Simulation models of new opto-mechanical systems are often based on engineering experience with older, potentially
dissimilar systems. This can result in inaccuracies in the model prediction. A method is needed to gauge the fidelity of
new system models in the initial design phases, often in the absence of hardware data. The Nyquist criterion is used to
develop a quantitative measure of model fidelity, called the Nyquist fidelity metric. The spatial Nyquist fidelity method
is presented which uses the Nyquist fidelity metric to both assess the fidelity of existing complex models and to
synthesize new multi-component models starting from architectural considerations such as geometric and material
properties of the system. This method also estimates the error bound on the output figures of merit based on the fidelity
levels and sensitivity analysis. The Nyquist fidelity method is applied to the Modular Optical Space Telescope (MOST),
the Thirty Meter Telescope, and the Stratospheric Observatory for Infrared Astronomy. It is shown in the MOST case
study that the Nyquist fidelity method provides a 40% improvement in computational time while assuring less than 5%
modal frequency error, and less than 2.2% error in the output figure of merit.
Analysis of complex interdisciplinary systems such as large telescopes is usually performed using simulation models due to the expense of hardware testbeds. The level of fidelity of these simulation models, or the relative closeness to which the model simulates reality for the behavior under investigation, is often not assessed in a quantitative manner. Rather, the model is described qualitatively as being of either "low" or "high" fidelity, often progressing from lower to higher fidelity models as the candidate designs are down-selected. This paper provides a quantitative assessment of fidelity for structural subsystems for large telescope models based on the Nyquist criterion, shows how it influences simulation accuracy, and applies it to a space telescope model. This metric will be useful for assessing the fidelity of an existing structural finite element telescope model or in creating a new model having sufficient fidelity (sufficient accuracy).
High quality multi-disciplinary integrated models are needed for
complex opto-mechanical spacecraft such as SIM and TPF in order to
predict the system's on-orbit behavior. One major activity in
early design is to examine the system's behavior over multiple
configurations using an integrated model. A three step procedure
for model tuning is outlined that consists of (1) applying
engineering insight to the model so that all physical systems are
present in the model, (2) using optimization to automatically
update system parameters that are uncertain in the model, and (3)
evaluating the model at several configurations using the updated
parameters. The key contribution of this work is the systematic
checking of the validity of the updated parameters by evaluating,
both in the model and the experiment, the system at different
configurations (step three). It is hypothesized that if the
simulation model and experimental data of the additional
configurations match well then the tuned system parameters were
indeed updated in a way that physically represents the system.
This three step process is applied to a testbed at the MIT Space
This paper examines the optimal placement of nodes for a Wireless Sensor Network (WSN) designed to monitor a critical facility in a hostile region. The sensors are dropped from an aircraft, and they must be connected (directly or via hops) to a High Energy Communication Node (HECN), which serves as a relay from the ground to a satellite or a high-altitude aircraft. The sensors are assumed to have fixed communication and sensing ranges. The facility is modeled as circular and served by two roads. This simple model is used to benchmark the performance of the optimizer (a Multi-Objective Genetic Algorithm, or MOGA) in creating WSN designs that provide clear assessments of movements in and out of the facility, while minimizing both the likelihood of sensors being discovered and the number of sensors to be dropped. The algorithm is also tested on two other scenarios; in the first one the WSN must detect movements in and out of a circular area, and in the second one it must cover uniformly a square region. The MOGA is shown again to perform well on those scenarios, which shows its flexibility and possible application to more complex mission scenarios with multiple and diverse targets of observation.
Future spaceborne interferometric arrays must meet stringent optical performance and tolerance requirements while exhibiting modularity and acceptable manufacture and integration cost levels. The Massachusetts Institute of Technology (MIT) Adaptive Reconnaissance Golay-3 Optical Satellite (ARGOS) is a wide-angle Fizeau interferometer spacecraft testbed designed to address these research challenges. Designing a space-based stellar interferometer, which requires tight tolerances on pointing and alignment for its apertures, presents unique multidisciplinary challenges in the areas of structural dynamics, controls, and multiaperture phasing active optics. In meeting these challenges, emphasis is placed on modularity in spacecraft subsystems and optics as a means of enabling expandability and upgradeability. A rigorous theory of beam-combining errors for sparse optical arrays is derived and flown down to the design of various subsystems. A detailed elaboration on the optics system and control system is presented based on the performance requirements and beam-combining error tolerances. The space environment is simulated by floating ARGOS on a frictionless air-bearing that enables it to track both fast and slow moving targets.
A multidisciplinary analysis is demonstrated for the NEXUS space telescope precursor mission. This mission was originally designed as an in-space technology testbed for the Next Generation Space Telescope (NGST). One of the main challenges is to achieve a very tight pointing accuracy with a sub-pixel line-of-sight (LOS) jitter budget and a root-mean-square (RMS) wavefront error smaller than λ/50 despite the presence of electronic and mechanical disturbances sources. The analysis starts with the assessment of the performance for an initial design, which turns out not to meet the requirements. Twentyfive design parameters from structures, optics, dynamics and controls are then computed in a sensitivity and isoperformance analysis, in search of better designs. Isoperformance allows finding an acceptable design that is well “balanced” and does not place undue burden on a single subsystem. An error budget analysis shows the contributions of individual disturbance sources. This paper might be helpful in analyzing similar, innovative space telescope systems in the future.
In order to better understand the technological difficulties involved in designing and building a sparse aperture array, the challenge of building a white light Golay-3 telescope was undertaken. The MIT Adaptive Reconnaissance Golay-3 Optical Satellite (ARGOS) project exploits wide-angle Fizeau interferometer technology with an emphasis on modularity in the optics and spacecraft subsystems. Unique design procedures encompassing the nature of coherent wavefront sensing, control and combining as well as various system engineering aspects to achieve cost effectiveness, are developed. To demonstrate a complete spacecraft in a 1-g environment, the ARGOS system is mounted on a frictionless air-bearing, and has the ability to track fast orbiting satellites like the ISS or the planets. Wavefront sensing techniques are explored to mitigate initial misalignment and to feed back real-time aberrations into the optical control loop. This paper presents the results and the lessons learned from the conceive, design and implementation phases of ARGOS. A preliminary assess-ment shows that the beam combining problem is the most challenging aspect of sparse optical arrays. The need for optical control is paramount due to tight beam combining tolerances. The wavefront sensing/control requirements appear to be a major technology and cost driver.
This paper presents a comprehensive framework for integrated modeling, simulation and analysis of optical telescopes. This framework is called DOCS (Dynamics-Optics-Controls-Structures) and supports model development, model integration, analysis and multidisciplinary design optimization of this class of precision opto-mechanical systems. First the research background and literature in this young filed is discussed. Next the structure and nominal process of an integrated modeling, simulation and analysis study for a generic optical telescope using the DOCS framework is discussed in detail. The major steps include subsystems modeling, model assembly, model reduction and conditioning, initial performance assessment, sensitivity analysis, uncertainty analysis, redesign, design optimization and isoperformance analysis. Such a comprehensive analysis is demonstrated for the NEXUS Space Telescope precursor mission. This mission was designed as a technology testbed for the Next Generation Space Telescope. The challenge is to achieve a very tight pointing accuracy with a sub-pixel line-of-sight (LOS) jitter budget and a root-mean-square (RMS) wavefront error smaller than λ/50 despite the presence of electronic and mechanical disturbance sources. The framework suggested in this paper has the potential for becoming a general prescription for analyzing future, innovative telescope projects. Significant challenges remain in enabling fast simulations for large models, analytical sensitivity analysis for all sub-models, incorporation of slow-varying thermal or impulsive transient effects and the effective use of experimental results.
As a cornerstone in NASA's Origins program, the primary goal of the Terrestrial Planet Finder (TPF) mission is to directly detect the existence of Earth-like planets around nearby stars. This paper presents a process and a software tool, based on a quantitative systems engineering methodology, to conduct architectural trade studies during the TPF mission conceptual design phase.
NGST represents a challenging problem from the point of view of maintaining a milli-arcsecond level pointing accuracy and diffraction limited wavefront performance in the presence of dynamic onboard disturbances during science observations in a cryogenic environment. A Dynamics-Optics-Controls- Structures framework is being developed in support of the NGST dynamics and controls modeling program.