ALBERT

Description: The Task Committee on Methods for Identification of Large Structures in Space was founded in Jul. 1984. The charter of the committee was to prepare a state-of-the-art report on methods of system identification applicable to large space structures (LSS). Funding to support preparation of the report was received in Aug. 1985 from the Air Force Rocket Propulsion Laboratory (now the Air Force Astronautics Laboratory), in the form of a contract to the ASCE. The report was completed, and published by AFRPL in Sep. 1986. The Task Committee consisted of ten members, including ASCE and AFRPL representatives. The membership represented Government, Industry, and Universities, and consisted of electrical, mechanical, and civil engineers, with backgrounds in Structural Dynamics, Optimization, and Controls. An effort was made to use consistent terminology and notation throughout the report which would be compatible with the terminology used in both the structures and controls communities.

Description: This paper describes an autonomous control concept for pointing and articulation of science instruments on the Eos (Earth observing system) NASA/NOAA platforms intended to be operational by the late 1990s. Key features of this concept include advanced control adaptation and tuning strategies which provide performance robustness over a wide range of system uncertainties and mission time criticality. System identification-control modification paradigms are synthesized to form an adaptation continuum over this extended regime of autonomous operations.

Description: Large space structures are characterized by a large number of modes, grouped frequencies, and small inherent damping. Model reduction techniques in time domain may not be effective due to small damping. The model truncation method is generally used. This method can not solve the problem of grouped frequencies, and will lose all the information about the higher order modes. A new method developed in this paper, which tries to minimize the error of interested transfer functions, makes use of all the information of the original system, and achieves improvement not only from a smaller error of transfer functions but also from better frequency distribution.

Description: A space flight experiment being developed by NASA's Office of Aeronautics and Space Technology (OAST) that uses the Space Station as a testbed to study techniques for determining the dynamic characteristics of large space structures (LSS) is described. The experiment is separate from the Space Station Program itself with research objectives outside the domain of Space Station Program objectives. A brief description of the experiment, in terms of the general objective and approach, is given along with a statement of the potential benefits to NASA and others. The bulk of material to follow deals with the experiment definition activity that is underway. The scope of an 'initial' definition study and preliminary results from supporting Space Station dynamics analyses is presented. The term initial is used to indicate that the study currently being conducted has limited objectives and is not expected to complete the required experiment definition. A follow on study is planned and is mentioned in the summary.

Description: One of the basic requirements in engineering analysis is the development of a mathematical model describing the system. Frequently comparisons with test data are used as a measurement of the adequacy of the model. An attempt is typically made to update or improve the model to provide a test verified analysis tool. System identification provides a systematic procedure for accomplishing this task. The terms system identification, parameter estimation, and model correlation all refer to techniques that use test information to update or verify mathematical models. The goal of system identification is to improve the correlation of model predictions with measured test data, and produce accurate, predictive models. For nonmetallic structures the modeling task is often difficult due to uncertainties in the elastic constants. A finite element model of the shell was created, which included uncertain orthotropic elastic constants. A modal survey test was then performed on the shell. The resulting modal data, along with the finite element model of the shell, were used in a Bayes estimation algorithm. This permitted the use of covariance matrices to weight the confidence in the initial parameter values as well as confidence in the measured test data. The estimation procedure also employed the concept of successive linearization to obtain an approximate solution to the original nonlinear estimation problem.

Description: Reliable structural dynamic models will be required as a basis for deriving the reduced-order plant models used in control systems for large space structures. Ground vibration testing and model verification will play an important role in the development of these models; however, fundamental differences between the space environment and earth environment, as well as variations in structural properties due to as-built conditions, will make on-orbit identification essential. The efficiency, and perhaps even the success, of on-orbit identification will depend on having a valid model of the structure. It is envisioned that the identification process will primarily involve parametric methods. Given a correct model, a variety of estimation algorithms may be used to estimate parameter values. This paper explores the effects of modeling errors and model deficiencies on parameter estimation by reviewing previous case histories. The effects depend at least to some extent on the estimation algorithm being used. Bayesian estimation was used in the case histories presented here. It is therefore conceivable that the behavior of an estimation algorithm might be useful in detecting and possibly even diagnosing deficiencies. In practice, the task is complicated by the presence of systematic errors in experimental procedures and data processing and in the use of the estimation procedures themselves.

Description: On-orbit system identification (ID) of large space systems is essential for various reasons. For example, the complex composite structure of such systems cannot be ground-tested; their structural dynamic characteristics must be known accurately in order to accomplish active control. Furthermore, such capability can be used to characterize/identify various disturbances. The identification process is consisted of four principal elements: (1) modeling, (2) the estimation algorithm, (3) input system, and (4) measurement system. These elements are highly correlated and all togerher determine the success of the identification problem. Accurate modeling of large space systems is the most important element of the identification process. Large flexible structures are non-linear and infinite dimensional systems with highly coupled parameters and low frequency packed modes. In addition, these systems are subject to stochastic and time-varying disturbances, they have structural parameters which can vary due to on-orbit assembly deployment, and operations. These systems are generally; however, represented by constant coefficient, finite order differential equations. The non-linearities, coupling and noise effects are also often neglected. Moreover, identification experiment designs which lead to highly complex optimization problems usually require the simultaneous choice of ID algorithm, sensor, and actuator type and placement. On-orbit bandwidth and power restrictions on excitation, limited data window, and restrictions on sensor/actuator type, placement and number, has led to practical questions of implementations.

Description: This paper examines the use of on-orbit identification based on Maximum Likelihood Estimation (MLE) to provide these high-order, high-accuracy control design models for large space structures (LSS's). First, it outlines a general MLE identification algorithm, together with a covariance-analysis procedure to assess algorithm performance in terms of systematic and stochastic errors. Next, it examines various simplifications appropriate for the LSS identification application. Simplified analytical performance results are presented, as are numerical results to support these analyses. Finally, a graphical interpretation of these results is given.

Description: This talk focuses on the determination of state-space models for large space systems using only the output data. The output data could be generated by the unknown or deliberate initial conditions of the space structure in question. We shall review some relevant fundamental work on the state-space modeling of sequential output data that is potentially applicable to large space structures. If formulated in terms of some generalized Markov parameters, this approach is in some sense similar to, but much simpler than, the Juang-Pappa Eigensystem Realization Algorithm (ERA) and the Ho-Kalman construction procedure.

Description: For the future space systems, on-orbit identification (ID) capability will be required to complement on-orbit control, due to the fact that the dynamics of large space structures, spacecrafts, and antennas will not be known sufficiently from ground modeling and testing. The computational requirements for ID of flexible structures such as the space station (SS) or the large deployable reflectors (LDR) are however, extensive due to the large number of modes, sensors, and actuators. For these systems the ID algorithm operations need not be computed in real-time, only in near real-time, or an appropriate mission time. Consequently the space systems will need advanced processors and efficient parallel processing algorithm design and architectures to implement the identification algorithms in near real-time. The MAX computer currently being developed may handle such computational requirements. The purpose is to specify the on-board computational requirements for dynamic and static identification for large space structures. The computational requirements for six ID algorithms are presented in the context of three examples: the JPL/AFAL ground antenna facility, the space station (SS), and the large deployable reflector (LDR).

Description: This proposal discusses a new nonlinear, nonparametric method for off-line modeling and on-line estimation of the deformation of a flexible structure undergoing rapid retargeting maneuvers. In these circumstances, the structural stiffness and damping coefficients depend on the angular acceleration omega(dot), the angular rate omega, and the square of the angular rate omega. In the single axis case, the excitation of the structure is represented by the vector u(exp T) = (omega(dot), omega(exp 2), 2(omega)), to which the structural dynamics responds as a 'bilinear' (i.e., parametrically excited) system. A similar technique for multiaxial rotations yields a bilinear model with respect to matrix valued excitations. Three methods of estimation and modeling are described in this proposal to achieve deformation state determination: (1) a method based on a feedback linearized procedure which gives an estimate of the state by means of observers installed in the deformable body; (2) off-line modeling of the deformation state of the structure by means of topological interpolators; and (3) an on-line structural state estimation method based on a combination of the two previous techniques.

Description: Parameter identification and modeling are key elements of a design and operational flight strategy for control of flexible space structures. The emphasis of the identification program is on on-orbit applications to spacecraft control. High performance robust controllers will result from advanced design synthesis techniques and on-orbit identification/system tuning. Near term goals for the program include development of an integrated on-line processing capability for flexible body parameter identification, and validation with physical structure experiments.

Description: Viewgraphs and discussion on on-orbit system parameter identification are included. Topics covered include: dynamic programming filter (DPF); cost function and estimator; frequency domain formulation structrual dynamic identification; and attributes of DPF.

Description: On-going research at The Aerospace Corporation studying the feasibility of applying adaptive control methodologies to the control of flexible space structures is described. A laboratory testbed was established to test system identification and control approaches. The laboratory set-up and controller design approach are discussed. The ARX least squares parameter estimation technique is analyzed in terms of frequency domain transfer function bias error. This analysis approach enables the determination of the effects of sampling rate, sensor type, and data prefiltering on the estimation performance. The ability to identify space structure dynamics over a range of frequencies is shown to be heavily dependent on these factors.

Description: Accurate mathematical models are clearly an important part of the design, modification, control, and damage assessment of large space structures. A critical part of model determination of any type of structure is the use of system identification (SI), a process for using measured excitation and response data to improve the form of a mathematical model or the values of the parameters in it. The reasons for using SI are manifold, but most of them involve uncertainties in either the form of or the parameters in the mathematical model. Some of the more common uncertainties are the following: the nature of the damping, the characteristics of sloppy or sticky joints, inelastic material properties, and parameters in a reduced Degree-of-Freedom (DOF) model. This paper contains a description of the SI algorithm followed by two illustrations: one in which all parameters in a shear building model are determined using pull back and quick release data, and another in which an equivalent reduced DOF model is obtained for a two story frame.

Description: This presentation will address an approach for using modal residues and roots, typically derived from vibration testing, to solve for the lumped parameters of a finite element model. The uniqueness of the approach is that the mass, stiffness, or damping matrices of a structure do not need to be known or estimated a-priori. Instead just the connectivity of the structure needs to be estimated. In addition, the presentation will demonstrate how models with orders larger than the number of modes obtained by the test program can be solved. The presentation will start by covering the basis of the approach, then illustrate a trivial application of how it works, and finally illustrate its use on a cantilevered beam using only a fraction of the mode to solve the mass, stiffness, and damping matrices.

Description: Virtually all space structure mission requirements include control of vibration, position, and possibly shape. In order to satisfy their requirements it is necessary to know the dynamic characteristics of the structure. Therefore, a means must be developed to continuously determine a dynamic model, or changes in the model, and to appropriately adapt the controls to these changes. The detection and location of physical damage would also be a very beneficial characteristic of such a procedure. A discussion outlining the methods to perform On-Orbit Model Determination is presented.

Description: For the past several years much effort has been given to the development of techniques for designing control systems for large space structures (LSS's). The main objective of these efforts has been to develop a LSS control methodology that produces designs that meet strenuous performance requirements and are robust to model inaccuracies. Unfortunately, performance and robustness are conflicting requirements. Because LSS's can not be fully tested on ground, it has become an accepted fact that the design of LSS control systems to meet performance requirements can not be completed until the LSS is placed on-orbit and tested and an accurate model is extracted from on-orbit test results. Modern MIMO sampled-data frequency response design techniques are viable candidates for designing LSS control systems. First, this paper presents techniques for performing MIMO system identification (ID) from test data. Then, techniques for improving the performance of the system ID process in the presence of noise are presented. Finally, practical utility of the system ID approaches are validated by the presentation of results obtained from application on the LSS Ground Test Facility at Marshall Space Flight Center.

Description: The Galileo scan platform is controlled in two degrees of freedom. A clock (Spin Bearing) actuator controls the relative position between the rotor and stator, and a cone actuator controls the position between the stator and the platform. Instruments on the platform are required to point to 140 micro-rad accuracy and 50 micro-rad per second stability. The system identification objectives were to identify dominant structural resonance frequencies, mode shapes, and damping ratio which exist in the transfer function between the clock actuator and the gyro sensor; and position the notch filter to limit undesirable actuator torque output to ensure stability and performance.

Description: Many modern spacecraft are complex multi-body dynamic systems where the bodies are connected by several active control systems for pointing and isolation. Mission requirements indicate that many structural modes and possibly some nonlinear effects will require characterization. Thus, even characterization at the subsystem level will become more difficult than usual. System level characterization difficulties will be compounded by the fact that only limited ground testing will be possible on the full up system, and flight testing will be restricted by an extremely limited measurements set. The object of the present discussion is the application of matrix majorant theory to the problem of assessing dynamic system performance when knowledge of the system is uncertain. We show how majorants provide an effective tool to relate required performance output to system identification test quality in terms of residual uncertainty in input-output relations, parameter values, nonlinearities, and interactions. The underlying machinery consists of the block-norm matrix which is a nonnegative matrix each of whose elements is the norm of a block of a suitably partitioned matrix. A matrix which bounds the block-norm matrix in the sense of nonnegative matrices, i.e., element by element is known as a majorant.

Description: An optimal on-orbit experiment is designed to extract the most information from an on-orbit test, subject to the constraints of the testing environment. However, simply jumping in and optimizing standard measures of information with respect to the experiment design can cause severe problems if attention is not paid to the specific needs and properties of the problem at hand. The actual criteria to be optimized depends on (among other things) the particular ID algorithm and parametrization being used. Two parametric techniques are the focus of this presentation: recursive prediction error method (RPEM) and maximum likelihood estimation (MLE).

Description: The research project which resulted in the AMI (Analytical Model Improvement) method was funded by the Langley Research Center from 1979 through 1985. The objective was to develop a method for obtaining improved dynamic models of structures by using both test data and numerical analysis. The research was successfully performed and the method was applied to a real structure having a realistic NASTRAN model with over 500 degrees of freedom. The method and its application are briefly discussed as well as other indirect benefits in related technical areas.

Description: The objective of this paper is to summarize and review several investigations on the assessment and control of structural damage in civil engineering. Specifically, the definition of structural damage is discussed. A candidate method for the evaluation of damage is then reviewed and demonstrated. Various ways of implementing passive and active control of civil engineering structures are next summarized. Finally, the possibility of applying expert systems is discussed.

Description: An eigenvector expansion method is utilized to predict eigenvalue and eigenvector derivatives due to geometric reconfiguration of a Gimbalflex fine-pointing/vibration isolation system called SAVI (Space Active Vibration Isolation). The eigenvector expansion method used is a modification of the classical method and allows for rigid body roots. Using the resulting modal derivatives, free-free nonlinear equations of motion are developed with Lagrange's Method. These equations represent a nonlinear plant model to be used in conjunction with a control system transient response simulation.

Description: Two different methods are proposed for identifying the structural properties of large orbiting space structures under ordinary service loads, and for assessing potential damage due to impact or other extreme loadings. It is shown that the behavior of a structure in a weightless environment is nonlinear due to unloaded or lightly loaded connections, an effect which not only complicates structural control, but makes the problem of system identification more difficult than for ground based systems. Both proposed methods assume that the structure is subjected to loads imposed by prescribed self stressing systems sufficient to produce repeatable internal force systems in the structure. The first method is based on statical response and requires a survey of structural displacements produced by the self stressing systems. The displacements do not have to be determined completely (i.e., in three directions at each connection), but more displacement information produces more accurate structural stiffness information. It is anticipated that displacement measurements will be taken using on-board laser measurement devices. The second technique employs dynamic stress wave measurement techniques using on-board loading devices and strain gages to track stress wave propagation through the space structure. This approach, which is in its early stages of development, relies on an analysis of transit times of impulsive stress waves and changes in transit times and wave forms due to changes in structural parameters.

Description: Uncertainties of a large space system (LSS) can be deterministic or stochastic in nature. The former may result in, for example, an energy spillover problem by which the interaction between unmodeled modes and controls may cause system instability. The stochastic uncertainties are responsible for mode localization and estimation errors, etc. We will address the effects of uncertainties on structural model formulation, use of available test data to verify and modify analytical models before orbiting, and how the system model can be further improved in the on-orbit environment.

Description: The objective of this project is to identify modal properties such as the eigenvalues and eigenfunctions of structures. The formal means for accomplishing this task, Structural Identification, is viewed as a two step procedure: (1) identify the eigensolution; and (2) using the identified eigensolution, identify the mass and stiffness. The eigensolution is identified as a correction on a postulated model based on erroneous parameters.

Description: The topics are presented in viewgraph form and include the following: directed energy systems - vibration issue; Neutral Particle Beam Integrated Space Experiment (NPB-ISE) opportunity/study objective; vibration sources/study plan; NPB-ISE spacecraft configuration; baseline slew analysis and results; modal contributions; fundamental pitch mode; vibration reduction approaches; peak residual vibration; NPB-ISE spacecraft slew experiment; goodbye ISE - hello Zenith Star Program.

Description: For a low Earth orbit (LEO) to geosynchronous orbit (GEO) mission scenario, it can be shown that both a chemically-propelled, aerobraked orbital transfer vehicle (OTV), and a high-thrust, nuclear OTV use approximately 50 percent less propellant than a comparable, chemical OTV. At the University of Virginia two teams worked on designs for these types of OTVs. One group formed WWSR Inc. and worked on the aerobraked OTV, which was named Project Orion. The other group, named MOVERS, collaborated on the design for the nuclear engine OTV. This report will briefly review the nature and specifics of their work. This will summarize each of these propellant systems and their corresponding cost savings. It will also highlight the strengths and weaknesses of each OTV concept.

Description: Students in Aerospace Engineering (ASE) and Mechanical Engineering (ME) at the University of Texas at Austin completed eight separate design projects under the sponsorship of the NASA/USRA Advanced Space Design Program. During the Fall semester of 1987, ASE and ME students worked on various aspects of a 'bootstrap' lunar base. ASE students concentrated on the overall definition of the base while the ME team designed a convertible lunar lander lower stage. Six design projects were executed during the spring semester of 1988. Aerospace engineering students designed a fast Mars mission crew transfer vehicle, an Earth-orbiting transportation node to support the lunar base, a lunar base construction shack/initial habitat vehicle, and carried out a systems assessment aimed at arresting ozone depletion in the Earth's atmosphere. The ME students designed a lunar surface personnel navigation system and investigated radiation shielding structures for a lunar base. The design objectives, a summary of the results, and selected comments are given concerning each of the projects. Due to the number of projects completed, the reader should refer to the text of the final project reports for additional and detailed information.

Description: The concept of utilizing a winged vehicle for the remote exploration of Mars was studied in the 1970's. This study, directed by NASA's Jet Propulsion Laboratory, considered an unmanned, instrumented platform to perform the necessary mission requirements to study the Mars environment. Two versions were considered. In one, the Marsplane was deployed during the entry maneuver and flew until its fuel was exhausted. Its mission ended with the subsequent landing. In the second version, the Marsplane could land and take off a limited number of times. In the past decade, technological advances in materials, aerodynamics, and propulsion systems have increased the feasibility of a Marsplane. It was the objective of this year's design project to examine the impact of these technological advances and to investigate (1) the feasibility of a manned Marsplane, and (2) the spacecraft system necessary to deliver the Marsplane to the surface of Mars.

Description: The overall goal for this NASA/USRA-sponsored 'Apollo Lightcraft Project' is to develop a revolutionary launch vehicle technology that can reduce payload transport costs by a factor of 1000 below the Space Shuttle Orbiter. The RPI design team proposes to utilize advanced, highly energetic, beamed-energy sources (laser, microwave) and innovative combined-cycle (airbreathing/rocket) engines to accomplish this goal. This second year focused on systems integration and analysis of the 'Apollo Lightcraft'. This beam-powered, single-stage-to-orbit vehicle is envisioned as the globe-trotting family shuttlecraft of the 21st century. Detailed investigations of the Apollo Lightcraft Project during the second year of study helped evolve the propulsion system design, while focusing on the following areas: (1) man/machine interface; (2) flight control systems; (3) power beaming system architecture; (4) reentry aerodynamics; (5) shroud structural dynamics; and (6) optimal trajectory analysis.

Description: The desire to understand and explore space has driven man to overcome the confines of the Earth's atmosphere and accept the challenge of spaceflight. With our increasing ability to travel, work, and explore in space comes a need for a better understanding of the hazards in this relatively new endeavor. One of the most important and immediate needs is to be able to predict the ignition, spread, and growth of fire on board spacecraft. Fire safety aboard spacecraft has always been a concern; however, with the increasing number and duration of proposed missions, it is imperative that the spacecraft be designed with a solid understanding of fire hazards, insuring that all risks have been minimized and extinguishment systems are available.

Description: Project Longshot presents a preliminary design for an unmanned probe to Alpha Centauri with a planned launch early in the 21st century. A 100-year estimated travel time was baselined for the mission on Space Report, Pioneering the Space Frontier. One of the elements of the Commission's solar and space physics plan is 'a long-life, high-velocity spacecraft to be sent out of the Solar System on a trajectory to the nearest star'. Adequate lead time is included in the scenario to allow the development of several enabling technologies and to ensure that the required space operations infrastructure will be in place.

Description: In this summary, we present the main conclusions and results of a design study conducted by a group of 25 students at the University of Maryland. The students, all participants in the spring 1988 courses ENEE488Q and ENEE418 in the Electrical Engineering Department, considered the problem of designing a free-flying space robot for applications in satellite servicing. In addition to the paper study, a subgroup of eight students undertook the ambitious task of designing and building a laboratory testbed for the concept of a free flyer. This subgroup has already completed the main fabrication of the mechanical hardware and most of the onboard electronics. When completed by the end of the summer, the testbed will consist of a seven-degree-of-freedom dual-armed planar robot that will float on an air table and carry out various commanded tasks. All the fabrication and testing is being conducted in the Intelligent Servosystems Laboratory at the University of Maryland. The project team split up into subgroups as follows: (1) articulation concepts 1 and 2, to study competing ideas for the overall systems architecture and manipulator design; (2) real time control systems group; (3) a computer graphics group to provide animations of a planar design; (4) a mechanical hardware design group for the laboratory testbed; and (5) a control systems hardware design group for the same.

Description: The University of Central Florida's design of an Integrated Refuse Management System for the proposed International Space Station is addressed. Four integratable subsystems capable of handling an estimated Orbiter shortfall of nearly 40,000 lbs of refuse produced annually are discussed. The subsystems investigated were: (1) collection and transfer; (2) recycle and reuse; (3) advanced disposal; and (4) propulsion assist in disposal. Emphasis is placed on the recycling or reuse of those materials ultimately providing a source of Space Station refuse. Special consideration is given to various disposal methods capable of completely removing refuse from close proximity of the Space Station. There is evidence that pyrolysis is the optimal solution for disposal of refuse through employment of a Rocket Jettison Vehicle. Additionally, design considerations and specifications of the Refuse Management System are discussed. Optimal and alternate design solutions for each of the four subsystems are summarized. Finally, the system configuration is described and reviewed.

Description: The project SLICK (Space Laser Interorbital Cargo Kite) involves conceptual designs of reusable space-based laser-powered orbital transfer vehicle (LOTV) for ferrying 16,000 kg cargo primarily between low Earth orbit (LEO) and geosynchronous earth orbit (GEO). The power of LOTV is beamed by a single 32-MW solar-pumped iodide laser orbiting the Earth at an altitude of one Earth radius. The laser engine selected for the LOTV is based on a continuous-wave, steady-state propulsion scheme and uses an array of seven discrete plasmas in a flow of hydrogen propellant. Both all-propulsive and aerobraked LOTV configurations were analyzed and developed. The all-propulsive vehicle uses a rigid 11.5-m aperture primary mirror and its engine produces a thrust of 2000 N at a specific impulse of 1500 sec. For the LEO-to-GEO trip, the payload ratio, m(sub payload/m(sub propellant)+m(sub dry vehicle) = 1.19 and the trip time is about 6 days. The aerobraked version uses a lightweight, retractable wrapped-rib primary mirror which is folded for aerobraking and a 20-m-diameter inflatable-ballute aeroshield which is jettisoned after aeromaneuver. Lifecycle cost analysis shows that the aerobraked configuration may have an economic advantage over the all-propulsive configuration as long as the cost of launching the propellant to LEO is higher than about $500/kg in current dollars.

Description: This is a brief description of the USRA-sponsored design project at the University of Arizona. Approximately eighty-percent of this effort was spent pursuing a novel engineering concept for the in-situ processing of orbital debris utilizing resources available in low Earth orbit (LEO); the other twenty-percent was devoted to discovering innovative additives for the anchoring of supersonic combustion zones that find direct use in the Aerospace Plane that is expected to use scramjets. The seriousness of the orbital debris problem is briefly described. Available 'solutions' are outlined from the literature. The engineering design is briefly mentioned, with an emphasis on the positive aspects of the space environment that should be used in an economical approach. The aspects of operating in microgravity, vacuum, and in utilizing solar energy are mentioned. A quantitative computer animation was developed to provide design data. Three specific dead spacecraft were identified for an initial cleanup mission. The design concept, which includes a solar processor, remote arm manipulators, and the gradual processing of the debris, is also described. This is followed by a description of hardware construction. Operation and actual processing of simulated debris parts (aluminum, for now) are demonstrated in the NASP task, construction of the new design for measuring the radiation from the key free radicals (as enhanced by the additives) is described. Immediate (1988) and long-range (through 1992) future plans are shown to clearly indicate the full engineering design strategy in the light of the national space program thrusts.

Description: The unmanned Multiple Exploratory Probe System (MEPS) is designed for Mars observations in preparation for manned missions to the planet early in the 21st century. MEPS will test vehicle systems, provide important data about the Martian surface and atmosphere, and assist the planning of manned missions. This mission will be a precursor to the manned missions. MEPS will consist of six primary systems. A Command Information Center (CIC) will be employed as an onboard mission control, communications link, and observation post. The Space Transportation Main Engine (STME) will be used to provide the thrust for Earth-Mars transit following vehicle construction near the Space Station. A polar lander/Orbital Transfer Vehicle (OTV) will be deployed during transit to achieve a polar orbit about Mars. A secondary propulsion will be used to place MEPS into orbit about Mars; this system and the aerobrake will circularize the orbit. Following orbit circulation, a satellite will be deployed to observe the Martian surface and atmosphere and to study the space environment. Polar and equatorial lander systems will land on Mars with rovers to collect surface and atmospheric samples while on-board laboratories will provide initial sample study. Two solid rocket booster/payload vehicles will launch samples into a low Mars orbit. The OTV will rendezvous with each payload capsule and then transfer the samples to Earth for hands-on observation.

Description: Techniques have been developed for the sub-pixel location of control points in satellite images returned by the Voyager spacecraft. The procedure uses digital imaging data in the neighborhood of the point to form a multipicture model of a piece of the surface. Comparison of this model with the digital image in each picture determines the control point locations to about a tenth of a pixel. At this level of precision, previously insignificant effects must be considered, including chromatic aberration, high level imaging distortions, and systematic errors due to navigation uncertainties. Use of these methods in the study of Jupiter's satellite Io has proven very fruitful.

Description: Normalized 18-GHz microwave brightness temperatures, T(B), and a vegetation index determined from satellite radiometer data are combined with climatically modeled surface moisture estimates to constrain a simple physically based soil moisture model. It is found that the normalized T(B) values correlated well with soil moisture when the data were segregated by vegetation index range, but less so when all the data were combined. By using the vegetation index parameter, the model is shown to account for about 70 percent of the variability in modeled surface soil moisture.

Description: A simple equation relating the Microwave Polarization Difference Index (MPDI) and the Normalized Difference Vegetation Index (NDVI) is proposed which represents well data obtained from Nimbus 7/SMMR at 37 GHz and NOAA/AVHRR Channels 1 and 2. It is found that there is a limit which is characteristic of a particular type of cover for which both indices are equally sensitive to the variation of vegetation, and below which MPDI is more efficient than NDVI. The results provide insight into the relationship between water content and chlorophyll absorption at pixel size scales.

Description: This study evaluates the potential of measuring/mapping forest damage in spruce-fir forests in the Green Mountains of Vermont and White Mountains of New Hampshire using Landsat Thematic Mapper (TM) data. The TM 1.65/0.83-micron (TM5/4) and 2.22/0.83-micron (TM7/4) band ratios were found to correlate well with ground-based measurements of forest damage (a measure of percentage foliar loss) at 11 spruce-fir stands located on Camels Hump, a mountain in northern Vermont. Images using 0.56 and 1.65-micron bands with 1.65/0.83-micron band ratios indicated locations of heavy conifer forest damage. Both 1.65/0.83 and 2.22/0.83-micron band ratios were used to quantify levels of conifer forest damage among individual mountains throughout many of the Green and White Mountains. Damage was found to be consistently higher for the Green than the White Mountains.

Description: An airborne pulsed laser system was used to obtain canopy height data over a southern pine forest in Georgia in order to predict ground-measured forest biomass and timber volume. Although biomass and volume estimates obtained from the laser data were variable when compared with the corresponding ground measurements site by site, the present models are found to predict mean total tree volume within 2.6 percent of the ground value, and mean biomass within 2.0 percent. The results indicate that species stratification did not consistently improve regression relationships for four southern pine species.

Description: The use of remote sensing data to monitor and analyze the arctic environment is examined. Landsat MSS, TM simulated, NS001, Seasat, and airborne radar are employed to investigate the Strand and Dune areas on the Arctic Coastal Plain in Alaska. The Strand area contains landforms associated with permafrost and the Dune area is dominated by eolian deposits consisting of large longitudinal dunes. The remote sensing data are compared to baseline geomorphic maps derived from aerial photography. It is observed that the multispectral data are better than the radar data for the detection and recognition of arctic landforms, and the NS001 data provided the highest spatial resolution and correlated well with the high-altitude aerial photography.

Description: Over large areas in the tropics, satellite imagery is the principal source of data on the area, current stature, and extent of disturbance of the forests. The information from imagery that covers large areas at low resolution is greatly enhanced when different types of imagery can be compared. The paper presents a comparison of data from Landsat MSS and from the Shuttle Imaging Radar (SIR-A) L band HH polarization data for sites in the Amazon Basin. Results indicate that SIR-A backscatter from the undisturbed forest was lower than that from some disturbed areas and from flooded forests and that SIR-A brightness, increases nonlinearly with the Landsat normalized difference vegetation index. It is hypothesized that the brightest radar returns in southern Amazonia are from newly cleared forests that are littered with standing and fallen tree boles that function as corner reflectors; and that backscatter will diminish from disturbed areas over time as fields are burned repeatedly.

Description: This study evaluated the utility of data collected by the high-spectral resolution airborne imaging spectrometer (AIS-2, tree mode, spectral range 0.8-2.2 microns) and the broad-band Daedalus airborne thematic mapper (ATM, spectral range 0.42-13.0 micron) in assessing forest decline damage at a predominantly Scotch pine forest in the FRG. Analysis of spectral radiance values from the ATM and raw digital number values from AIS-2 showed that higher reflectance in the near infrared was characteristic of high damage (heavy chlorosis, limited needle loss) in Scotch pine canopies. A classification image of a portion of the AIS-2 flight line agreed very well with a damage assessment map produced by standard aerial photointerpretation techniques.

Description: Visible IR Intelligent Spectrometer (VIRIS) reflectance data have been found to have similar features that are related to air-pollution-induced forest decline and visible damage in both the red spruce of Vermont and the Norway spruce of Baden-Wuerttemberg; the similarity suggests a common source of damage. Spectra of both species include a 5-nm blueshifting of the red-edge inflection point, while pigment data for both species indicate a loss of total chlorophylls. The blue shift of the chlorophyll absorption maximum, as well as the increased red radiance and decreased near-IR radiance of the damaged spruce, may be used to delineate and map damage areas.

Description: Recent advances in imaging spectroscopy for remote sensing applications are discussed, reviewing the results of recent investigations. The advantages offered by the higher spectral resolution of imaging spectroscopy relative to scanners such as Landsat MSS and TM are explained; the design and performance of the Airborne Imaging Spectrometer (Vane et al., 1984) are described and illustrated with drawings, photographs, and sample images; data processing and analysis techniques are outlined; and applications to geological and botanical research are considered.

Description: The effects of look angle and wavelength variation in geologically applied radar images are examined and the applications of observations of these effects for the study of other planets are discussed. Seasat, SIR-A, SIR-B, and airborne radar images and multiple look angle and multiwavelength scatterometer data are used. It is found that smaller look angle radar data can provide good discrimination among certain diverse materials which are not distinguishable at larger look angles, such as subpixel fault scarps and volcanic dykes. Discriminant analyses of scatterometer data of all geological targets observed gave best results with minimum data by using all wavelengths available and small look angles. The results provide information on the nature of radar images which could be valuable in interpreting radar images of Venus.

Description: Four doubled CO2 experiments with the GISS general circulation model are compared to investigate the consistency of changes in water availability over the United States. The experiments compare the influence of model sensitivity, model resolution, and the sea-surface temperature gradient. The results show that the general mid-latitude drying over land is dependent upon the degree of mid-latitude eddy energy decrease, and thus the degree of high-latitude temperature change amplification. There is a general tendency in the experiments for the northern and western United States to become wetter, while the southern and eastern portions dry. However, there is much variability from run to run, with different regions showing different degrees of sensitivity to the parameters tested. The results for the western United States depend most on model resolution; those for the central United States, on the sea-surface temperature gradient and the degree of mid-latitude ocean warming; and those for the eastern United States, on model sensitivity. The changes in particular seasons depend on changes in other seasons, and will therefore be sensitive to the realism of the ground hydrology parameterization.

Description: Recent research on the remote sensing of forest leaf and canopy biochemical contents suggests that the shortwave IR region contains this information; laboratory analyses of dry ground leaves have yielded reliable predictive relationships between both leaf nitrogen and lignin with near-IR spectra. Attention is given to the application of these laboratory techniques to a limited set of spectra from fresh, whole leaves of conifer species. The analysis of Airborne Imaging Spectrometer data reveals that total water content variations in deciduous forest canopies appear as overall shifts in the brightness of raw spectra.

Description: A control law is presented for three-axis rotational maneuvers of a spacecraft (orbiter)-beam-tip body (antenna or a reflector) configuration based on nonlinear inversion and modal velocity feedback. Using invertibility and functional reproducibility results, a decoupling attitude control law is presented such that, in the closed-loop system, the attitude angles of the spacecraft are independently controlled using the control moments acting on the space vehicle. This controller asymptotically decouples the flexible dynamics from the rigid one and also allows the decomposition of the elastic dynamics into two subsystems representing the transverse deflections of the beam in two orthogonal planes. These low-order subsystems are used for derivation of a modal velocity feedback stabilizer using the force and moment actuators at the end body. Simulation results are presented to show that, in the closed-loop system, attitude control and elastic mode stabilization are accomplished in spite of the parameter uncertainty and disturbance torque input in the system.

Description: Biomass of growing vegetation over large semiarid regions can be estimated by digital manipulation of data from the AVHRR on NOAA polar-orbiting satellites. Here, the African Sahel is classified using a methodology which incorporates both the normalized difference and CAUSE procedures for the monitoring of vegetation during drought conditions. Preliminary analysis of color IR photographs taken on Space Shuttle missions indicates that such photographs can be digitized, registered to maps and other images, and utilized to fill temporal gaps in the historical record of data from unmanned satellites.

Description: Spacecraft glow poses a contamination threat to low orbital altitude optical sensor systems. The complexity of the phenomena entails a multifaceted approach to system design for vehicle glow minimization. In the case of Space Shuttle cloud glow, which involves line and band emission, filtering and careful optical sensor wavelength selection may also prove useful; Space Shuttle thruster glow mitigation entails the limitation of thruster firings during sensor operations. Careful selection of instrument baffle materials and coatings, as well as control of surface temperatures, are recommended as ways of limiting glow impact for instruments directed in the direction of vehicle movement.

Description: The dimensional stability of ceramic coated thermal protection materials developed for use on advanced entry vehicles is evaluated. Dimensional stability of these ceramic materials were studied as a function of temperature and pressure during exposure to simulated atmospheric entry in an arc-jet facility.

Description: A field investigation using thermal remote sensing was performed to test the feasibility of detecting the surface topography of granite bedrock beneath a thin cover of alluvium. Imagery of a region of the Mojave Desert were taken with an airborne multispectral scanner with thermal spectral bandwidths of 10.4 and 12.5 microns an instantaneous field of view of 2.5 mrad. It is suggested that a buried high thermal diffusivity horizon measurably lowers the surface temperature of the overlying lower diffusivity material during the peak of the annual heating cycle.

Description: Multidirectional reflectance factor measurements acquired in the summer of 1986 are used to make estimates of big bluestem grass albedo, evaluating the variation of albedo with changes in solar zenith angle and phenology. On any given day, the albedo was observed to increase by at least 19 percent as solar zenith angle increased. Changes in albedo were found to correspond to changes in the green leaf area index of the grass canopy. Estimates of albedo made using reflectance data acquired within only one or two azimuthal planes and at a restricted range of view zenith angle were evaluated and compared to 'true' albedos derived from all available reflectance factor data. It was found that even a limited amount of multiple direction reflectance data was preferable to a single nadir reflectance factor for the estimation of prarie grass albedo.

Description: Laboratory reflectance measurements of 10 soils were used to determine the relationship between soil moisture and three spectral indices: the TM5/7 ratio and the Wetness(R) and Brightness(R) features of the reflectance factor TM Tasseled Cap transformation. Response of the indices to dry mass water percentage was approximately linear for individual soils, except for Wetness(R) and Brightness(R) at high moisture content. Soil differences in the slopes of the Wetness(R)- and Brightness(R)-moisture content relationships were almost entirely eliminated by expressing water content as the percentage of water retained at 0.1 bar (10 kPa) tension (relative water content). The resultant soil lines were offset from one another by the differences in dry soil index value. Slope of the TM5/7 response was not completely normalized by expressing moisture status as relative water content, because slope appeared to vary with dry soil ratio value. Sensitivity to the effects of illumination angle was negligible for the TM5/7 ratio, somewhat greater for Wetness(R) and greatest for Brightness(R).

Description: A comparison is made between linear discriminant analysis and supervised classification results based on signatures from the Landsat TM, the Thermal Infrared Multispectral Scanner (TIMS), and airborne SAR, alone and combined into extended spectral signatures for seven sedimentary rock units exposed on the margin of the Wind River Basin, Wyoming. Results from a linear discriminant analysis showed that training-area classification accuracies based on the multisensor data were improved an average of 15 percent over TM alone, 24 percent over TIMS alone, and 46 percent over SAR alone, with similar improvement resulting when supervised multisensor classification maps were compared to supervised, individual sensor classification maps. When training area signatures were used to map spectrally similar materials in an adjacent area, the average classification accuracy improved 19 percent using the multisensor data over TM alone, 2 percent over TIMS alone, and 11 percent over SAR alone. It is concluded that certain sedimentary lithologies may be accurately mapped using a single sensor, but classification of a variety of rock types can be improved using multisensor data sets that are sensitive to different characteristics such as mineralogy and surface roughness.

Description: A radar image of the Lake Champlain West quadrangle in the Adirondack Mountains of the U.S. is synthesized and used to test the surface integral approach to radarclinometry. It is shown that the surface integral approach to radarclinometry possesses an inherent instability that can be avoided only if the radar reflectance function possesses a shallow slope over the range of operation and if terrain slopes are bounded to prevent their being either parallel or perpendicular to the Poynting vector of the radar irradiance. It is found that the noise associated with real SAR systems makes this instability worse. It is concluded that the value of the surface integral approach to radarclinometry shows little promise.

Description: The optimal experiment design for on-orbit identification of modal frequency and damping parameters in large flexible space structures is discussed. The main result is a separation principle for D-optimal design which states that under certain conditions the sensor placement problem is decoupled from the input design problem. This decoupling effect significantly simplifies the overall optimal experiment design determination for large MIMO structural systems with many unknown modal parameters. The error from using the uncoupled design is estimated in terms of the inherent damping of the structure. A numerical example is given, demonstrating the usefulness of the simplified criteria in determining optimal designs for on-orbit Space Station identification experiments.

Description: A band ratioing method is developed which directs the retention of topographic expression and albedo information so that they remain depicted as prominent variations in image intensity. Band data are adjusted so that ratio values for each surface material coherently increase with increasing pixel bispectral radiance for all three ratios of the color composite. The retained topographic expression and albedo information do not significantly distort the ratio-enhanced band-variant reflectance information, and the resultant images are similar to chromaticity-enhanced band composite images, but require only simple arithmetic processing steps.

Description: Landsat-5 digital numbers have been used to compute the at-satellite planetary reflectance on spectrally similar zones on the Meares and Schwanda glaciers in Alaska and the Grossglockner glacier group in Austria. The patterns of TM-derived reflectances in the ablation areas of the Grossglockner glacier group and the Meares Glacier compare favorably with published reflectance curves measured on the surface of glacier ice, though the surface reflectance of snow-covered ice is higher than the Landsat-derived reflectance for the glaciers studied. The accuracy of the at-satellite planetary reflectances is shown to be affected by topographic and atmospheric effects and by the anisotropic nature of snow reflectance.

Description: A model for radiative transfer in heterogeneous three-dimensional canopies such as those found in forests is proposed. Its use in estimating important biophysical variables such as leaf area index and canopy architecture from bidirectional canopy reflectance data is discussed. The model and its use in estimating canopy parameters through its inversion are validated with measured canopy reflectance data for corn canopies.

Description: This paper proposes a stop-gap nonoptimum vehicle for transferring astronauts from a tumbling stranded spacecraft to a nearby rescue spacecraft. The design is limited to the use of available or 'soon-to-be' available flight-qualified hardware and consists of three major components: the manned maneuvering unit, the personnel rescue enclosure, and the apogee kick motor capture device. The apogee kick motor capture device is modified to serve as the connection between the manned maneuvering unit and the personnel rescue enclosure. The performance of this interim rescue vehicle is analyzed with NASA flight simulation software to test the feasibility of the design. Results show that the control system of the manned maneuvering unit adequately limits uncommanded rotations during all simulated maneuvers in the primary control mode but not during transverse translations in the backup control mode. Impingement of thruster plumes on the personnel rescue enclosure is shown to be of some importance in certain maneuvers. The satellite stabilization mode of the control system is found to have significant rotational-to-translational coupling that has associated adverse effects on flying qualities, making the mode undesirable for the rescue mission.

Description: The passing of large orbital vehicles through the space environment often generates such emissions as glows on or near the vehicle surface and halos surrounding the vehicle. These induced emissions may affect observations made with the optical instrumentation carried by the vehicles. The glows' causative mechanisms appear to be a complex function of altitude, time in orbit, materials, insolation, and vehicular size and orientation. Attention is presently given to contamination environment data obtained for the instrument suite carried by the Spacelab 1 Space Shuttle mission.

Description: Measurements of the atmospheric effect on the spectral signature of surface cover were conducted during hazy conditions over the Chesapeake Bay and its eastern shore. In the experiment the upward radiance was measured by an airborne scanning radiometer in nine spectral bands between 465 and 773 nm, above and below the haze layer. Simultaneous measurements of the aerosol optical thickness and its vertical distribution were conducted. The results of the measurements are used to study the spectral dependence of the atmospheric effect on remote sensing of water bodies and vegetated fields (forest, corn field, and pasture), and to verify theoretical predictions. It is suggested that the radiances over dark areas (e.g., water in the near IR and forest in the visible) can be used to derive the aerosol optical thickness as is done over oceans with the CZCS satellite images. Combined with climatological information, the derived optical thickness can be used to perform corrections of the atmospheric effect. Examples of the derivation of the aerosol optical thickness and correction of the upward radiances are given.

Description: Interferometric aperture synthesis is presented as an alternative to real aperture measurements of the earth's brightness temperature from low earth orbit. The signal-to-noise performance of a single interferometric measurement is considered, and the noise characteristics of the brightness temperature image produced from the interferometer measurements are discussed. The sampling requirements of the measurements and the resulting effects of the noise in the measurements on the image are described. The specific case of the electronically steered thinned array radiometer (ESTAR) currently under construction is examined. The ESTAR prototype is described in detail sufficient to permit a performance evaluation of its spatial and temperature resolution. Critical aspects of an extension of the ESTAR sensor to a larger spaceborne system are considered. Of particular importance are the number and placement of antenna elements in the imaging array. A comparison of the implementation methodologies of radio astronomy and earth remote sensing is presented along with the effects of the source brightness distribution, the antenna array configuration and the method used for array scanning.

Description: Landsat Thematic Mapper (TM) and aircraft-borne Thematic Mapper simulator (TMS) data were collected over two areas of natural vegetation in southern California exposed to gradients of pollutant dose, particularly in photochemical oxidants: the coastal sage scrub of the Santa Monica Mountains in the Los Angeles basin, and the yellow pine forests in the southern Sierra Nevada. In both situations, natural variations in canopy closure, with subsequent exposure of understory elements (e.g.,rock or soil, chaparral, grasses, and herbs), were sufficient to cause changes in spectral variation that could obscure differences due to visible foliar injury symptoms observed in the field. TM or TMS data are therefore more likely to be successful in distinguishing pollution injury from background variation when homogeneous communities with closed canopies are subjected to more severe pollution-induced structural and/or compositional change. The present study helps to define the threshold level of vegetative injury detectable by TM data.

Description: The spectral canopy reflectance, biomass, and projected leaf-area index (LAI) of widely dispersed plots of a North American coastal plant were measured in order to study potential impacts of continental-scale environmental variability on the assumptions underlying remote vegetation analysis. Systematic changes in the canopy geometry and resultant near-infrared reflectance of this plant were noted. Mean infrared canopy reflectances of canopies in the northern half of the range were shown to nearly double those of the southern half. It is suggested that the difference results from divergent canopy morphologies, with the northern canopies presenting greater horizontally projected LAIs per unit biomass than southern canopies.

Description: Improved estimates of soil wetness were obtained using observations from both the NIMBUS-7 Scanning Multichannel Microwave Radiometer (SMMR) and the NOAA-7 Advanced Very High Resolution Radiometer (AVHRR). SMMR 66 GHz frequency, horizontal polarization, brightness temperature T(BH) was first correlated with soil wetness, as computed using an Antecedent Precipitation Index (API) model, for a number of SMMR ground resolution areas involving a fairly wide range of vegetation densities. The API generally accounted for more than 70 percent of the observed temporal variability in T(BH), with linear correlations being significant at the 1 percent level. The regression slope of T(BH) versus API correlated well, at the 1 percent level, with a vegetation index derived from AVHRR visible and near-infrared observations. The regression intercept was found to correlate less satisfactorily, but was significant at the 5 percent level. These linear regression results were used to develop a diagnostic model for soil wetness using SMMR and AVHRR data only.

Description: Accuracies of solutions (structural temperatures and thermal stresses) obtained from different thermal and structural FEMs set up for the Space Shuttle Orbiter (SSO) are compared and discussed. For studying the effect of element size on the solution accuracies of heat-transfer and thermal-stress analyses of the SSO, five SPAR thermal models and five NASTRAN structural models were set up for wing midspan bay 3. The structural temperature distribution over the wing skin (lower and upper) surface of one bay was dome shaped and induced more severe thermal stresses in the chordwise direction than in the spanwise direction. The induced thermal stresses were extremely sensitive to slight variation in structural temperature distributions. Both internal convention and internal radiation were found to have equal effects on the SSO.

Description: An algorithm for automatic atmospheric correction of satellite imagery of the earth's surface is proposed which is applicable to low-resolution and high-resolution imagery of land areas. The algorithm is based on the satellite image being corrected and on the climatology of the area, and it requires that some pixels in the image correspond to dense dark vegetation as the surface cover. The algorithm is sensitive to the assumed reflectance of the dense dark vegetation, and the accuracy of the corrected surface reflectance is expected to be + or - 0.01. Using the method, aerosol optical thicknesses were derived from clear and hazy Landsat MSS images in the Washington, D.C. and Chesapeake Bay region, and the results are found to agree well with simultaneous sunphotometer ground measurements.

Description: Nitrous oxide flux and nitrogen turnover were measured in three types of Amazonian forest ecosystems within Reserva Florestal Ducke near Manaus, Brazil. Nitrogen mineralization and nitrate production measured during 10-day laboratory incubations were 3-4 times higher in clay soils associated with 'terra firme' forests on ridge-top and slope positions than in 'campinarana' forests on bottomland sand soils. In contrast, nitrous oxide fluxes did not differ significantly among sites, but were highly variable in space and time. The observed frequency distribution of flux was positively skewed, with a mean overall sites and all sampling times of 1.3 ng N2O-N/sq cm per hr. Overall, the flux estimates were comparable to or greater than those of temperature forests, but less than others reported for Amazoonia. Results from a field fertilization experiment suggest that most nitrous oxide flux was associated with denitrification of soil nitrate.

Description: The roles of leaf anatomy, moisture and pigment content, and number of leaf layers on spectral reflectance in healthy, pollution-stressed, and water-stressed conifer needles were examined experimentally. Jeffrey pine (Pinus jeffreyi) and giant sequoia (Sequoiadendron gigantea) were exposed to ozone and acid mist treatments in fumigation chambers; red pine (Pinus resinosa) needles were artificially dried. Infrared reflectance from stacked needles rose with free water loss. In an air-drying experiment, cell volume reductions induced by loss of turgor caused near-infrared reflectance (TM band 4) to drop after most free water was lost. Under acid mist fumigation, stunting of tissue development similarly reduced band 4 reflectance. Both artificial drying and pollutant fumigation caused a blue shift of the red edge of spectral reflectance curves in conifers, attributable to chlorophyll denaturation. Thematic mapper band ratio 4/3 fell and 5/4 rose with increasing pollution stress on artificial drying. Loss of water by air-drying, freeze-drying, or oven-drying enhanced spectral features, due in part to greater scattering and reduced water absorption. Grinding of the leaf tissue further enhanced the spectral features by increasing reflecting surfaces and path length. In a leaf-stacking experiment, an asymptote in visible and infrared reflectance was reached at 7-8 needle layers of red pine.

Description: One of the goals for the Space Station is to achieve greater autonomy, and have less reliance on ground commanding than previous space missions. This means that the crew will have to take an active role in scheduling and rescheduling their activities onboard, perhaps working from preliminary schedules generated on the ground. Scheduling is a time intensive task, whether performed manually or automatically, so the best approach to solving onboard scheduling problems may involve crew members working with an interactive software scheduling package. A project is described which investigates a system that uses knowledge based techniques for the rescheduling of experiments within the Materials Technology Laboratory of the Space Station. Particular attention is paid to: (1) methods for rapid response rescheduling to accommodate unplanned changes in resource availability, (2) the nature of the interface to the crew, (3) the representation of the many types of data within the knowledge base, and (4) the possibility of applying rule-based and constraint-based reasoning methods to onboard activity scheduling.

Description: Contamination control requirements for the Space Station have been evolving over the last few years. Workshops, comments by experimenters and continuing analysis have resulted in recommending changes to the November 19, 1986 version of Space Station External Contamination Control Requirements, JSC 30426. These are summarized and presented, so that the requirements can be revised as soon as possible, to minimize costly design impacts on the Space Station.

Description: Quartz crystal microbalance sensors are recommended to verify and monitor surface deposition on the early transverse boom as well as the later dual-keel Space Station configurations. Performance and placement of these sensors are discussed and compared to imposed maximum mass deposition rate requirements at the science instrument and critical power locations. Additional measurements are suggested to gain further knowledge on properties of the deposited material.

Description: We recommend that a stereo camera system be utilized as a diagnostic for the particulate environment surrounding the Space Station. This system should have sufficient sensitivity to identify contaminated periods, to isolate the effects of sources and activities and to determine optical clearing times. A reasonable compromise between sensitivity and other operational constraints is recommended. Sensitivity comparable to the film camera systems should suffice, but long periods of unattended operation and remotely controlled exposure sequences are essential requirements.

Description: At 340 km, for typical conditions, the neutral atmospheric density is several times 10E8/cc and is thus more abundant than the ionized component by several factors of 10. At that altitude, the principal series is atomic oxygen with 10 percent N2, and 1 percent He, and trace amounts of O2, H, N, NO, and Ar. The constituent densities are highly variable with local time, latitude, and geophysical indices. The physical interaction with surfaces at orbital velocity leads to large buildup of density on forward faces and great depletions in the wakes of objects. Chemical reactions lead to major modifications in constituent densities as in the case of the conversion of most colliding oxygen atoms to oxygen bearing molecules. The neutral environment about an orbiting body is thus a complex product of many variables even without a source of neutral contaminants. The addition of fluxes of gases emanating from the orbiting vehicle, as will be the case for the Space Station, with the associated physical and chemical interactions adds another level of complexity to the character of the environment and mandates a sophisticated measurement capability if the neutral environment is to be quantitatively characterized.

Description: The measurement and monitoring of infrared emission in the environment of the Space Station has a twofold importance - for the study of the phenomena itself and as an aid in planning and interpreting Station based infrared experiments. Spectral measurements of the infrared component of the spacecraft glow will, along with measurements in other spectral regions, provide data necessary to fully understand and model the physical and chemical processes producing these emissions. The monitoring of the intensity of these emissions will provide background limits for Space Station based infrared experiments and permit the determination of optimum instrument placement and pointing direction. Continuous monitoring of temporal changes in the background radiation (glow) will also permit better interpretation of Station-based infrared earth sensing and astronomical observations. The primary processes producing infrared emissions in the Space Station environment are: (1) Gas phase excitations of Station generated molecules ( e.g., CO2, H2O, organics...) by collisions with the ambient flux of mainly O and N2. Molecular excitations and generation of new species by collisions of ambient molecules with Station surfaces. They provide a list of resulting species, transition energies, excitation cross sections and relevant time constants. The modeled spectrum of the excited species occurs primarily at wavelengths shorter than 8 micrometer. Emissions at longer wavelengths may become important during rocket firing or in the presence of dust.

Description: This paper describes the Work Package 3 activities in the area of neutral contamination monitoring for the Space Station. Goddard Space Flight Center's responsibilities include the development of the Attached Payload Accommodations Equipment (APAE), the Polar Orbiting Platform (POP), and the Flight Telerobotic Servicer (FTS). GSFC will also develop the Customer Servicing Facility (CSF) in Phase 2 of the Space Station.

Description: The objective of this work is to develop a practical sensor analytic redundancy management scheme for flexible spacecraft and to demonstrate it using the SCOLE experimental apparatus. The particular scheme to be used is taken from previous work on the Grid apparatus by Williams and Montgomery.

Description: The combined problem of slew maneuver control and vibration suppression of NASA Spacecraft Control Laboratory Experiment (SCOLE) is considered. The coupling between the rigid body modes and flexible modes together with the effect of the control forces on the flexible antenna is discussed. The nonlinearities in the equations are studied in terms of slew maneuver angular velocities.

Description: Control design approaches for SCOLE experimentation at Langley Research Center are considered; the following future topics are discussed: (1) Effects of Actuator Dynamics; (2) Refinement of STAC; (3) System Identification; and (4) Experimentation.

Description: A discussion of Slewing and Vibration Control makes the following conclusions: (1) A 2-stage approach is feasible and promising for rapid slewing and precision pointing of SCOLE; (2) Not all bang-bang type of time-minimized slew maneuvers will excite large structural vibrations in SCOLE; and (3) Modal dashpots can be a concentrated high-power vibration control, as well as the usual diffuse (broadband, low-power (low-authority) control. The following recommendations are made: (1) Limit the magnitude of applied forces on reflector to either the 25-lb limit of vernier thrusters on the real Space Shuttle or the 150-lb level equivalent to the cold-gas jets of laboratory SCOLE; (2) to complete stage 2, add an integrated design of LQF/LTR (Linear-Quadratic-Gaussian/Loop-Transfer Recovery) and Modal Dashpots; and, (3) Validate the 2-stage approach using the SCOLE laboratory facility with a comprehensive sequence of integrated designs and experiments coupling nonlinear rigid-body motions with flexible-body dynamics.

Description: A class of nonlinear damping models is introduced with application to flexible flight structures characterized by low damping. Approximate solutions of engineering interest are obtained for the model using the classical averaging technique of Krylov and Bogoliubov. The results should be considered preliminary pending further investigation.

Description: Requirements for space structures of increasing size, complexity, and precision have engendered a search for thermal design verification methods that do not impose unreasonable costs, that fit within the capabilities of existing facilities, and that still adequately reduce technical risk. This requires a combination of analytical and testing methods. This requires two approaches. The first is to limit thermal testing to sub-elements of the total system only in a compact configuration (i.e., not fully deployed). The second approach is to use a simplified environment to correlate analytical models with test results. These models can then be used to predict flight performance. In practice, a combination of these approaches is needed to verify the thermal/structural design of future very large space systems.

Description: A prototype docking mechanism for the Space Station was designed and fabricated for NASA. This docking mechanism is actively controlled and uses a set of electromechanical actuators for alignment and load attenuation. Dynamic tests are planned using the Marshall Space Flight Center's 6-DOF Motion Simulator. The proposed tests call for basic functionality verification as well as complete hardware-in-the-loop docking dynamics simulations.

Description: The idea of using spray-on foam insulation as a passive thermal and micrometeorite protection system is explored. The benefits of applying an exterior coating of foam insulation can be: (1) the foam can provide a thermally stable shield that can assist in reducing the strain on traditional space radiator systems and can also act as a passive thermal guard, allowing a greater fault tolerance if the standard system should fail; (2) the foam can act as an ablative shell diminishing the effects of natural and manmade debris striking the structure; (3) the foam is lightweight - about 1/2 ounce per sq ft; (4) the foam is low cost and easy to maintain; and (5) the foam is a stable material that does not react when exposed to earth or lunar environments.