Lunar Soft Landing Research Articles

Trajectory optimization for lunar soft landing with a Hamiltonian-based adaptive mesh refinement strategy

In this study, the problem of fuel-optimal lunar soft landing trajectory optimization using variable-thrust propulsion is considered. First, the lunar soft landing trajectory optimization problem with three-dimensional kinematics and dynamics model, boundary conditions, and path constraints strictly described is formulated. Then, the formulated trajectory optimization problem is solved by the simultaneous dynamic optimization approach. With bounds imposed on the magnitude of engine thrust, the optimal control solutions typically have a “bang-bang” thrust profile. The general simultaneous dynamic optimization approach has difficulty handling breakpoints in the control profiles. A novel adaptive mesh refinement strategy based on a constant Hamiltonian profile is proposed to address the difficulty of locating breakpoints in the thrust profile. Two cases are simulated. The engine of the first case is throttleable between zero and full thrust. The engine of the second case is throttleable between 10% and 60% of full thrust, and at full thrust. Union property of R-function method is utilized to express the thrust profile of the second case in the trajectory optimization problem. Simulation results show that the enhanced simultaneous dynamic optimization approach with adaptive mesh refinement strategy can effectively capture the breakpoints in the optimal thrust profile and obtain more refined lunar soft landing optimal solutions, compared with the results obtained by the general simultaneous dynamic optimization approach.

Innovative hazard detection and avoidance strategy for autonomous safe planetary landing

Autonomous hazard detection and avoidance (AHDA) is one of the key technologies for future safe planetary landing missions. In this paper, we address the latest progress on planetary autonomous hazard detection and avoidance technologies. First, the innovative autonomous relay hazard detection and avoidance strategy adopted in Chang’e-3 lunar soft landing mission and its flight results are reported in detail. Second, two new conceptual candidate schemes of hazard detection and avoidance are presented based on the Chang’e-3 AHDA system and the latest developing technologies for the future planetary missions, and some preliminary testing results are also given. Finally, the related supporting technologies for the two candidate schemes above are analyzed.

The control strategy of Lunar landing craft avoiding obstacles with minimum fuel consumption

A comprehensive evaluation method and its terminal control strategy based on a simulated annealing algorithm are presented in this paper, which have application in avoiding the obstacles for lunar soft landing. Unlike many fuel-irrelevant researches or some classical strategies only concluding the routes, an innovative strategy considering the minimum fuel consumption and the obstacle avoidance strategy together is proposed, in view of a dynamics theory for minimum fuel consumption and computer simulation. The combined process consists of a coarse obstacle avoidance and a precise one, and in this paper, we take Chang-e’3 for example—all the following data and craft structures are based on Chang-e’3. Firstly, in order to optimize the algorithm for avoiding obstacles during the landing process, we need to create an index system to evaluate the degree of safety of the whole area divided into several sub-blocks. In the coarse obstacle avoidance section, at first, the digital picture taken from an elevation of 2400 m above the surface is divided into 100 m×100 m sub-blocks, and “the highest elevation”, “the lowest elevation”, “the minimum slope angle” are set as three indicators evaluating the landing safety of each area. The safety greyscale image can be drawn and the safety degree can be evaluated quantitatively by Cauchy membership function. Secondly, we develop a control scheme with high computational efficiency and apply it to select the safest landing area automatically. Taking the computational simulated trajectory into consideration, we adapt the simulated annealing algorithm to obtain the better landing area, in the process of which, a nonlinear programming model is introduced with the object function of minimum fuel consumption. With the parameters of the simulated annealing algorithm—the cooling coefficient a =0.999 and the terminal temperature E =10 - 30, the program nicely finishes with a high time efficiency. The entire operation of trajectory simulating, the safest landing area analyzing, the obstacle avoidance, the descent stages transforming and the comprehensive consideration of minimum fuel consumption are all based on computational simulation. Thirdly, according to the algorithm simulation, the landing trajectory including a specific landing coordinate is obtained. It takes 25.7 s for Chang’e-3 to complete the soft landing of coarse obstacle avoidance, with 63.86 kg fuel consumption. We also simulate with a given change of the initial conditions, in order to test and verify the validity of the algorithm, and results turn out to be very desirable. Similarly, the optimal control strategy for precise obstacle avoidance is also obtained, and in this section, the time cost and fuel consumption are both negligibly small. Finally, we focus on the error rate of the algorithm and provide a method to diminish the error rate—by adjusting weights of the three indicators which are used to evaluate the safety of sub-blocks. The algorithm can better deal with the effect of topographic diversity.

月球软着陆自主障碍识别与避障制导方法

An autonomous hazard detection and roughne hazard-avoidance guidance method for soft lunar landing is presented to mitigate the landing risks due to the uncertainty and hazards of the lunar terrain, such as surface roughness and slope. The method is composed of roughne hazard detection and a landing site selection algorithm. The high-precision relative autonomous navigation algorithm is based on image data, and the autonomous hazard-avoidance guidance algorithm is based on an adjusted thrust engine. The theoretic deduction and simulation analysis shows that a safe landing site is acquired and autonomous pinpoint hazard avoidance is achieved using the presented method. The method can be used to improve the achievement of a soft landing and has been successfully used in the Change-3 soft landing mission.

Lunar Circle Pit Recognition Based on Multi-level De-noising Hough Transform

In order to solve the problem of selecting lunar soft landing spot, this study doped out a solution to the problem. Collect images of the lunar surface and analyze them. According to the fact that the speed and accuracy of selecting lunar soft landing spot are highly required in the field of aviation, this study proposed circle pit recognition method based on Multi-level de-noising Hough transform. In this method, do halved sampling process repeatedly on the original images in order to get a set of variable resolution image of pyramid like. Search the circle pit position in the lowest resolution image roughly. And then refine soft landing point in the high resolution images in order to get the lunar circle pit position accurately. Do experiments using the images of the lunar surface which were collected by Chang 'e-3. What's more, selecting the appropriate landing point selection mechanism to select a soft landing point and verify results. After doing all of these, compare this method with five kinds of traditional methods. The experimental results shows the method which is proposed in this study effectively improves the speed and accuracy of Hough transform circle detection, with strong robustness and high efficiency, so as to improve the speed and accuracy of selected lunar soft landing point.

Lunar Soft - Landing Trajectory of Mechanics Optimization Based on the Improved Ant Colony Algorithm

Based on research carried out for the most fuel-lunar soft landing trajectory optimization problem. First, by improving the function approximation method, the lunar soft landing trajectory optimization problem into a parameter optimization problem, and the optimization variables and state variables have a clear physical meaning. Then use the decimal ant colony algorithm adds local search strategy to study the optimization problem. Finally, the optimization algorithm to optimize term direction angle simulation and error analysis.

Lunar soft landing with minimum-mass propulsion system using H2O2/kerosene bipropellant rocket system

Minimum-mass propulsion specifications using a H2O2/kerosene bipropellant rocket system for a small lunar lander were derived. A multivariable optimization was conducted with propulsion specifications and propellant consumptions obtained by solving optimal control problems for a lunar soft landing. In this paper, the optimal specifications, trajectory, and mass budget are presented.

Terminal Sliding Mode Control of a Lunar Lander with Electric Propulsion

This paper presents an attitude control method based on electric propulsion systems for the lunar lander that considers the important characteristics of nonlinearity and uncertainty of lunar soft landing maneuvers with large attitudes. The attitude control law is designed according to the terminal sliding mode variable structure control method. A soft lunar landing utilizing the proposed control method is simulated, and the results show that this attitude control system demonstrates superior global robustness, consumes less propellant, and can achieve higher precision than a conventional chemical propulsion-based control system. For a lunar lander with a pulse plasma thruster as the propulsion system, the attitude control precision of the system is 0.002 degrees when the attitude control force is 0.1 Newtons. When a conventional chemical, not electric, propulsion thruster is used, if the attitude control force decreases by one order of magnitude, then the control precision of the lunar lander decreases 10-fold. This study demonstrates that a terminal sliding mode variable structure control method combined with low level thrust electric propulsion can improve the precision of lunar soft landings.

Geometrical Guidance Algorithm for Soft Landing on Lunar Surface

Abstract Augmented design of a guidance algorithm previously developed for intercepting target in a missile-target engagement scenario, but catering to powered descent soft landing on lunar surface is the main focus of this paper. When it comes to lunar soft landing guidance formulation, it is required that the Lander reach the desired position with terminal velocity constraints. An existing circular guidance law developed for missile guidance is modified to be applied for the fine braking phase of lunar powered descent. Presently in the algorithm, at the beginning of each guidance cycle, there will be an assumed varying circular path from missile to target and the guidance solution lies in finding the acceleration towards the centre of the circle so that the missile moves towards the target. The design augmentation proposed for lunar landing introduces a quadratic acceleration term opposite to the instantaneous tangential velocity vector to ensure terminal conditions. Coefficients of the quadratic acceleration profile are determined by the length of the circle as well as the magnitude of instantaneous tangential velocity. Finally a simple compensation scheme based on comparison of actual velocity from sensors and expected velocity from the algorithm is also proposed to take care of the variation in gravity and error in initial mass estimate.

Automatic Graben Detection in Lunar Images Using Hessian Technique

Lunar surface exploration is increasing rapidly with priority being given to precision of lunar soft landing. Lunar soft landing is achieved when craters and grabens are used as navigational landmarks. Grabens are formed from localized tensional stress fields or from near-surface dike emplacement. These tectonic features tend to have consistently straight or accurate parallel-striking walls bounded by steep, inward-dipping normal faults. Aiming at navigational application, a novel approach for automatic graben detection based on Hessian technique has been implemented on Digital Terrain Model (DTM) of lunar image. The Hessian technique uses gradient change as a key parameter to identify grabens. Adaptive Binarization using Otsu method is used to extract graben features from the Hessian image. Features such as small grabens and craters are removed using morphological operations, resulting in significant appearance of grabens. The experiment is conducted in different DTM images of lunar surface and the results indicate 90 % of the grabens are detected. The statistical results are evaluated based on visual interpretation, for both automatic and manual graben detection. It is observed that the proposed automatic graben detection technique gives better results than the manual detection.

Integrated translational and rotational control for the terminal landing phase of a lunar module

For lunar soft landing missions, precise landing with a low touch-down velocity and a vertical attitude is of great importance and challenge for a lunar module in the terminal landing phase. To this end, this paper presents a novel descending control strategy, by which the position and attitude of a lunar module are able to simultaneously meet the requirements. During descent of the lunar module, since the required thrust vector depends on the attitude maneuvering, the translation dynamics and the rotation dynamics of the lunar module are in fact mutually coupled, which leads to a six degree-of-freedom (DOF) control problem. Therefore, we formulated the coupled translation and rotation dynamics for a lunar module equipped with a main thruster providing body-fixed forces only in one direction and attitude actuators providing torques about three mutually perpendicular axes in the body frame. By using a state transformation, a filter-based backstepping control strategy is subsequently proposed, in which a second-order filter is utilized to overcome the “explosion of terms” problem existing in standard backstepping. The stability of the closed-loop system is guaranteed based on Lyapunov stability theory and singular perturbation theory. Numerical simulation of a lunar landing scenario shows the effectiveness and the robustness of the proposed control strategy.

Fuel near-optimal guidance law for the powered descending phase of a lunar module

This article proposes a fuel near-optimal robust guidance law for a lunar module during the powered descending phase in the lunar soft landing mission. At first, a three-dimensional kinetic model is introduced based on the defined target coordinate frame, where a single thruster misalignment and the moon rotation are considered. Next, with a computational method in conjunction with a time transform, a fuel optimal open-loop guidance law is derived for the nominal model without any disturbance. Then, to attenuate disturbances caused by thrust error and external disturbances, an additional robust H∞ feedback controller is designed to track the obtained reference trajectory and meet the constraints of control input, by solving a convex optimization problem with linear matrix inequality conditions. Finally, numerical simulation illustrates the effectiveness of the proposed approach.

Two-Dimensional Trajectory Optimization for Soft Lunar Landing Considering a Landing Site

This paper addresses minimum-fuel, two-dimensional trajectory optimization for a soft lunar landing from a parking orbit to a desired landing site. The landing site is usually not considered when performing trajectory optimization so that the landing problem can be handled. However, for precise trajectories for landing at a desired site to be designed, the landing site has to be considered as the terminal constraint. To convert the trajectory optimization problem into a parameter optimization problem, a pseudospectral method was used, and C code for feasible sequential quadratic programming was used as a numerical solver. To check the reliability of the results obtained, a feasibility check was performed.

Two-dimensional Trajectory Optimization of a Soft Lunar Landing from a Parking Orbit Considering a Landing Site

AbstractIn this paper, minimum-fuel, two-dimensional trajectory optimization from a parking orbit to the desired landing site is presented. The landing site is usually not considered when performing the trajectory optimization. However, to design the precise trajectories to land at the desired site, the landing site has to be considered as the terminal constraint. To convert the trajectory optimization problem into a parameter optimization problem, a pseudospcetral (PS) method is used, and CFSQP is used as a numerical solver. To check that the results obtained are good solutions, the feasibility check is performed.

GNC system scheme for lunar soft landing spacecraft

A pinpoint autonomous GNC system scheme for lunar soft landing is proposed. First, the descriptions of the mission outline for lunar soft landing are provided. Second, the GNC system design of the spacecraft is proposed. The baseline is a 3-axis stabilized system during all the phases of the mission. Orbit maneuvers are achieved by an assembled constant thrust or a throttleable main engine. The attitude control actuators are thrusters. The attitude and orbit determination is performed by sun sensors, star sensors, Inertial Measurement Unit (IMU), altimeter, velocimeter and lunar imaging sensors. Third, some proposed autonomous navigation and guidance methods including hazard detection and avoidance for lunar soft landing are analyzed. Finally, we draw some conclusions.

A new strategy for lunar soft landing

A new strategy based on a windlass device is proposed to deal with the terminal descent of a lunar soft landing mission in this paper. The lunar lander considered here consists of a rover and an assist module (AM) which is equipped with three decelerating thrusters and a lateral thruster. The rover and the AM are connected by a tether (mass is negligible) which winds around the windlass. The dynamics of the terminal descent process is modeled. The guidance laws for the terminal descent phase are derived. Both fuel optimality and lander safety are considered. The design of the terminal guidance laws in this paper are separated into three stages: firstly, finding the activation point of the decelerating thrusters to minimize fuel consumption; secondly, determining the tensile force of the tether for deployment; and finally, designing the tether’s tensile force to make the rover track the desired velocity-altitude profile. The proposed soft landing strategy needs only the measurements of altitude and vertical velocity. Therefore, it is easy to implement. It effectively prevents the decelerating thrusters’ plume from damaging the lander due to ground interference. Moreover, it is robust against the uncertainties of initial altitude and vertical velocity. Numerical simulation results illustrate the feasibility of the proposed soft landing strategy.

B1 月面軟着陸実験の実現に向けて

For further precise investigation of the moon following on SELENE planned for the launch in 2004, exploring the lunar surface is inevitable. This paper describes the critical technologies such as guidance & navigation, obstacle detection, power supply, mobirity system, etc. and has clarified some technical issues for achieving lunar soft landing and exploration. Besides R&D implementation plan including ground test to develop these technologies are discussed. According to the plan the first Japanese soft landing experiment onto the moon is expected to be realized in 2006.

推力方向制約付き月軟着陸の最適化

The purpose of this paper is to obtain the optimum trajectory for the fuel minimum guidance phase of lunar soft landing experiments NASDA is planning. In this phase the lander descends to a few km altitude from 15km altitude realized by the Hohmann transfer from the circular orbit of 100km altitude. Characteristics of the phase are thrust direction constraints near the terminal point, which are (A) thrust direction at the terminal point must be vertical and (B) change rate of the thrust direction must be smaller than the specified value. It was obtained the minimum thrust should be used for the latter half of the interval just before the terminal point where the thrust direction is changing largely. The optimum thrust direction except the region where the thrust direction is changing largely coincided with the one for no constraints. This fact will ease the derivation of guidance law.

The Surveyor Variable and Fixed Focal-Length Lenses

The Surveyor spacecraft has TV cameras equipped with specially designed versatile variable focal-length lenses. Each lens is capable of withstanding extreme environments under the most adverse conditions of an earth launch, earth-moon journey, lunar soft landing and the vacuum and temperature variations of the moon. Each lens assembly is an optically integrated unit consisting of a 25mm to 100mm variable focal-length lens with earth-commanded motor drives for focal length, 4 ft to infinity focus and f/4 to f/22 adjustable iris mechanisms; a reflex viewing system behind the lens for automatic exposure control; solenoid-actuated focal-plane shutter; and motor-driven color filter wheel to provide color picture capability. Effects of radiation on optical transmission, optical performance in a vacuum, outgassing of particles on the lens surfaces, selection of lubricants and an all air-spaced optical design were among the design problems that were solved.

Design considerations for Surveyor guidance.

Design considerations for midcourse guidance and terminal descent system of Surveyor lunar soft landing spacecraft