remote sensing Technical Note Localization of the Chang’e-5 Lander Using Radio-Tracking and Image-Based Methods Jia Wang 1, Yu Zhang 1, Kaichang Di 2,3 , Ming Chen 1, Jianfeng Duan 1, Jing Kong 1, Jianfeng Xie 1, Zhaoqin Liu 2, Wenhui Wan 2,*, Zhifei Rong 1, Bin Liu 2 , Man Peng 2 and Yexin Wang 2 1 Beijing Aerospace Control Center (BACC), Beijing 100094, China; [email protected] (J.W.); [email protected] (Y.Z.); [email protected] (M.C.); [email protected] (J.D.); [email protected] (J.K.); [email protected] (J.X.); [email protected] (Z.R.) 2 State Key Laboratory of Remote Sensing Science, Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100101, China; [email protected] (K.D.); [email protected] (Z.L.); [email protected] (B.L.); [email protected] (M.P.); [email protected] (Y.W.) 3 CAS Center for Excellence in Comparative Planetology, Hefei 230026, China * Correspondence: [email protected]; Tel.: +86-10-64807987 Abstract: Chang’e-5, China’s first unmanned lunar sample-return mission, was successfully landed in Northern Oceanus Procellarum on 1 December 2020. Determining the lander location precisely and timely is critical for both engineering operations and subsequent scientific research. Localization of the lander was performed using radio-tracking and image-based methods. The lander location was determined to be (51.92◦W, 43.06◦N) by both methods. Other localization results were compared for cross-validation. The localization results greatly contributed to the planning of the ascender lifting off from the lander and subsequent maneuvers, and they will contribute to scientific analysis of the returned samples and in situ acquired data. Citation: Wang, J.; Zhang, Y.; Di, K.; Keywords: Chang’e-5; lander localization; radio-tracking; descent images; image-based localization Chen, M.; Duan, J.; Kong, J.; Xie, J.; Liu, Z.; Wan, W.; Rong, Z.; et al. Localization of the Chang’e-5 Lander Using Radio-Tracking and 1. Introduction Image-Based Methods. Remote Sens. 2021, 13, 590. https://doi.org/ Chang’e-5, China’s first unmanned lunar sample-return mission, was launched from 10.3390/rs13040590 Wenchang Space Launch Center in Hainan Province on 24 November 2020. The spacecraft entered an elliptical lunar orbit on 28 November 2020 and moved to a near-circular lunar Academic Editor: Shengbo Chen orbit on 29 November 2020. After separating from the orbiter–re-entry capsule combination Received: 12 January 2021 on 30 November 2020, the lander–ascender combination successfully landed in Northern Accepted: 3 February 2021 Oceanus Procellarum of the Moon at 23:11 (UTC +8) on 1 December 2020 [1]. Subsequently, Published: 7 February 2021 the lander–ascender combination completed collection of lunar samples using a drill and a mechanical arm on 2 December 2020, and the ascender carried the samples and lifted itself Publisher’s Note: MDPI stays neutral to the lunar orbit on 3 December 2020. On 6 December 2020, the ascender and re-entry with regard to jurisdictional claims in capsule completed a rendezvous and docking after a series of sophisticated maneuvers, published maps and institutional affil- and the ascender delivered the sealed sample container into the re-entry capsule. The iations. orbiter–re-entry capsule combination travelled in a lunar orbit for nearly six days and entered the Moon-Earth transfer orbit after two injection maneuvers on 12 and 13 December. Finally, the re-entry capsule returned to Earth at the preset landing site in North China’s Inner Mongolia autonomous region at 1:59 (UTC +8) on 17 December 2020 [2], marking Copyright: © 2021 by the authors. the complete success of the mission, which is also the first lunar sample-return mission Licensee MDPI, Basel, Switzerland. of the world since 1976. Figure1 shows a diagram of the descending, landing, ascending, This article is an open access article rendezvous and docking processes. distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Remote Sens. 2021, 13, 590. https://doi.org/10.3390/rs13040590 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, 590 2 of 11 Remote Sens. 2021, 13, x FOR PEER REVIEW 2 of 11 orbiter-reentry capsule combination Seperation of lander- ascender combination 15km Deceleration 2km Approaching Hovering and 100m hazard avoidance 0m Rendezvous and Orbiting Descending and landing Surface operation Ascending docking Figure 1. FigureDiagram 1. ofDiagram descending, of descending, landing, ascending, landing, ascending, rendezvous rendezvous and docking. and docking. Determining theDetermining lander location the lander precisely location and timely precisely is critical and timely for both is critical engineering for both engineering operations andoperations subsequent and scie subsequentntific research. scientific For research.example, For the example, lander location the lander is a location key is a key pa- parameter for makingrameter and for makingfine-tuning and the fine-tuning plans of theascender plans oftaking ascender off from taking the off lander from on the lander on the the lunar surfacelunar and surface rendezvous and rendezvous and dockin andg with docking the orbiter–re-entry with the orbiter–re-entry capsule combi- capsule combination nation on orbit.on The orbit. lander The lo landercalization localization result, when result, associated when associated with a geologic with a geologic map of mapthe of the area, is area, is also importantalso important to provide to provide the geologic the geologic context context of the collected of the collected samples. samples. Lander localizationLander methods localization can be methods broadly can divided be broadly into two divided categories: into two the categories:radio- the radio- tracking methodtracking and image-based method and method; image-based both have method; been used both for have determining been used the for lo- determining the cations of lunarlocations and Mars of lunarlanders and of Marsprevious landers missions of previous [3,4]. In missions China, [landing3,4]. In China, position landing position research of extraterrestrialresearch of extraterrestrial bodies started bodies from the started Chang’e-1 from the mission Chang’e-1 [5]. Chang’e-1 mission [5 ]. Chang’e-1 spacecraft collidedspacecraft on the collided Moon in on a thecontrolled Moon inmanner a controlled in 2009. manner The radio-tracking in 2009. The radio-trackingteam team used the deep usedspace the network deep space and interferometry network and interferometry system to track system the entire to track collision the entire pro- collision process cess of Chang’e-1of Chang’e-1 and gave andthe gaveposition the positionof the final of the signal final signalvanishing vanishing point point(the collision (the collision point) [6]. point) [6]. Chang’e-3Chang’e-3 is China’s is China’s first firstsoft softlanding landing mission mission and andlanded landed in Mare in Mare Imbrium Imbrium in in December December 2013.2013. The Thedeep deep space space network network and and interferometry interferometry system system in China in China were were used used to track the to track the landerlander after after landing, landing, and and the the positioning calculationcalculation ofof the the extraterrestrial extraterrestrial lander based on lander based onradio radio measurement measurement was was realized realized for for the the first first time time in China in China [7,8 ].[7,8]. In January In Janu- 2019, Chang’e-4 ary 2019, Chang’e-4landed landed on the on far the side far of side the of Moon. the Moon. Since theSince ground the ground station station cannot cannot directly observe the directly observelander, the lander, it is impossible it is impossible to locate to locate the lander the lander directly directly by radio by radio measurement. measure- ment. Image-based localization of the lunar lander was applied in both Chang’e-3 and Image-basedChang’e-4 localization missions of the using lunar the lander descent was images applied and otherin both images Chang’e-3 taken byand the landers and Chang’e-4 missionsbase maps using generated the descent from images Chang’e-2 and other images images and/or take Lunarn by Reconnaissance the landers and Orbiter Camera base maps generated(LROC) from Narrow Chang’e-2 Angle images Camera and/or (NAC) Lunar images Reconnaissance [9–16]. A number Orbiter of teams Cam- involved in the era (LROC) Narrowwork andAngle the Camera results supported(NAC) images mission [9–16]. operations A number and of scientific teams involved investigations in in different the work and theturnaround results supported times. mission operations and scientific investigations in dif- ferent turnaround times.In the Chang’e-5 mission, we performed lander localization using both the radio- In the Chang’e-5tracking mission, method andwe image-basedperformed lander method localization immediately using after both landing. the radio- In the radio-tracking method, the measurement data of Unified X-band (UXB) and Very Long Baseline Interferom- tracking method and image-based method immediately after landing. In the radio-track- etry (VLBI) were used for statistical positioning calculations. In image-based localization, ing method, the measurement data of Unified X-band (UXB) and Very Long Baseline In- descent images taken by the lander during the descending and landing process and the terferometry (VLBI) were used for statistical positioning calculations. In image-based lo- base map produced from Chang’e-2 images were used to determine the lander location; an calization, descent images taken by the lander during the descending and landing process LROC NAC base map was also used for cross-validation. and the base map produced from Chang’e-2 images were used to determine the lander The descending and landing trajectory of Chang’e-5 is different from that of Chang’e-3 location; an LROC NAC base map was also used for cross-validation.