Hans-Peter Roeser

Active damping of the SOFIA Telescope assembly

The NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA) employs a 2.5-meter reflector telescope in a Boeing 747SP. The telescope is housed in an open cavity and is subjected to aeroacoustic and inertial disturbances in flight. To meet pointing requirements, SOFIA must achieve a pointing stability of approximately 0.5 arcseconds RMS. An active damping control system is being developed for SOFIA to reduce image jitter and image degradation due to resonance of the telescope assembly. Our paper discusses the history of the active damping design for SOFIA, from early concepts to the current implementation which has recently completed a ground and flight testing for proof-of-concept. We describe some milestones in the analysis and testing of the telescope assembly which guided the development of the vibration control system. The control synthesis approach and current implementation of the active damping control system is presented. Finally, we summarize the performance observed in early flight tests and the steps that are currently foreseen to completing the development of this system.

Development of a safe mode attitude control for a FPGA based micro satellite

This paper presents the development of the safe mode attitude controller for the micro-satellite Flying Laptop (FLP). The safe mode for the FLP is defined as the situation when the satellite is able to continue its normal operation with reduced functionality on the occurrence of serious anomaly in the software or the hardware of the satellite, which cannot be automatically processed and corrected on board. This mode is used to ensure the satellites vital functions (e.g. on board power) while in the meantime, the failure is being analyzed on ground. In safe mode, the satellite is controlled through autonomous software dedicated for such a condition. The sensors used for this mode should reflect high reliability and the sensor outputs need to be available all the time, however only a coarse pointing knowledge is required during this mode.

Modeling of the SOFIA secondary mirror controller

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5m infrared telescope built into a Boeing 747SP. During observations the telescope will not only be subject to aircraft vibrations and maneuver loads - by opening a large door to give the observatory an unhindered view of the sky, there will also be aerodynamic and aeroacoustic disturbances. A critical factor in the overall telescope performance is the SOFIA Secondary Mirror Assembly (SMA). The 35cm silicon carbide mirror is mounted on the Secondary Mirror Mechanism (SMM), which has five degrees-offreedom and consists of two parts: The slow moving base for focusing and centering, and on top of that the Tilt Chop Mechanism (TCM) for chopping with a frequency of up to 20Hz and a chop throw of up to 10arcmin. The development of the controller that is used to meet the stringent performance requirements relys heavily on a state space model of the system. A pole-placement controller is compared to an optimal LQG control approach which makes use of the model to calculate all required system states in real-time. The paper explains the modeling of the TCM with linear differential equations and the optimization via a grey-box model approach with system identification data. Simulated data is then compared to measurements taken on ground and in flight.

Preparation of the pointing and control system of the SOFIA Airborne Telescope for early science missions

During observation flights the telescope structure of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is subject to disturbance excitations over a wide frequency band. The sources can be separated into two groups: inertial excitation caused by vibration of the airborne platform, and aerodynamic excitation that acts on the telescope assembly (TA) through an open port cavity. These disturbance sources constitute a major difference of SOFIA to other ground based and space observatories and achieving the required pointing accuracy of 1 arcsecond cumulative rms or better below 70 Hz in this environment is driving the design of the TA pointing and control system. In the current design it consists of two parts, the rigid body attitude control system and a feed forward based compensator of flexible TA deformation. This paper discusses the characterization and control system tuning of the as-built system. It is a process that integrates the study of the structural dynamic behavior of the TA, the resulting image motion in the focal plane, and the design and implementation of active control systems. Ground tests, which are performed under controlled experimental conditions, and in-flight characterization tests, both leading up to the early science performance capabilities of the observatory, are addressed.

Enhanced Evaluation of Recombination Coefficient Measurements in Plasma Wind Tunnels

This paper describes the methodology of determining the recombination coefficients for candidate materials of the catalytic based sensor system PHLUX. The methodology was broadened in terms of evaluation of the specific heat flux on catalytic surfaces. This leads to the possibility of calculating species concentration and atom net fluxes on and at the surface directly. Recombination coefficients with an indication of temperature and pressure level at which they were determined is given. Total emissivities for these materials for temperatures up to 2300K are provided.

Reflection properties of vegetation and soil with a new BRDF database

At least in the visible and near infrared spectral region the basic principles about the bi-directional reflectance distribution function (BRDF) have achieved an advanced state. This basic knowledge has been summarized by international well known scientists and firstly published in a book. A CD-ROM data base is attached to the book. For this paper these data have been used for a very first statistical synopsis. The measurement results of different scientists for similar targets and similar geometric conditions differ partly to a great extent. Hence mean values and variances cannot presently be used to describe the BRDF of different surfaces. The anisotropy factor defined by Sandmeier seems appropriate to be used in some investigations. The data sets are not sufficient for serious statistical analyses. Further measurements are essential to overcome the gap from examples to confidential statistical evaluations. In the last section the measurement methods planned by the University of Stuttgart for a ground measuring device and by target pointing of a small satellite are discussed.

SOFIA's secondary mirror assembly: in-flight performance and control approach

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5m infrared telescope built into a Boeing 747 SP. In 2014 SOFIA reached its Full Operational Capability milestone and nowadays takes off about three times a week to observe the infrared sky from altitudes above most of the atmosphere’s water vapor content. An actively controlled 352mm SiC secondary mirror is used for infrared chopping with peak-to-peak amplitudes of up to 10 arcmin and chop frequencies of up to 20Hz and also as actuator for fast pointing corrections. The Swiss-made Secondary Mirror Mechanism (SMM) is a complex, highly integrated and compact flexure based mechanism that has been performing with remarkable reliability during recent years. Above mentioned capabilities are provided by the Tilt Chopper Mechanism (TCM) which is one of the two stages of the SMM. In addition the SMM is also used to establish a collimated telescope and to adjust the telescope focus depending on the structure’s temperature which ranges from about 40°C at takeoff in Palmdale, CA to about −40◦C in the stratosphere. This is achieved with the Focus Center Mechanism (FCM) which is the base stage of the SMM on which the TCM is situated. Initially the TCM was affected by strong vibrations at about 300 Hz which led to unacceptable image smearing. After some adjustments to the PID-type controller it was finally decided to develop a completely new control algorithm in state space. This pole placement controller matches the closed loop system poles to those of a Bessel filter with a corner frequency of 120 Hz for optimal square wave behavior. To reduce noise present on the position and current sensors and to estimate the velocity a static gain Kalman Filter was designed and implemented. A system inherent delay is incorporated in the Kalman filter design and measures were applied to counteract the actuators’ hysteresis. For better performance over the full operational temperature range and to represent an amplitude dependent non-linearity the underlying model of the Kalman filter adapts in real-time to those two parameters. This highly specialized controller was developed over the course of years and only the final design is introduced here. The main intention of this contribution is to present the currently achieved performance of the SOFIA chopper over the full amplitude, frequency, and temperature range. Therefore a range of data gathered during in-flight tests aboard SOFIA is displayed and explained. The SMM’s three main performance parameters are the transition time between two chop positions, the stability of the Secondary Mirror when exposed to the low pressures, low temperatures, aerodynamic, and aeroacoustic excitations present when the SOFIA observatory operates in the stratosphere at speeds of up to 850 km/h, and finally the closed-loop bandwidth available for fast pointing corrections.

Low Cost Control and Simulation Environment for the "Flying Laptop", a University Microsatellite

The Institute of Space Systems at the University of Stuttgart is preparing the Flying Laptop, a microsatellite for technology evaluation and Earth observation. The university environment mandates low cost, which rules out engineering models. Thus a model-based development approach is used to verify concepts during the development phase and to support on-board software tests. The applied simulation infrastructure should use professional software to train students on an appropriate environment. However, the software has also to be intuitive and easy to learn in order that students can be quickly productive in the limited time that they have available during their theses. The solution was to combine established industry toolsets, supplied at low cost. The system simulation infrastructure Model-based Development and Verification Environment (MDVE) was developed by Astrium as real time simulator and is commanded via an ESA SCOS-2000 mission control system. The spacecraft can be visualized by connecting the real-time 3D visualization software Celestia to the simulator. Efficient commanding is ensured by the procedure editor and execution engine Manufacturing and Operations Information System (MOIS) which replaces the operator and enables automated tests.

An Explicit Guidance Method for a Lifting Interplanetary Re-Entry Vehicle

Future long-term plans for the robotic or human exploration of solar system bodies demand new and innovative concepts for the design of vehicles which can enter a planetary atmosphere and land on its surface safely. There has been and continues to be, considerable interest and research into the development of new trajectory generators and guidance systems for aerospace vehicles. Vehicle approaching a planet at hyperbolic speed passes through its atmosphere with very high thermal and mechanical loads. Vehicle and mission design process requires a thorough investigation of this flight phase and its sensitivity to environmental influences and uncertainties as well as vehicle properties. This report describes a guidance scheme combining onboard flight path prediction and optimization utilizing non-linear programming techniques. The optimization program makes use of a complex optimization method to find an optimized set of control parameters for a prescribed cost function and restrictions only once at the beginning of a mission phase, whereas the guidance program makes use of a simplified and fast method of Gradient Projection Algorithm (GPA) in order to have less computation load onboard.

Preliminary System Simulation Environment of the University Micro-Satellite Flying Laptop

The Institute of Space Systems at the Universitat Stuttgart builds a\break micro-satellite for the purpose of technology evaluation and scientific experiments. The complexity of the project and its limited human resources require efficient modern development and engineering techniques. To support the satellite development process, EADS Astrium Friedrichshafen has sponsored a Model-based Development and Verification Environment infrastructure, to be used for test and design verification. This paper focuses on the set-up of the Model-based Development and Verification Environment simulator, in particular on the configuration of the simulated satellite environment, the creation of equipment models and operational facilities to control the simulated satellite and the simulator. Orbit and thermal simulations were conducted to demonstrate the functionality of the environment setup under consideration.