Kuo-Chia Liu

Reaction Wheel Disturbance Modeling, Jitter Analysis, and Validation Tests for Solar Dynamics Observatory

The Solar Dynamics Observatory (SDO) aims to study the Suns influence on the Earth by understanding the source, storage, and release of the solar energy, and the interior structure of the Sun. During science observations, the jitter stability at the instrument focal plane must be maintained to less than a fraction of an arcsecond for two of the SDO instruments. To meet these stringent requirements, significant amount of analysis and test effort have been devoted to predicting the jitter induced from various disturbance sources. One of the largest disturbances onboard is the reaction wheel. This paper presents the SDO approach on reaction wheel disturbance modeling and jitter analysis. It describes the verification and calibration of the disturbance model, and ground tests performed for validating the reaction wheel jitter analysis. To mitigate the reaction wheel disturbance effects, the wheels will be limited to operate at low wheel speeds based on the current analysis. An on-orbit jitter test algorithm is also presented in the paper which will identify the true wheel speed limits in order to ensure that the wheel jitter requirements are met.

The neutron star interior composition explorer (NICER): mission definition

Over a 10-month period during 2013 and early 2014, development of the Neutron star Interior Composition Explorer (NICER) mission [1] proceeded through Phase B, Mission Definition. An external attached payload on the International Space Station (ISS), NICER is scheduled to launch in 2016 for an 18-month baseline mission. Its prime scientific focus is an in-depth investigation of neutron stars—objects that compress up to two Solar masses into a volume the size of a city—accomplished through observations in 0.2–12 keV X-rays, the electromagnetic band into which the stars radiate significant fractions of their thermal, magnetic, and rotational energy stores. Additionally, NICER enables the Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) demonstration of spacecraft navigation using pulsars as beacons. During Phase B, substantive refinements were made to the mission-level requirements, concept of operations, and payload and instrument design. Fabrication and testing of engineering-model components improved the fidelity of the anticipated scientific performance of NICER’s X-ray Timing Instrument (XTI), as well as of the payload’s pointing system, which enables tracking of science targets from the ISS platform. We briefly summarize advances in the mission’s formulation that, together with strong programmatic performance in project management, culminated in NICER’s confirmation by NASA into Phase C, Design and Development, in March 2014.

Precision telescope pointing and spacecraft vibration isolation for the Terrestrial Planet Finder Coronagraph

The Terrestrial Planet Finder Coronagraph is a visible-light coronagraph to detect planets that are orbiting within the Habitable Zone of stars. The coronagraph instrument must achieve a contrast ratio stability of 2e-11 in order to achieve planet detection. This places stringent requirements on several spacecraft subsystems, such as pointing stability and structural vibration of the instrument in the presence of mechanical disturbance: for example, telescope pointing must be accurate to within 4 milli-arcseconds, and the jitter of optics must be less than 5 nm. This paper communicates the architecture and predicted performance of a precision pointing and vibration isolation approach for TPF-C called Disturbance Free Payload (DFP)* . In this architecture, the spacecraft and payload fly in close-proximity, and interact with forces and torques through a set of non-contact interface sensors and actuators. In contrast to other active vibration isolation approaches, this architecture allows for isolation down to zero frequency, and the performance of the isolation system is not limited by sensor characteristics. This paper describes the DFP architecture, interface hardware and technical maturity of the technology. In addition, an integrated model of TPF-C Flight Baseline 1 (FB1) is described that allows for explicit computation of performance metrics from system disturbance sources. Using this model, it is shown that the DFP pointing and isolation architecture meets all pointing and jitter stability requirements with substantial margin. This performance relative to requirements is presented, and several fruitful avenues for utilizing performance margin for system design simplification are identified.

Passive isolator design for jitter reduction in the Terrestrial Planet Finder Coronagraph

Terrestrial Planet Finder (TPF) is a mission to locate and study extrasolar Earthlike planets. The TPF Coronagraph (TPF-C), planned for launch in the latter half of the next decade, will use a coronagraphic mask and other optics to suppress the light of the nearby star in order to collect visible light from such planets. The required contrast ratio of 5e-11 can only be achieved by maintaining pointing accuracy to 4 milli-arcseconds, and limiting optics jitter to below 5 nm. Numerous mechanical disturbances act to induce jitter. This paper concentrates on passive isolation techniques to minimize the optical degradation introduced by disturbance sources. A passive isolation system, using compliant mounts placed at an energy bottleneck to reduce energy transmission above a certain frequency, is a low risk, flight proven design approach. However, the attenuation is limited, compared to an active system, so the feasibility of the design must be demonstrated by analysis. The paper presents the jitter analysis for the baseline TPF design, using a passive isolation system. The analysis model representing the dynamics of the spacecraft and telescope is described, with emphasis on passive isolator modeling. Pointing and deformation metrics, consistent with the TPF-C error budget, are derived. Jitter prediction methodology and results are presented. Then an analysis of the critical design parameters that drive the TPFC jitter response is performed.

Solar Dynamics Observatory On-Orbit Jitter Testing, Analysis, and Mitigation Plans

The recently launched Solar Dynamics Observatory (SDO) has two science instruments onboard that required sub-arcsecond pointing stability. Significant effort has been spent pre-launch to characterize the disturbances sources and validating jitter level at the component, sub-assembly, and spacecraft levels. However, an end-to-end jitter test emulating the flight condition was not performed on the ground due to cost and risk concerns. As a result, the true jitter level experienced on orbit remained uncertain prior to launch. Based on the pre-launch analysis, several operational constraints were placed on the observatory aimed to minimize the instrument jitter levels. If the actual jitter is below the analysis predictions, these operational constraints can be relaxed to reduce the burden of the flight operations team. The SDO team designed a three-day jitter test, utilizing the instrument sensors to measure pointing jitter up to 256 Hz. The test results were compared to pre-launch analysis predictions, used to determine which operational constraints can be relaxed, and analyzed for setting the jitter mitigation strategies for future SDO operations.

Solar Dynamics Observatory Guidance, Navigation, and Control System Overview

Angela M. Russo, Scott R. Starin, and Melissa F. VessNASA Goddard Space Flight Center Code 591, Greenbelt, Maryland 20771AbstractThe Solar Dynamics Observatory (SDO) was designed and built at the Goddard Space Flight Center, launchedfrom Cape Canaveral on February 11, 2010, and reached its final geosynchronous science orbit on March 16, 2010.The purpose of SDO is to observe the Sun and continuously relay data to a dedicated ground station. SDO remainsSun-pointing throughout most of its mission for the instruments to take measurements of the Sun. The SDO attitudecontrol system (ACS) is a single-fault tolerant design. Its fully redundant attitude sensor complement includessixteen coarse Sun sensors (CSSs), a digital Sun sensor (DSS), three two-axis inertial reference units (IRUs), andtwo star trackers (STs). The ACS also makes use of the four guide telescopes included as a part of one of the scienceinstruments. Attitude actuation is performed using four reaction wheels assemblies (RWAs) and eight thrusters, witha single main engine used to provide velocity-change thrust for orbit raising. The attitude control software has fivenominal control modes, three wheel-based modes and two thruster-based modes. A wheel-based Safehold running inthe attitude control electronics box improves the robustness of the system as a whole. All six modes are designed onthe same basic proportional-integral-derivative attitude error structure, with more robust modes setting their integralgains to zero. This paper details the final overall design of the SDO guidance, navigation, and control (GNCAtmospheric Imaging Assembly (AIA), led by Lockheed Martin Space and Astrophysics Laboratory; and ExtremeUltraviolet Variability Experiment (EVE), led by the University of Colorado. The basic mission is to observe theSun for a very high percentage of the 5-year mission (10-year goal) with long stretches of uninterrupted observationsand with constant, high-data-rate transmission to a dedicated ground station to be located in White Sands, NewMexico. These goals guided the design of the spacecraft bus that will carry and service the three-instrument payload.Overarching design goals for the bus are geosynchronous orbit, near-constant Sun observations with the ability to flythrough eclipses, and constant HGA contact with the dedicated ground station. A three-axis stabilized ACS isneeded both to point at the Sun accurately and to keep the roll about the Sun vector correctly positioned with respectto the solar north pole. This roll control is especially important for the magnetic field imaging of HM I.The mission requirements have several general impacts on the ACS design. Both the AIA and HMI instrumentsare very sensitive to the blurring caused by jitter. Each has an image stabilization system (ISS) with some ability tofilter out high frequency motion, but below the bandwidth of the ISS the control system must compensate fordisturbances within the ACS bandwidth or avoid exciting jitter at higher frequencies.Within the ACS bandwidth, the control requirement imposed by AIA is to place the center of the solar disk nomore than 2 arc

Solar Dynamics Observatory (SDO) HGAS Induced Jitter

This paper presents the results of a comprehensive assessment of High Gain Antenna System induced jitter on the Solar Dynamics Observatory. The jitter prediction is created using a coupled model of the structural dynamics, optical response, control systems, and stepper motor actuator electromechanical dynamics. The paper gives an overview of the model components, presents the verification processes used to evaluate the models, describes validation and calibration tests and model-to-measurement comparison results, and presents the jitter analysis methodology and results.

Dynamic stability with the disturbance-free payload architecture as applied to the Large UV/Optical/Infrared (LUVOIR) mission

The need for high payload dynamic stability and ultra-stable mechanical systems is an overarching technology need for large space telescopes such as the Large Ultraviolet / Optical / Infrared (LUVOIR) Surveyor. Wavefront error stability of less than 10 picometers RMS of uncorrected system WFE per wavefront control step represents a drastic performance improvement over current space-based telescopes being fielded. Previous studies of similar telescope architectures have shown that passive telescope isolation approaches are hard-pressed to meet dynamic stability requirements and usually involve complex actively-controlled elements and sophisticated metrology. To meet these challenging dynamic stability requirements, an isolation architecture that involves no mechanical contact between telescope and the host spacecraft structure has the potential of delivering this needed performance improvement. One such architecture, previously developed by Lockheed Martin called Disturbance Free Payload (DFP), is applied to and analyzed for LUVOIR. In a noncontact DFP architecture, the payload and spacecraft fly in close proximity, and interact via non-contact actuators to allow precision payload pointing and isolation from spacecraft vibration. Because disturbance isolation through non-contact, vibration isolation down to zero frequency is possible, and high-frequency structural dynamics of passive isolators are not introduced into the system. In this paper, the system-level analysis of a non-contact architecture is presented for LUVOIR, based on requirements that are directly traceable to its science objectives, including astrophysics and the direct imaging of habitable exoplanets. Aspects of architecture and how they contribute to system performance are examined and tailored to the LUVOIR architecture and concept of operation.

Verification of the Solar Dynamics Observatory High Gain Antenna Pointing Algorithm Using Flight Data

The Solar Dynamics Observatory (SDO), launched in 2010, is a NASA-designed spacecraft built to study the Sun. SDO has tight pointing requirements and instruments that are sensitive to spacecraft jitter. Two High Gain Antennas (HGAs) are used to continuously send science data to a dedicated ground station. Preflight analysis showed that jitter resulting from motion of the HGAs was a cause for concern. Three jitter mitigation techniques were developed and implemented to overcome effects of jitter from different sources. These mitigation techniques include: the random step delay, stagger stepping, and the No Step Request (NSR). During the commissioning phase of the mission, a jitter test was performed onboard the spacecraft, in which various sources of jitter were examined to determine their level of effect on the instruments. During the HGA portion of the test, the jitter amplitudes from the single step of a gimbal were examined, as well as the amplitudes due to the execution of various gimbal rates. The jitter levels were compared with the gimbal jitter allocations for each instrument. The decision was made to consider implementing two of the jitter mitigating techniques on board the spacecraft: stagger stepping and the NSR. Flight data with and without jitter mitigation enabled was examined, and it is shown in this paper that HGA tracking is not negatively impacted with the addition of the jitter mitigation techniques. Additionally, the individual gimbal steps were examined, and it was confirmed that the stagger stepping and NSRs worked as designed. An Image Quality Test was performed to determine the amount of cumulative jitter from the reaction wheels, HGAs, and instruments during various combinations of typical operations. The HGA-induced jitter on the instruments is well within the jitter requirement when the stagger step and NSR mitigation options are enabled.

Requirements formulation and dynamic jitter analysis on Fourier-Kelvin stellar interferometer

The Fourier-Kelvin Stellar Interferometer (FKSI) has been proposed to detect and characterize extra solar giant planets. The baseline configuration for FKSI is a two-aperture, structurally connected nulling interferometer, capable of providing null depth less than 10-4 in the infrared. The objective of this paper is to summarize the process for setting the top level requirements and the jitter analysis performed on FKSI to date. The first part of the paper discusses the derivation of dynamic stability requirements, necessary for meeting the FKSI nulling demands. An integrated model including structures, optics, and control systems has been developed to support dynamic jitter analysis and requirements verification. The second part of the paper describes how the integrated model is used to investigate the effects of reaction wheel disturbances on pointing and optical path difference stabilities.