Toralf Boge

On-Orbit Servicing Missions: Challenges and Solutions for Spacecraft Operations

The DLR German Space Operations Center (GSOC) is currently involved in the preparation of two On-Orbit Servicing missions, DEOS and OLEV. Due to the many new challenges within those missions the ground segment design requires new concepts. Accordingly, the paper presents the challenges and solutions regarding the communication architecture including teleoperation and extended contact time. Additionally, we discuss a method of vision based navigation which bridges the gap between absolute and purely geometric navigation. Finally, an integrated system test including GSOC’s new EPOS facility is described.

Using advanced industrial robotics for spacecraft Rendezvous and Docking simulation

One of the most challenging and risky missions for spacecraft is to perform Rendezvous and Docking (RvD) autonomously in space. To ensure a safe and reliable operation, such a mission must be carefully designed and thoroughly verified before a real space mission can be launched. This paper describes a new, robotics-based, hardware-in-the-loop RvD simulation facility which uses two industrial robots to simulate the 6-DOF dynamic maneuvering of the docking satellites. The facility is capable of physically simulating the final approaching within 25-meter range and the entire docking/capturing process in a satellite on-orbit servicing mission. The paper briefly discusses the difficulties of using industrial robots for HIL contact dynamics simulation and how these problems are solved.

Analytical and Experimental Stability Investigation of a Hardware-in-the-Loop Satellite Docking Simulator

The European Proximity Operations Simulator of the DLR-German Aerospace Center is a robotics-based simulator that aims at validating and verifying a satellite docking phase. The generic concept features a robotics tracking system working in closed loop with a force/torque feedback signal. Inherent delays in the tracking system combined with typical high stiffness at contact challenge the stability of the closed-loop system. The proposed concept of operations is hybrid: the feedback signal is a superposition of a measured value and of a virtual value that can be tuned in order to guarantee a desired behavior. This paper is concerned with an analytical study of the system’s closed-loop stability, and with an experimental validation of the hybrid concept of operations in one dimension. The robotics simulator is modeled as a second-order loop-delay system and closed-form expressions for the critical delay and associated frequency are derived as a function of the satellites’ mass and the contact dynamics stiffness and damping parameters. A numerical illustration sheds light on the impact of the parameters on the stability regions. A first-order Pade approximation provides additional means of stability investigation. Experiments were performed and test results are described for varying values of the mass and the damping coefficients. The empirical determination of instability is based on the coefficient of restitution and on the observed energy. There is a very good agreement between the critical damping values predicted by the analysis and observed during the tests. The contact duration shows also a very good fit between analysis and experiment. In addition, results from a one-dimensional contact experiment carried on an air-floating testbed are successfully emulated using the proposed hybrid docking simulator. This illustrates the flexibility of the hybrid simulator, where various contact dynamics can be emulated without changing any hardware elements.

EPOS - Using Robotics for RvD Simulation of On-Orbit Servicing Missions

Increasing complexity and costs of satellite missions promote the idea of extending the operational lifetime or improving functionalities/performance of a satellite in orbit instead of simply replacing it by a new one. Further, satellites in orbit can severely be affected by aging or degradation of their components and systems as well as by consumption of available resources. These problems may be solved by satellite on-orbit servicing (OOS) missions. One of the critical issues of such a mission is to ensure a safe and reliable Rendezvous and Docking (RvD) operation performed autonomously in space. Due to the high risk associated with an RvD operation, it must be carefully analyzed, simulated and verified in detail before the real space mission can be launched. This paper describes a ground-based hardware-in-the-loop RvD simulation facility. Designed and built on 2-decade experience of RvD experiment and testing, this unique, high-fidelity simulation facility is capable of physically simulating the final approach within 25-meter range and the docking/capture process of an on-orbital servicing mission.

Autonomous Navigation for On-Orbit Servicing

On-orbit servicing missions induce challenges for the rendezvous and docking system since a typical target satellite is not specially prepared for such a mission, can be partly damaged or even freely tumbling with lost attitude control. In contrast to manned spaceflight or formation flying missions, new sensors and algorithms have to be designed for relative navigation. Dependent on the distance to the target, optical sensors such as mono and stereo cameras as well as 3D sensors like laser scanners can be employed as rendezvous sensors. Navigation methods for far and close range and different verification methods are discussed.

Control of industrial robots for hardware-in-the-loop simulation of satellite docking

One of the most challenging and risky missions for spacecraft is to perform Rendezvous and Docking (RvD) autonomously in space. To ensure a safe and reliable operation, such a mission must be carefully designed and thoroughly verified before a real space mission can be launched. This paper describes the impact-contact dynamics simulation capability of a new, robotics-based, hardware-in-the-loop (HIL) RvD simulation facility which uses two industrial robots to simulate 6-DOF dynamic maneuvering of two docking satellites. The facility is capable of physically simulating the final approaching within 25-meter range and the entire docking/capturing process in a satellite on-orbit servicing mission. The paper briefly discusses the difficulties of using industrial robots for HIL contact dynamics simulation and how these problems are solved. Admittance control strategy is proposed to control the robotic system to make the robot dynamically behave like the spacecraft during a physical interception. The control strategy works as an outer loop on the top of the existing control system of the industrial robot and hence, it does not require altering the joint control hardware and software which are inaccessible for an industrial robot. A simulation study has shown that the methodology can accurately simulate the impact-contact dynamics behavior of the spacecraft in a docking operation.

Discrete Multiobjective Optimization Methodology applied to the Mixed Actuators Problem and Tested in a Hardware-in-the-loop Rendevzous Simulator

The spacecraft control problem using a set of actuators with conflicting characteristics is investigated in this paper. A novel approach, called actuator multiobjective command method, based on a discrete multiobjective optimization technique is proposed. The method is included in a coupled translational and attitude control system applied to the final approach rendezvous. Furthermore, all elements of the guidance, navigation and control loop have been developed and implemented in a simulation framework. A reaction control system, a set of reaction wheels, and a set of magnetic torqrods are the group of actuators used in this work. The discrete multiobjective problem is formulated with four objectives: torque error, fuel and electrical charge consumption, disturbance of coupling, and risk of utilization. The decision variable represent the command torque to the actuators. In addition, the hardware-in-the-loop rendezvous and docking simulation facility of the German Aerospace Center has been used to test the proposed method under realtime conditions. Results indicate that a mixed actuators methodology can achieve better performance with respect to those using the same type of actuators.

Multi-Objective Optimization Applied to Real-Time Command Problem of Spacecraft Thrusters

A novel approach to solve the real-time command problem of spacecraft thrusters, called the thruster multi-objective command method, is proposed in this paper. The reaction control system technology uses a set of thrusters in a special setup to simultaneously provide force and torque to the spacecraft. The thruster management function calculates all the candidate solutions that solve the thruster coupling problem. Then, a discrete multi-objective optimization method selects at every control cycle the best combination of thrusters and their firing time duration, which simultaneously optimizes a group of four objectives: the force error, the torque error, the propellant mass consumption, and the total number of pulses. The proposed method is included in a coupled translational and attitude control system applied to the final approach rendezvous scenario. Furthermore, all elements of the guidance, navigation, and control loop are accurately designed and implemented in a simulation framework. Results indicate...

A Novel Navigation and Sensor Strategy for Far, Mid and Close Range Rendezvous to a Cooperative Geostationary Target Spacecraft

In this article, we propose a novel concept for rendezvous to a geostationary target spacecraft. For an approach to a cooperative target in the geostationary orbit (GEO), we develop a strategy for far, mid and close range rendezvous. The concept of the iBOSS project (Intelligent Building Blocks for On-Orbit Satellite Servicing) serves as reference scenario, where a servicing satellite with rendezvous and berthing capability approaches a client satellite in GEO for possible payload manipulation or lifetime extension. In our work, we assume a scenario similar to the iBOSS case. We assume the client to be a cooperative, geostationary communication satellite which is attitude-stabilized. Different rendezvous sensors and equipment are analyzed for suitable use during a mission like iBOSS. The selected rendezvous concept is based on a camera system which consists of three cameras with different field-of-view to cover all rendezvous phases. During the entire approach, it has to be ensured, that the intensity of the light reflected by the target or emitted at the targets surface and received by the camera is large enough such that the target is visible in the camera images. In a detailed visibility analysis, we estimate the magnitude of the irradiance of the light received by the camera. Further, we present methods for relative navigation during far, mid and close range rendezvous. The results of the visibility analysis and a performance evaluation of the proposed navigation methods show that the presented concept can be employed for rendezvous to a cooperative target in GEO.

Using Industrial Robots for Hardware-in-the-Loop Simulation of Spacecraft Rendezvous and Docking

One of the most challenging and risky operations for spacecraft is to perform proximity Rendezvous and Docking (R&D) autonomously in space. To ensure a safe and reliable operation, such a mission must be carefully designed and thoroughly verified before a real space mission can be launched. This paper describes the control strategy for achieving high fidelity contact dynamics simulation of a new, robotics-based, hardware-in-the-loop (HIL) R&D simulation facility which uses two industrial robots to simulate the 6-DOF dynamic maneuvering of the two docking satellites. The facility is capable of physically simulating the final approaching within a 25-meter range and the entire docking or capturing process in a satellite on-orbit servicing mission. The paper discusses the difficulties of using industrial robots for HIL contact dynamics simulation and the proposed robot control strategy for dealing with these difficulties.Copyright