Tech Briefs

Algorithm for Training a Recurrent Multilayer Perceptron

An improved algorithm has been devised for training a recurrent multilayer perceptron (RMLP) for optimal performance in predicting the behavior of a complex, dynamic, and noisy system multiple time steps into the future. [An RMLP is a computational neural network with self-feedback and cross-talk (both delayed by one time step) among neurons in hidden layers]. Like other neural-network-training algorithms, this algorithm adjusts network biases and synaptic-connection weights according to a gradientdescent rule. The distinguishing feature of this algorithm is a combination of global feedback (the use of predictions as well as the current output value in computing the gradient at each time step) and recursiveness. The recursive aspect of the algorithm lies in the inclusion of the gradient of predictions at each time step with respect to the predictions at the preceding time step; this recursion enables the RMLP to learn the dynamics. It has been conjectured that carrying the recursion to even earlier time steps would enable the RMLP to represent a noisier, morecomplex system.

Posted in: Briefs, Information Sciences

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Electrodeionization Using Microseparated Bipolar Membranes

Low concentrations of ions do not inhibit further removal of ions. An electrochemical technique for deionizing water, now under development, is intended to overcome a major limitation of prior electrically–based waterpurification techniques. The limitation in question is caused by the desired decrease in the concentration of ions during purification: As the concentration of ions decreases, the electrical resistivity of the water increases, posing an electrical barrier to the removal of the remaining ions. In the present technique, this limitation is overcome by use of electrodes, a flow-field structure, and solid electrolytes configured to provide conductive paths for the removal of ions from the water to be deionized, even when the water has already been purified to a high degree.

Posted in: Briefs, Materials

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Safer Electrolytes for Lithium-Ion Cells

A number of nonvolatile, low-flammability liquid oligomers and polymers based on aliphatic organic carbonate molecular structures have been found to be suitable to be blended with ethylene carbonate to make electrolytes for lithium-ion electro- chemical cells. Heretofore, such electrolytes have often been made by blending ethylene carbonate with volatile, flammable organic carbonates. The present nonvolatile electrolytes have been found to have adequate conductivity (about 2 mS/cm) for lithium ions and to remain liquid at temperatures down to —5 °C. At normal charge and discharge rates, lithiumion cells containing these nonvolatile electrolytes but otherwise of standard design have been found to operate at current and energy densities comparable to those of cells now in common use. They do not perform well at high charge and discharge rates — an effect probably attributable to electrolyte viscosity. Cells containing the nonvolatile electrolytes have also been found to be, variously, nonflammable or at least self-extinguishing. Hence, there appears to be a basis for the development of safer high-performance lithium-ion cells.

Posted in: Briefs, Materials

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Analyzing Dynamics of Cooperating Spacecraft

A software library has been developed to enable high-fidelity computational simulation of the dynamics of multiple spacecraft distributed over a region of outer space and acting with a common purpose. All of the modeling capabilities afforded by this software are available independently in other, separate software systems, but have not previously been brought together in a single system. A user can choose among several dynamical models, many high-fidelity environment models, and several numerical-integration schemes. The user can select whether to use models that assume weak coupling between spacecraft, or strong coupling in the case of feedback control or tethering of spacecraft to each other. For weak coupling, spacecraft orbits are propagated independently, and are synchronized in time by controlling the step size of the integration. For strong coupling, the orbits are integrated simultaneously. Among the integration schemes that the user can choose are Runge-Kutta Verner, Prince-Dormand, Adams-Bashforth- Moulton, and Bulirsh-Stoer. Comparisons of performance are included for both the weak- and strong-coupling dynamical models for all of the numerical integrators. The library was designed for ease of integration with high-fidelity environment models already in use in the Flight Dynamics Analysis Branch, which is one of seven institutional support branches within the Mission Engineering and Systems Analysis Division at Goddard Space Flight Center.

Posted in: Briefs, TSP, Software

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Spacecraft Attitude Maneuver Planning Using Genetic Algorithms

A key enabling technology that leads to greater spacecraft autonomy is the capability to autonomously and optimally slew the spacecraft from and to different attitudes while operating under a number of celestial and dynamic constraints. The task of finding an attitude trajectory that meets all the constraints is a formidable one, in particular for orbiting or fly-by spacecraft where the constraints and initial and final conditions are of time-varying nature. This approach for attitude path planning makes full use of a priori constraint knowledge and is computationally tractable enough to be executed onboard a spacecraft. The approach is based on incorporating the constraints into a cost function and using a Genetic Algorithm to iteratively search for and optimize the solution. This results in a directed random search that explores a large part of the solution space while maintaining the knowledge of good solutions from iteration to iteration. A solution obtained this way may be used "as is" or as an initial solution to initialize additional deterministic optimization algorithms. A number of representative case examples for time-fixed and time-varying conditions yielded search times that are typically on the order of minutes, thus demonstrating the viability of this method. This approach is applicable to all deep space and planet Earth missions requiring greater spacecraft autonomy, and greatly facilitates navigation and science observation planning.

Posted in: Briefs, TSP, Software

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Forensic Analysis of Compromised Computers

Directory Tree Analysis File Generator is a Practical Extraction and Reporting Language (PERL) script that simplifies and automates the collection of information for forensic analysis of compromised computer systems. During such an analysis, it is sometimes necessary to collect and analyze information about files on a specific directory tree. Directory Tree Analysis File Generator collects information of this type (except information about directories) and writes it to a text file. In particular, the script asks the user for the root of the directory tree to be processed, the name of the output file, and the number of subtree levels to process. The script then processes the directory tree and puts out the aforementioned text file. The format of the text file is designed to enable the submission of the file as input to a spreadsheet program, wherein the forensic analysis is performed. The analysis usually consists of sorting files and examination of such characteristics of files as ownership, time of creation, and time of most recent access, all of which characteristics are among the data included in the text file.

Posted in: Briefs, TSP, Software

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Document Concurrence System

The Document Concurrence System is a combination of software modules for routing users expressions of concurrence with documents. This system enables determination of the current status of concurrences and eliminates the need for the prior practice of manually delivering paper documents to all persons whose approvals were required. This system runs on a server, and participants gain access via personal computers equipped with Web-browser and electronic-mail software. A user can begin a concurrence routing process by logging onto an administration module, naming the approvers and stating the sequence for routing among them, and attaching documents. The server then sends a message to the first person on the list. Upon concurrence by the first person, the system sends a message to the second person, and so forth. A person on the list indicates approval, places the documents on hold, or indicates disapproval, via a Web-based module. When the last person on the list has concurred, a message is sent to the initiator, who can then finalize the process through the administration module. A background process running on the server identifies concurrence processes that are overdue and sends reminders to the appropriate persons.

Posted in: Briefs, Software

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