'e-3 (CE-3) is significant for verifying orbital control strategy, optimizing orbital planning, accurately determining the landing site of CE-3 and analyzing the geological background of the landing site. Due to complexities involved in the landing process, there are some differences between the planned trajectory and the actual trajectory of CE-3. The landing camera on CE-3 recorded a sequence of the landing process with a frequency of 10 frames per second. These images recorded by the landing camera and high-resolution images of the lunar surface are utilized to calculate the position of the probe, so as to reconstruct its precise trajectory. This paper proposes using the method of trajectory reconstruction by Single Image Space Resection to make a detailed study of the hovering stage at a height of 100 m above the lunar surface. Analysis of the data shows that the closer CE-3 came to the lunar surface, the higher the spatial resolution of images that were acquired became, and the more accurately the horizontal and vertical position of CE-3 could be determined. The horizontal and vertical accuracies were 7.09 m and 4.27 m respectively during the hovering stage at a height of 100.02 m. The reconstructed trajectory can reflect the change in CE-3's position during the powered descent process. A slight movement in CE-3 during the hovering stage is also clearly demonstrated. These results will provide a basis for analysis of orbit control strategy, and it will be conducive to adjustment and optimization of orbit control strategy in follow-up missions.

A new lunar absolute control point: established by images from the landing camera on Chang'e-3

The establishment of a lunar control network is one of the core tasks in selenodesy, in which defining an absolute control point on the Moon is the most important step. However, up to now, the number of absolute control points has been very sparse. These absolute control points have mainly been lunar laser ranging retroreflectors, whose geographical location can be observed by observations on Earth and also identified in high resolution lunar satellite images. The Chang'e-3 (CE-3) probe successfully landed on the Moon, and its geographical location has been monitored by an observing station on Earth. Since its positional accuracy is expected to reach the meter level, the CE-3 landing site can become a new high precision absolute control point. We use a sequence of images taken from the landing camera, as well as satellite images taken by CE-1 and CE-2, to identify the location of the CE-3 lander. With its geographical location known, the CE-3 landing site can be established as a new absolute control point, which will effectively expand the current area of the lunar absolute control network by 22%, and can greatly facilitate future research in the field of lunar surveying and mapping, as well as selenodesy.

Analysis of the geomorphology surrounding the Chang'e-3 landing site

Chang'e-3 (CE-3) landed on the Mare Imbrium basin in the east part of Sinus Iridum (19.51°W, 44.12°N), which was China's first soft landing on the Moon and it started collecting data on the lunar surface environment. To better understand the environment of this region, this paper utilizes the available high-resolution topography data, image data and geological data to carry out a detailed analysis and research on the area surrounding the landing site (Sinus Iridum and 45 km×70 km of the landing area) as well as on the topography, landform, geology and lunar dust of the area surrounding the landing site. A general topographic analysis of the surrounding area is based on a digital elevation model and digital elevation model data acquired by Chang'e-2 that have high resolution; the geology analysis is based on lunar geological data published by USGS; the study on topographic factors and distribution of craters and rocks in the surrounding area covering 4 km×4 km or even smaller is based on images from the CE-3 landing camera and images from the topographic camera; an analysis is done of the effect of the CE-3 engine plume on the lunar surface by comparing images before and after the landing using data from the landing camera. A comprehensive analysis of the results shows that the landing site and its surrounding area are identified as typical lunar mare with flat topography. They are suitable for maneuvers by the rover, and are rich in geological phenomena and scientific targets, making it an ideal site for exploration.

Scientific data products and the data pre-processing subsystem of the Chang'e-3 mission

The Chang'e-3 (CE-3) mission is China's first exploration mission on the surface of the Moon that uses a lander and a rover. Eight instruments that form the scientific payloads have the following objectives: (1) investigate the morphological features and geological structures at the landing site; (2) integrated in-situ analysis of minerals and chemical compositions; (3) integrated exploration of the structure of the lunar interior; (4) exploration of the lunar-terrestrial space environment, lunar surface environment and acquire Moon-based ultraviolet astronomical observations. The Ground Research and Application System (GRAS) is in charge of data acquisition and pre-processing, management of the payload in orbit, and managing the data products and their applications. The Data Pre-processing Subsystem (DPS) is a part of GRAS. The task of DPS is the pre-processing of raw data from the eight instruments that are part of CE-3, including channel processing, unpacking, package sorting, calibration and correction, identification of geographical location, calculation of probe azimuth angle, probe zenith angle, solar azimuth angle, and solar zenith angle and so on, and conducting quality checks. These processes produce Level 0, Level 1 and Level 2 data. The computing platform of this subsystem is comprised of a high-performance computing cluster, including a real-time subsystem used for processing Level 0 data and a post-time subsystem for generating Level 1 and Level 2 data. This paper describes the CE-3 data pre-processing method, the data pre-processing subsystem, data classification, data validity and data products that are used for scientific studies.

Echo simulation of lunar penetrating radar: based on a model of inhomogeneous multilayer lunar regolith structure

Lunar Penetrating Radar (LPR) based on the time domain Ultra-Wideband (UWB) technique onboard China's Chang'e-3 (CE-3) rover, has the goal of investigating the lunar subsurface structure and detecting the depth of lunar regolith. An inhomogeneous multi-layer microwave transfer inverse-model is established. The dielectric constant of the lunar regolith, the velocity of propagation, the reflection, refraction and transmission at interfaces, and the resolution are discussed. The model is further used to numerically simulate and analyze temporal variations in the echo obtained from the LPR attached on CE-3's rover, to reveal the location and structure of lunar regolith. The thickness of the lunar regolith is calculated by a comparison between the simulated radar B-scan images based on the model and the detected result taken from the CE-3 lunar mission. The potential scientific return from LPR echoes taken from the landing region is also discussed.

Preface: The Chang'e-3 lander and rover mission to the Moon

The Chang'e-3 (CE-3) lander and rover mission to the Moon was an intermediate step in China's lunar exploration program, which will be followed by a sample return mission. The lander was equipped with a number of remote-sensing instruments including a pair of cameras (Landing Camera and Terrain Camera) for recording the landing process and surveying terrain, an extreme ultraviolet camera for monitoring activities in the Earth's plasmasphere, and a first-ever Moon-based ultraviolet telescope for astronomical observations. The Yutu rover successfully carried out close-up observations with the Panoramic Camera, mineralogical investigations with the VIS-NIR Imaging Spectrometer, study of elemental abundances with the Active Particle-induced X-ray Spectrometer, and pioneering measurements of the lunar subsurface with Lunar Penetrating Radar. This special issue provides a collection of key information on the instrumental designs, calibration methods and data processing procedures used by these experiments with a perspective of facilitating further analyses of scientific data from CE-3 in preparation for future missions.