Stephan Theil

Odyssey: A Solar System Mission

The Solar System Odyssey mission uses modern-day high-precision experimental techniques to test the laws of fundamental physics which determine dynamics in the solar system. It could lead to major discoveries by using demonstrated technologies and could be flown within the Cosmic Vision time frame. The mission proposes to perform a set of precision gravitation experiments from the vicinity of Earth to the outer Solar System. Its scientific objectives can be summarized as follows: (1) test of the gravity force law in the Solar System up to and beyond the orbit of Saturn; (2) precise investigation of navigation anomalies at the fly-bys; (3) measurement of Eddington’s parameter at occultations; (4) mapping of gravity field in the outer solar system and study of the Kuiper belt. To this aim, the Odyssey mission is built up on a main spacecraft, designed to fly up to 13 AU, with the following components: (a) a high-precision accelerometer, with bias-rejection system, measuring the deviation of the trajectory from the geodesics, that is also giving gravitational forces; (b) Ka-band transponders, as for Cassini, for a precise range and Doppler measurement up to 13 AU, with additional VLBI equipment; (c) optional laser equipment, which would allow one to improve the range and Doppler measurement, resulting in particular in an improved measurement (with respect to Cassini) of the Eddington’s parameter. In this baseline concept, the main spacecraft is designed to operate beyond the Saturn orbit, up to 13 AU. It experiences multiple planetary fly-bys at Earth, Mars or Venus, and Jupiter. The cruise and fly-by phases allow the mission to achieve its baseline scientific objectives [(1) to (3) in the above list]. In addition to this baseline concept, the Odyssey mission proposes the release of the Enigma radio-beacon at Saturn, allowing one to extend the deep space gravity test up to at least 50 AU, while achieving the scientific objective of a mapping of gravity field in the outer Solar System [(4) in the above list].

Testbed for on-orbit servicing and formation flying dynamics emulation

The testbed for on-orbit servicing and formation flying dynamics emulation is a facility to emulate the force and momentum-free dynamics of multi-spacecraft missions on ground. The facility consists of a large granite table and several air cushion vehicles that float on this table. The granite table has a size of 4m x 2.5m and a thickness of 0.6m with a very high evenness. The air-cushion vehicles consist of two parts: a lower translation platform and an upper attitude platform. Both platforms are connected by a spherical air bearing. The translation platform carries the flat air bearings which enable the vehicle to float on the table, high pressure air tanks to support the flat and the spherical air bearings and a vertical linear actuator. With this actuator it is possible to adjust the altitude of the spherical air bearing and thereby the altitude of the attitude platform. All components needed for a control loop to actuate the air cushion vehicles are mounted on the attitude platform. These are 12 proportional cold gas thrusters with high pressure air tanks, 3 reaction wheels, an onboard computer with real-time operating system and Wireless LAN for communications and software upload, an inertial measurement unit (IMU) and a sensor for an infrared tracking system. In addition the attitude platform carries a power system with batteries to support all components with electrical energy and a balancing system to move the center of mass in the center of the spherical air bearing. Due to the linear air bearing the system can emulate 2 translational degrees of freedom. The third translational degree of freedom (parallel to gravity vector) can be actuated by the vertical linear actuator. As the attitude platform can tilt and rotate, 3 rotational degrees of freedom are emulated. So there is a total of 5+1 degrees of freedom that can be emulated by the testbed. One purpose of this testbed is the testing of control algorithms for multi-spacecraft missions. It can be used for precise formation control as well as path-planning and formation acquisition. In addition to the usage of real hardware as in Hardware-in-the-Loop simulations, the force and momentum-free dynamics are not simulated but are real (emulation). This is also a foundation for the second task of the testbed: the emulation of contact dynamics for Rendezvous and Docking missions. Models of docking adapters can be mounted on the attitude platform of the air-cushion vehicles and docking maneuvers with real contact dynamics can be tested on ground. A further purpose of the testbed is the qualification of dedicated relative navigation sensors and inter-spacecraft communication systems. These components can also be mounted on the attitude platform and tested in an agile environment. This paper describes the design and configuration of the testbed for on-orbit servicing and formation flying dynamics emulation. Results showing the quality of the force and momentum-free environment are presented. First control, navigation and thruster actuation algorithms have been developed and their design and the result of their usage within the testbed are also shown. Finally possible scenarios for the usage of this testbed are presented and an outlook on further developments and improvements is given.

TRON - Hardware-in-the-Loop Test Facility for Lunar Descent and Landing Optical Navigation

Abstract Future exploration missions require advanced optical sensors for precise navigation and landing site evaluation. The Testbed for Robotic Optical Navigation (TRON) is a Hardware-in-the-Loop test environment, with the purpose to support the development of optical navigation technology, and to qualify breadboards to TRL 4, and to qualify flight models to TRL 5-6. In this paper the design and ongoing realization of TRON is discussed. The first application of TRON is to simulate relevant parts of the lunar landing. After illustrating the concept, the building blocks of the laboratory are explained in detail. These are the simulation of the scaled dynamics via a 7-DOF robot, the simulation of the optical environment via a black out system and a lighting system, and the simulation of the terrain geometry via scaled 3D terrain models. With modifications TRON can also provide relevant environments for Mars, asteroids and moons.

Hybrid Navigation System for the SHEFEX-2 Mission

In October 2005 the first experiment of the DLR hypersonic SHarp Edge Flight EXperiment Program was successfully launched at the Andoya Rocket Range in northern Norway with the purpose to investigate possible new shapes for future launcher or re-entry vehicles applying a shape with facetted surfaces and sharp edges. For 2010 the execution of the SHEFEX-2 experiment is planned. It shall focus on hypersonic flight control using steerable canard fins while new thermal-protection system concepts will also be a subject of investigation. The accurate control of the vehicle using the canards requires a high accuracy in knowledge of the angle of attack and the side slip angle. Both angles can only be derived from the flight path and an accurate inertial attitude measurement. The first can be achieved by using GPS measurements. The second can not be provided by an Inertial Navigation System due to the fact that drifts due to launch vibrations are exceeding the requirement. Therefore a star tracker is foreseen to update the attitude information shortly before re-entry. Since the case of SHEFEX-2 describes a re-entry scenario which is applicable to other re-entry missions the need arises to develop an integrated navigation system which can provide the navigation solution with needed accuracy. This navigation system shall combine the measurements from inertial measurement unit (IMU), GPS receiver and star tracker. A further extension to include other sensors shall be foreseen. The paper will describe the concept of the integrated navigation. A special focus is also laid on the analysis how the star tracker can be integrated into the SHEFEX-2 probe.

Adaptive Disturbance-Based High-Order Sliding-Mode Control for Hypersonic-Entry Vehicles

In this paper, an adaptive, disturbance-based sliding-mode controller for hypersonic-entry vehicles is proposed. The scheme is based on high-order sliding-mode theory, and is coupled to an extended...

Design of robust drag-free controllers with given structure

In this paper the problem of designing a robust controller with given structure for a plant describing a drag-free satellite is addressed. From recent experiences in drag-free control design we first derive an uncertain plant set representative of many drag-free missions with nonspherical test masses. The design plant is uncertain and a performance requirement is imposed on the absolute acceleration of the test mass along a measurement axis. The v-gap metric is first used to derive a simplified uncertain design plant. Then the main performance requirement is broken down into requirements on the uncertain closed loop behavior of the simplified system. The fulfillment of this new set of requirements guarantees robust achievement of the overall system goal. Then optimal single-input–singleoutput controllers are designed that robustly achieve the desired level of performance. The method proposed allows one to properly account for the uncertainties in the system retaining the decentralized structure of the controller suggested by the peculiar features of the design plant.

Hybrid Navigation System for Spaceplanes, Launch and Re-Entry Vehicles

In 2010 the German Aerospace Center (DLR) will conduct the second experiment of the DLR hypersonic SHarp Edge Flight EXperiment Program SHEFEX-2. The purpose is to investigate possible new shapes for future launcher or re-entry vehicles with faceted surfaces and sharp edges and to demonstrate key technologies for re-entry like hypersonic ight control using steerable canard ns. Accurate control of the vehicle using the canards requires a highly accurate knowledge of the angle of attack and the side slip angle. Both angles can only be derived from the ight path and an attitude measurement. The rst can be achieved using GPS measurements. The second can be provided only by the most accurate Inertial Navigation Systems (INS) because drifts due to launch vibrations exceed the accuracy requirements. Therefore, a star tracker will be used to update the attitude information shortly before entry. The SHEFEX-2 mission describes an entry scenario which is applicable to other entry missions. There is a general need to develop a high accuracy integrated navigation system which can be used for multiple missions. This navigation system should combine the measurements from an inertial measurement unit (IMU), GPS receiver and star tracker with the option to include additional sensors. This paper will describe the concept of the integrated navigation system with a focus on integrating the star tracker into the SHEFEX-2 experiment.

Hybrid Jacobian Computation for Fast Optimal Trajectories Generation

Nowadays the new, increased capabilities of CPUs have constantly encouraged researchers and engineers towards the investigation of numerical optimization as an analysis and synthesis tool in order to generate optimal trajectories and the controls to track them. In particular, one of the most promising techniques is represented by direct collocation methods. Among these, Pseudospectral Methods are gaining popularity for their straightforward implementation and some useful properties, like the possibility to remove the Runge phenomenon present in traditional interpolation techniques and the “spectral” convergence observable in the case of smooth problems. Experience shows that the quality of the results and the computation time are strongly affected by the jacobian matrix describing the transcription of the optimal control problem as an NLP. In this paper, the structure of the Jacobian matrix is analyzed, taking advantage of the sparse nature of such matrices. Additionally, its systematic “hybridization” will be discussed and implemented in order to speed up the simulations. Two different problems will be then described and solved with this approach and the results will be shown. Finally, a quantitative analysis of the performances deriving from the use of the hybrid jacobian compared to a traditional numerical technique will be shown as well.

ASTROD and ASTROD I: Progress Report

Over the next decade the gravitational physics community will benefit from dramatic improvements in many technologies critical to the tests of gravity and gravitational-wave detection. The highly accurate deep space navigation, interplanetary laser ranging and communication, interferometry and metrology, high precision frequency standards, precise pointing and attitude control, together with the drag-free technologies will revolutionize the field of the experimental gravitational physics. Deep-space laser ranging will be ideal for gravitational-wave detection, and testing relativity and measuring solar-system parameter to an unprecedented accuracy. ASTROD I is such a mission with single spacecraft; it is the first step of ASTROD (Astrodynamical Space Test of Relativity using Optical Devices) with 3 spacecraft. In this paper, we will present the progress of ASTROD and ASTROD I with emphases on the acceleration noises, mission requirement, charging simulation, drag-free control and low-frequency gravitational-wave sensitivity.

Attitude Determination for the SHEFEX 2 Mission Using a Low Cost Star Tracker

After the successfully launched hypersonic SHarp Edge Flight EXperiment (SHEFEX) in 2005, DLR has scheduled a second re-entry experiment SHEFEX 2 for 2010. The novel flight control system based on steerable canard fins requires a precise knowledge of the inertial spacecraft attitude. For this purpose, a star tracker is to be conceived and integrated in an inertial navigation system, updating the attitude information before re-entry. The developed star tracker is considered to be a low cost and low accuracy sensor that is suitable for the proposed mission and the attitude accuracy requirements. This sensor is based on an off-the-shelf camera and a PC104 computer communicating with the navigation computer. The attitude determination software consists of a processing chain including the camera control, the image processing, the star identification and the attitude estimation. The emphasis of the work is on developing a simple but robust system. Therefore, different algorithm concepts for each element of the chain have been implemented, tested and compared. Furthermore, star image simulation software has been developed, allowing performance and speed tests of the single blocks and the processing chain as a unit. The paper presents the star tracker system, details some newly developed parts and discusses the test results.