Heike Benninghoff

Efficient Image Segmentation and Restoration Using Parametric Curve Evolution with Junctions and Topology Changes

Curve evolution schemes are introduced for image segmentation based on a region-based contour model allowing for junctions, vector-valued images, and topology changes. Together with an a posteriori denoising in the segmented homogeneous regions, this leads to a fast and efficient method for image segmentation and restoration. An uneven spread of mesh points is avoided by using the tangential degrees of freedom. Several numerical simulations on artificial test problems and on real images illustrate the performance of the method.

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.

Segmentation and Restoration of Images on Surfaces by Parametric Active Contours with Topology Changes

In this article, a new method for segmentation and restoration of images on two-dimensional surfaces is given. Active contour models for image segmentation are extended to images on surfaces. The evolving curves on the surfaces are mathematically described using a parametric approach. For image restoration, a diffusion equation with Neumann boundary conditions is solved in a postprocessing step in the individual regions. Numerical schemes are presented which allow to efficiently compute segmentations and denoised versions of images on surfaces. Also topology changes of the evolving curves are detected and performed using a fast sub-routine. Finally, several experiments are presented where the developed methods are applied on different artificial and real images defined on different surfaces.

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.

Image Segmentation Using Parametric Contours With Free Endpoints

In this paper, we introduce a novel approach for active contours with free endpoints. A scheme for image segmentation is presented based on a discrete version of the Mumford-Shah functional where the contours can be both closed and open curves. Additional to a flow of the curves in normal direction, evolution laws for the tangential flow of the endpoints are derived. Using a parametric approach to describe the evolving contours together with an edge-preserving denoising, we obtain a fast method for image segmentation and restoration. The analytical and numerical schemes are presented followed by numerical experiments with artificial test images and with a real medical image.

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.

Segmentation of Three-Dimensional Images with Parametric Active Surfaces and Topology Changes

In this paper, we introduce a novel parametric finite element method for segmentation of three-dimensional images. We consider a piecewise constant version of the Mumford–Shah and the Chan–Vese functionals and perform a region-based segmentation of 3D image data. An evolution law is derived from energy minimization problems which push the surfaces to the boundaries of 3D objects in the image. We propose a parametric scheme which describes the evolution of parametric surfaces. An efficient finite element scheme is proposed for a numerical approximation of the evolution equations. Since standard parametric methods cannot handle topology changes automatically, an efficient method is presented to detect, identify and perform changes in the topology of the surfaces. One main focus of this paper are the algorithmic details to handle topology changes like splitting and merging of surfaces and change of the genus of a surface. Different artificial images are studied to demonstrate the ability to detect the different types of topology changes. Finally, the parametric method is applied to segmentation of medical 3D images.

Rendezvous Simulation for On-Orbit Servicing Missions Using Advanced Robotic Technology

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. Additionally this paper presents first results of hardware in the loop simulations for a rendezvous process to a non-cooperative target.