The photometric study of light scattering from the surface of alumina powder and interpretations by Hapke formula

A laboratory experiment helps to understand the light scattering property of regolith like samples with known compositions and other physical parameters. The laboratory data so obtained can be compared with the existing in situ data on celestial objects like asteroids. Further, it may be analyzed with the help of various theoretical models to understand the light scattering processes from regolith more clearly. In this work we have performed laboratory based photometry of the light scattered from the surfaces of powdered alumina (Al₂O₃) at various tilt angles of the sample and at large phase angles, with the particles having diameter 0.3 μm. The wavelength of observation was 632.8 nm. These data have been fitted by a surface scattering model originally suggested by Hapke. Instead of using empirical Henyey–Greenstein phase function to fix the values of albedo and phase function to be used within Hapke formula, we have used Mie theory for the same. This approach helped us to determine the single particle properties such as particle diameter and complex refractive index from surface scattering phase curve alone. Mie theory depends only on the size parameter X(=2π(radius/wavelength)) and complex refractive index (n, k) of the material. Since the absorption coefficient (k) for alumina is known to be very low but not exactly zero, the best fit to the experimental data was obtained by least square technique with k as a free parameter, as the other parameters are known. Finally, we compare our results with other published results and discuss the scope of application of the method we adopted.     

Ballistic missile track initiation from satellite observations

An algorithm is presented to initiate tracks of a ballistic missile in the initial exoatmospheric phase, using line of sight (LOS) measurements from one or more moving platforms (typically satellites). The major feature of this problem is the poor target motion observability which results in a very ill-conditioned estimation problem. The Gauss-Newton iterative least squares minimization algorithm for estimating the state of a nonlinear deterministic system with nonlinear noisy measurements has been previously applied to the problem of angles-only orbit determination using more than three observations. A major shortcoming of this approach is that convergence of the algorithm depends strongly on the initial guess. By using the more sophisticated Levenberg-Marquardt method in place of the simpler Gauss-Newton algorithm and by developing robust new methods for obtaining the initial guess in both single and multiple satellite scenarios, the above mentioned difficulties have been overcome. In addition, an expression for the Cramer-Rao lower bound (CRLB) on the error covariance matrix of the estimate is derived. We also incorporate additional partial information as an extra pseudomeasurement and determine a modified maximum likelihood (ML) estimate of the target state and the associated bound on the covariance matrix. In most practical situations, probabilistic models of the target altitude and/or speed at the initial point constitute the most useful additional information. Monte Carlo simulation studies on some typical scenarios were performed, and the results indicate that the estimation errors are commensurate with the theoretical lower bounds, thus illustrating that the proposed estimators are efficient     

START: System Testability Analysis and Research Tool

START, a software package for automatic test sequencing and testability analysis of complex, hierarchically described modular systems, is described, and its use in modeling systems is examined. START uses algorithms based on information theory, heuristic search, and graph theory to solve various faces of the test sequencing and testability analysis problems. A system is modeled in the failure space as a hierarchical directed graph with nodes denoting modules and testpoints and with AND nodes denoting redundancy. Interconnections among the nodes denote their immediate functional dependencies. START supports hierarchical testing in accordance with the maintenance strategy; a failure source may be isolated to a component or a module at any level. Other practical features include options to integrate diagnosis with repair (after partial diagnosis) in order to optimize test time, test cost, or test and repair cost. An interactive menu-mouse graphical interface serves as a high-level front end to these algorithms and enables the user to graphically enter and modify hierarchical functional models of systems. START presents the gist of the outputs of the testability analysis algorithms as a concise testability report consisting of important figures of merit     

Multi-signal flow graphs: a novel approach for system testabilityanalysis and fault diagnosis

In this paper, we present a comprehensive methodology for a formal, but intuitive, cause-effect dependency modeling using multi-signal directed graphs that correspond closely to hierarchical system schematics and develop diagnostic strategies to isolate faults in the shortest possible time without making the unrealistic single fault assumption. A key feature of our methodology is that our models lend naturally to real-world necessities, such as system integration and hierarchical troubleshooting