Available online at www.sciencedirect.com ScienceDirect Procedia Engineering00 (2014) 000–000 www.elsevier.com/locate/procedia “APISAT2014”, 2014 Asia-Pacific International Symposium on Aerospace Technology, APISAT2014 Research progress and future development on mission planning technologies of planetary rover Wang Qionga,b,*, Yu Dengyunc, Jia Yangb a Lunar Exploration and Space Engineering Center,No.12 Chegongzhuang Street, Xicheng District, Beijing 100037, China b Beijing Institute of Spacecraft System Engineering, No. 5142 Mailbox, Beijing 100094, China c China Aerospace Science and Technology Corporation, No.16 Fucheng Road, Haidian District, Beijing 100048, China Abstract Roving exploration becomes a major method of further exploration to target planet after preliminary circling exploration. Mission planning is to search feasible action sequence and trajectory of the rover from initial state to target state, satisfying complex constraints such as terrain, energy, thermal control and communications. Hence it is the basis and key of controlling rover to explore the planet safely and effectively. Global path planning and local path planning could be seen as a special case of mission planning. In this paper existing planetary rovers, prototypes and related navigation technologies are briefly summarized, followed by an introduction of various mission planning algorithms. Finally several areas for future research are specified. © 2014 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA). Keywords:Exploration robot; Mobile robot; Mission planning; Path planning 1. Introduction Planetary rover is a kind of spacecraft which could adapt planetary surface environment, move on the planetary surface, and carry out exploration, sampling, transport and such complex missions [1]. For short hereafter it is called rover. It has two kinds of function: (1) to help human accomplish tasks that are too difficult or uneconomical for human, (2) to improve precision, expand range and increase means of scientific exploration. * Corresponding author. Tel.: +86-10-88306151; fax: +86-10-88306170. E-mail address: wangq2006@163.com 1877-7058 © 2014 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA). 2 Wang Qiong/ Procedia Engineering00 (2014) 000–000 By roving exploration much deeper and more precise results could be obtained than by orbital exploration, and much larger range could be reached than by fixed landing exploration. These results could complement and prove each other. Hence, roving exploration becomes a major method of further exploration to target planet after preliminary circling exploration, and is taken seriously by many countries. Various prototypes were developed and several rovers were successfully employed in planet exploration missions, including three manned lunar vehicles, three robotic lunar rovers and four robotic mars rovers. Moreover, ESA ExoMars mars rover, Japanese SELENE-2 lunar rover, Indian Chandrayaan-2 lunar rover and several Google X-Prize participant rovers are under developing. These programs promoted the development of mission planning technology. In this paper, existing planetary rovers, prototypes and related navigation technologies are overviewed, followed by an introduction of various mission planning algorithms. Finally several areas for future research are specified. 2. Outline of planetary rovers 2.1. Robotic rovers that successfully carried out planet exploration missions Lunokhod-1 and Lunokhod-2 were developed by the Soviet Union. Lunokhod-1 landed on the moon on November 1971, becoming the first lunar rover in human history. Lunokhod-2 landed on January 1973. Their objectives were gathering images of lunar surface, carrying out soil physical test and chemical analysis in 90-day lifetime. In fact, Lunokhod-1 had worked for 322 days and Lunokhod-2 for 125 days. Sojourner, Spirit (MER-A), Opportunity (MER-B), Curiosity came from the USA. Sojourner landed on mars on July 1997 and became the first mars rover of human. It was designed to travel 10m in 7-day lifetime, but finally it traveled 104m around the lander in 83-sol life, during which wheel-soil interaction was determined and autonomous navigation and hazard-avoiding technologies were tested. The lessons learned were used for improving future rover designs. Spirit and Opportunity landed on mars both on January 2004. Their designed lives are both 90 sols, but actually Spirit had worked for more than seven years and Opportunity is still working. They had found the evidence of past water activities on the mars. Curiosity is the most powerful planetary rover so far. It landed on the Gale Crater region of mars on August 2012. Its designed life is two Martian year and now has completed more than half after finding that Mars once had environmental conditions favorable for microbial life. Chinese rover Yutu landed on Mare Imbrium of lunar on December 2013. The scientific objectives of Yutu mainly include lunar surface topography and geological survey, lunar surface material composition and resource survey. Its designed life is three months, but it encountered operational difficulties in the second lunar day and still gathered some useful data in the following several months. 2.2. Robotic rover prototypes Through the development of various robotic rover prototypes, experiences, lessons and technical reserves were accumulated for formal missions. Rocky series have superior mobility performance because of their six-wheeled Rocker-bogie suspension system. Among them, Rocky 7 and FIDO are the most mature. Rocky 7 [2] improved Sojourner to test new technologies for long distance traverse. FIDO [3] tested a series of new technologies for MER missions including navigation and control, ground planning, command and visualization tools. Marsokhod [4] comprises three parts, which could relatively rotate or incline. Hence it could passively adapt rough terrains. Marsokhod was used for test platform by several institutes. RATLER [5] was a test platform of lunar rover tele-operation technology. It consists of two parts that could relatively rotate, and could keep four wheels in touch with the ground. Nomad [6] was developed to test the capability of long-distance, long-duration exploration. A transforming chassis along with an averaging suspension system and self-contained wheel modules were adopted by Nomad. Hyperion and Zoe [7] were test platforms of sun-synchronous lunar polar exploration. In order to keep the rover always under the sun, its paths could be planned according to current rover position, current time and solar azimuth. A tracked mobility system was adopted by Nanokhod [8], which obtained energy and swapped data from/with the lander through umbilical cables. SHRIMP [9] adopted a unique chassis layout including a front wheel, a rear wheel and four middle wheels. Wang Qiong/ Procedia Engineering 00 (2014) 000–000 3 In China, various robotic lunar rover prototypes were developed by some industrial departments, institutes and universities. Among them, a six-wheeled, Rocker-bogie lunar rover prototype developed by China Academy of Space Technology was relatively the most mature. It was deployed in the desert to perform several demonstrations and experiments of new technologies, which was finally applied on Yutu rover. 2.3. Manned planetary vehicles Lunar Roving Vehicle (LRV) was a four-wheeled, two-seater lunar vehicle used for transporting astronauts and equipments on lunar surface. LRV was used in Apollo 15 mission and landed on the moon on July 1971, as the first manned lunar vehicle in human history. Subsequently LRV was used in Apollo 16, Apollo 17 missions. Space Exploration Vehicle (SEV) [10] is a new generation of modularized, multi-mission space vehicle, which could be used for in-space missions as well as surface exploration on planetary bodies. The SEV surface concept has a small pressurized cabin mounted on a twelve-wheeled chassis. ATHLETE [11] was a six-wheel-legged all-terrain mobility platform, whose main function was transporting large modules such as habitation modules. RAMA [12] was a pressurized four-wheeled planetary vehicle designed by ESA. It could work independently or combine other vehicles as a base. Moreover, MOBEV series, LOTRAN, Rover First, Daylight Rover, LPMR, MSTS and other concepts and prototypes of manned planetary vehicle were proposed or developed. 3. Outline of navigation technologies on planetary rovers Rover navigation is that rover senses environment and its own status by sensors, and realizes autonomous move to target in environment with obstacles. It mainly focuses on three aspects [13]: (1) to obtain the rover position, direction and environment information by some detection methods, i.e., environment sensing; (2) to process those obtained information and build environment model by some algorithms, i.e., environment modeling; (3) to find an optimum or approximate optimum free path according to some criterion, i.e., path planning. In environment sensing aspect, stereo vision based sensing modes were mostly adopted. Particularly, considering the platform safety, close-range hazard avoidance sensors were deployed. Laser sensors were adopted in the past, such as laser stripe sensors on Sojourner, laser rangefinder on Ratler. But on the latest rovers such as Opportunity, Spirit, Curiosity and Yutu, hazard-avoidance cameras based on binocular stereovision became essential. Correspondingly, in environment modelling aspect, three dimensional scene reconstruction methods based on stereovision were mostly adopted. Environment images obtained by stereo cameras were used to generate navigation map, after a series of processing steps including image correction, feature extraction, stereo matching, 3-D information estimation. DEM was the most common navigation map and other types of map also existed. For example, a traversability map indicating dangerous zones and safe zones was generated based on DEM by FIDO [3]. A more complicated goodness map was produced by Nomad to indicate how good it is to occupy a cell [6]. In path planning aspect, due to the onboard resource limitation, algorithm practicality depends on whether a good balance among complexity, real-time performance and optimality could be reached. Early rovers only had simple intelligence, and most works should be done through tele-operation by ground staffs with very low efficiency. For instance, Lunokhod was completely navigated and controlled by ground operators; Sojourner could only perform simple local reactive hazard-avoidance, and its waypoints were chosen by ground operators. As the emergence of stronger sensors, onboard computers and more advanced GNC algorithms, rovers gradually had an improved capability of autonomous navigation and control, as well as less manual interventions and higher efficiency. For example, onboard global path planning algorithm Field D* and local path planning algorithm GESTALT were adopted on Opportunity, Spirit and Curiosity [14]. Ground operators needed only assign exploration targets and tasks of the next day. Rovers could plan paths, navigate themselves and move to targets autonomously. 4. Research progress of mission planning algorithms Mission planning problem is to find a behavior sequence and a travel path from initial state to target state, given initial conditions (including initial rover state, time and environment conditions), behavior constraints set and target 4 Wang Qiong/ Procedia Engineering00 (2014) 000–000 set (including target rover state, time, etc.) [15]. Rover state includes position, energy level, temperature level, communications conditions and hold of other resources. The transitions between states are realized by behaviors, including sensing, moving, charging, scientific exploration, data transmission, hibernation, and so on. Path planning could be seen as a special case of mission planning if we only consider position for rover states, moving for behaviors, terrain obstacles for constraints. According to the acquaintance of environment information, path planning comprises global path planning based on complete prior information and local path planning based on realtime sensing environment information. In practical use, mission planning usually deals with cases of low resolution, more than hundred meters’ distance and dozens of hours’ duration. Its outputs are strategic waypoints and behavior sequence, which could provide initial point, target point and basic planning direction for global path planning and local path planning within certain range. Global path planning focuses on path searching, and deals with cases of medium resolution, dozens of meters’ distance and several hours’ duration. Local path planning focuses on obstacle avoiding, and deals with cases of high resolution, several meters’ distance and dozens of minutes’ duration. Because planetary rovers commonly have a wheeled mobility system with few degrees of freedom, its configuration space has small dimensions. Moreover, planetary rovers work in the complex unstructured environment of planetary surface, having limited resources but high safety demands, therefore deterministic algorithms are usually adopted to achieve completeness even optimality and certain bounds on the quality of path solution. In this paper, deterministic algorithms are mainly introduced. 4.1. Mission planning algorithms RCW [16] is the mission planner of Sojourner. In RCW, path planning and behavior planning are almost independent. Operators used RCW to estimate safe paths and select waypoints at intervals of 1-2 meters as intermediate navigation goals, based on terrain traversability and geometric constraint of line-of-sight between Sojourner and the lander. Behavior planning is performed by simple constraint checks, and interaction effects between various behaviors are not considered. MAPGEN [17] is the mission planner for MER missions. In early stage of the missions, operators used orbital imagery and rover stereo imagery to manually select waypoints and estimate the optimal path according to terrain traversability and sun, solar array and terrain geometry data. After adding Field D* algorithm into onboard software, operators only need assign exploration targets and tasks of the next day, and the rover would plan path and generate waypoints. MAPGEN adopts Simple Temporal Network (STN) constraint reasoning and scheduling technology to plan behaviors. During arranging the traverse behavior, distance between waypoints is used to estimate duration, energy distribution and other constraints. Hence, traverse behavior is loosely coupled with other behaviors, and interaction effects between behaviors get more consideration. But MAPGEN also determines path first, then arranges behaviors along the path. The effects of behaviors on path selection are not taken into account, so path planning and behavior planning are still independent. CLEaR [18] is a planning and execution system of position-distributed science measurements. Goal selection was performed by operators, and paths between goals were given by the path planner. But the paths are only used to estimate traverse distances and calculate corresponding expect time and energy costs, as constraints for behavior planning. Through the field tests of Rocky 7, Rocky 8 and FIDO, this path planning algorithm was found to be too simple. It was ignorant of time, resources limitations and geometric constraints, hence the eventual paths followed by the rovers differ greatly to the generated paths, and the estimated time and resource costs were also inaccurate. In CLEaR, traverse and other behaviors were closely interleaved, and time and resource allocations of each behavior strongly affected the feasibility of others. Derived from MAPGEN, ENSEMBLE [19] is the mission planner of Curiosity rover. It unifies the disparate tools in MER, while providing a better user interface. Its core planner EUROPA2, the next generation version of EUROPA used in MAPGEN, supports modeling of state and other resources. These above-mentioned planners can be classified as mission planning methods based on resource scheduling, focusing on behavior planning to solve resource conflicts. Path planning is only used for resource usage estimation and plays an auxiliary role. The idea that independently plan paths and behaviors is not the same as practical Wang Qiong/ Procedia Engineering 00 (2014) 000–000 5 situation of rover operation on planetary surface. Therefore, the mission planning results are less instructive for exploration activities due to large estimation error. Another research direction is path-based mission planning, which treats mission planning as a class of generalized path planning problems. The main idea is generalizing the path planning algorithms from lowdimensional state space to high-dimensional state space, and performing constraint checks while searching paths. In these “Traverse—Judge—Insert behavior—Traverse” reactive planning algorithms, behaviors are inserted along the generated path according to trigger conditions. ISE [7], core of the TEMPEST planner, generalized D* algorithm to high-dimensional state space and conducted backward mission-level path planning. Peng Song et al. [20] improved and generalized A* algorithm, and proposed Real-time Greedy algorithm. Pei S et al. [21] applied SD* Lite algorithm into mission planning, whose highlight is generating path in any direction. Xie Yuan et al. [22] utilized the predictable global dynamic characteristic of lunar surface environment, and proposed a Predictable Dynamic Environment algorithm and introduced a flexible plan concept. This algorithm was applied in Yutu mission. Song Pei [23] proposed a mission planning method based on predicate logic, which was also used in Yutu mission. These algorithms plan path and behaviors at the same time, and take into consideration of the behaviors’ effects on path selection. They do well on path optimality, but in return, the results of behavior planning are only feasible solutions other than optimal solutions. 4.2. Global path planning algorithms Global path planning algorithms are mostly based on geometrical model search. The geometrical model of rover environment space is built at first, and then some graph search algorithm is applied to obtain the optimal path. Geometrical modeling methods mainly include roadmap methods and cell decomposition methods. Graph search algorithms include Breadth-First, Depth-First, Iterative Deepening, Dijkstra, A* and other improved algorithms. 4.2.1. Roadmap methods Visibility graph method [24] builds a visibility map that guarantees two connected points visible to each other, and searches the shortest path on this map. The method is simple and intuitional, but has a complex process of graph generation as well as low search efficiency. Tangent graph method [25] deletes lots of useless connection lines from visibility graph, hence greatly reduces the search space. The method could find out the shortest path, but resulting path is close to the obstacles leading to poor safety. Voronoi diagram method [26] limits the rover to keeping the same distance with obstacles. This reduces the dimensions of configuration space and is conducive to searching feasible path. Compared with Tangent graph method, Voronoi diagram method generates longer path, but keeps the rover far away from obstacles. MAKLINK graph method [27] deletes unnecessary connections from visibility graph, and constructs lots of largest convex polygons to generate connected graph. The method is flexible because that change of initial point and target point would not reconstruct the graph, but construction of convex polygons is not unique, hence the resulting path is indeterminate and non-optimal. 4.2.2. Cell decomposition methods Cell decomposition methods decompose environment space into connected and non-overlapping cells, and search an optimal free path on the adjacency graph constructed by these cells [28]. These methods could be divided into two classes: exact cell decomposition and approximate cell decomposition. The former is complete but has a high modeling cost. The latter is easy to construct, save and use, hence it is widely applied. But it has low storage efficiency especially in environments containing large free space or obstacles. Moreover, due to the eightneighborhood limitation of rover heading, generated path is not optimal [15]. Aiming at above disadvantages of grid method, various multi-resolution grid methods were proposed, such as framed-quadtree [29], weighted framedquadtree and weighted framed-octree [30], variable resolution grid [31], compact mesh representation [32]. 4.2.3. Graph search algorithms Dijkstra algorithm [33] is a classical shortest-path search algorithm, which introduces evaluation function based on Breadth-First algorithm. A* algorithm [34] adds heuristic function to Dijkstra. A* is best-first, efficient and easy 6 Wang Qiong/ Procedia Engineering00 (2014) 000–000 to implement and guarantees to find out the best solution, hence it is widely used. But local changes of environment will cause re-planning from scratch. Based on A*, various improved algorithms were proposed. One sort of algorithms is reforming A* into anytime algorithms, which could rapidly give a initial feasible solution and find out better solutions as the computing time increases. Anytime Repairing A* [35], Restarting Weighted A* [36], Anytime Window A* [37], Beam-Stack Search [38], ITSA* [39], Anytime Nonparametric A* [40] are of this class. Another sort is solving the problem of eight-neighborhood limit on grid map. A* with Post-Smoothing algorithm [41] post-processes the results of A* to get a smoother path. Theta* algorithm [42] searches shortcuts by visibility check between parent node and successive nodes. The algorithm could find out any-direction path, and has a good performance on optimality and time efficiency. In addition, there are other improvement ideas. For example, Block A* algorithm [43] takes the strategy of trading space for time. It operates lots of grids simultaneously by using local distance database, and gets a faster computation speed than A*. 4.3. Local path planning algorithms Common local path planning algorithms include Artificial Potential Field (APF) algorithms, Bug-like algorithms, dynamic heuristic re-planning algorithms and terrain-evaluation-based algorithms. 4.3.1. Artificial Potential Field algorithms APF [44] virtualizes the rover environment as an artificial potential field, and the repulsions from obstacles and attraction from target co-affect the rover motion. It is simple, computational efficient and easy for real-time control. However, APF still has many disadvantages such as local minimum problem, goals non-reachable and oscillations. Many improved algorithms are proposed to tackle local minimum problem. These works can be classified into four approaches according to their control strategy [45]. The first kind of approach changes the repulsive potential functions (including its angle), such as reference [46]. The second kind of approach provides the additional control force, such as reference [47]. The third kind of approach adds virtual obstacle (or virtual goal) for repelling (or attracting) the mobile robots from the local minima (or to the target), such as reference [48]. The fourth kind of approach is to employ intelligent techniques, such as “follow wall” strategy in [49]. 4.3.2. Bug-like algorithms Bug-like algorithms is a kind of simple local path planning algorithms that guarantee of finding local optimal solutions. In the classical Bug1 and Bug2 algorithms [50], the rover only has two behavior modes: move on a straight line towards goal and follow a boundary. Because of the simplicity and huge application potential, many improved algorithms are proposed, including Alg, DistBug, Com, Class1, Rev, TangentBug, Ave, VisBug, OneBug, LeaveBug, CautiousBug, Bug2+, PointBug, ABug, Ibug, KBug, DHBug, etc [51]. Most of them focus on increasing efficiency. For example, TangentBug [52] introduces omni-directional range sensing data and searches local shortcuts to bypass obstacles based on local tangent map. CautiousBug [53] adopts conservative spiral search to choose the bypassing direction and reduces path length greatly. But these algorithms adopt over-ideal assumptions such as global positioning and regarding the rover as a point, hence generally they fit for indoor structural environment but not for planetary surface environment. Some algorithms tried to relax the assumptions. For instance, IBug [54] regards the target as virtual signal tower and uses signal strength sensor to guide the rover towards stronger signal with no need of accurate rover position and heading. WedgeBug [55] introduces the virtual rotary scan action of sensors to reduce the meaningless move of rover, and adopts visual dead-reckoning technology to enhance autonomous navigation capability. The algorithm is more practical in planetary surface environment and has been tested on the Rocky 7 prototype. 4.3.3. Dynamic heuristic replanning algorithms Aiming at the shortcoming of A* that re-plan from the scratch when environment locally changes, a lot of improved heuristic and incremental algorithms are proposed to get better results in unknown, partially known or dynamic environments. D* algorithm [56] generalizes A* into dynamic environment. As a re-planning algorithm, D* only needs revise local changed nodes, hence greatly improves search efficiency. But D* is hard to understand, analyze, realize and extend. Lifelong Planning A*(LPA*) [57] is the incremental version of A* and is a plan reuse Wang Qiong/ Procedia Engineering 00 (2014) 000–000 7 algorithm. It improves efficiency and keeps the simplicity and convenience of A* as well. Based on LPA*, D* Lite [58] was proposed. It is a simplified version of D* and generates the same path as D*, but is simpler and more extensible. Therefore, subsequent D*-like algorithms are mostly based on D* Lite. One category of algorithms is solving the problem of eight-neighborhood limit on grid map. Field D* [14] adopts linear interpolation to eliminate the non-smooth path problem caused by discrete state transfer between adjacent grids. It was applied on Opportunity, Spirit and Curiosity. SD* Lite [59] checks the visibility between nodes, and uses Euclid distance to estimate its cost. Incremental Phi* [60] is the incremental version of Basic Theta*, taking both advantages of any-angle algorithms with optimal path and that of incremental algorithms with high efficiency. Another category is improving search efficiency. DD* Lite [61] uses dominance relationship between states to exclude regions that need not be explored, thus improves efficiency in large state space. Improved Dynamic A* [62] further reduces the re-computation in D* Lite because of terrain changes and remarkably speeds up the computation. There is also a category of algorithms that synthesizes the advantages of multiple algorithms. Anytime Dynamic A* [63] integrates the anytime responding feature of ARA* and the incremental re-planning feature of D* Lite. ROAMAN [3] synthesize the conciseness of environment representation of configuration space method and the high efficiency of D*. It was tested in FIDO rover prototype. 4.3.4. Terrain-evaluation-based algorithms The basic idea of terrain-evaluation-based algorithms is carrying out statistic analysis on local terrain around the rover and then determining the best path. Morphin [64] evaluates the goodness, certainty, traversability and other measures of each rover-sized terrain patch, generates goodness map and finds the most traversable path among several possible paths in front of the rover. The algorithm was tested on Ratler and Hyperion prototypes. It could nicely deal with environment modeling error and positioning error, and works well together with global planning algorithms such as D*. GESTALT [65] improves Morphin on several areas, including patch shape, computation method of goodness, merging method of goodness maps, voting standard for forward arcs and so on. The algorithm is more practical and was used in the local hazard-avoidance module of Opportunity, Spirit and Curiosity. Probabilistic Terrain Analysis (PTA) [66] uses probabilistic techniques to integrate range measurements over time, and relies on efficient statistical tests for distinguishing drivable from non-drivable terrain. However it is a binary detection algorithm providing a simple go/no-go solution. Loh J E et al. [67] proposed a new roughness metric to create a roughness map and provide planetary rovers with qualitative information about the terrain and difficulty of traversal. It is computationally efficient, and runs in linear time. 5. Conclusion and outlook Mission planning technology is the basis and key of safe and efficient roving exploration on planetary surface, hence much research has been done on it. In order to satisfy the demands of future planetary roving exploration mission, mission planning technology should be studied further on the following aspects: (1) Mission-level joint path-behavior planning algorithm under complex constraints. Current mission planning algorithms either focus on behavior planning too much, or on path planning too much. How to balance the two better and realize optimal planning of paths and behaviors at the same time, needs further studies. (2) Combination of multi-layer planning algorithms. The coarse-to-fine, macro-to-micro hierarchical planning structure is a classical structure of robotic intelligent control. 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