Implicit Formulations of Bounded-Impulse Trajectory Models for Preliminary Interplanetary Low-Thrust Analysis

The bounded-impulse approach to low-thrust interplanetary trajectory optimization is widely used. In an effort to efficiently implement this approach using NASAs OpenMDAO optimization software, the authors have implemented implicit formulations of the forward shooting/backwards-shooting methods commonly used in bounded-impulse models. These implicit approaches allow for vectorization of the underlying calculations which can significantly reduce runtime in interpreted languages. An implicit approach may be either converged by using an underlying nonlinear solver to converge the state propagation, or as a constraint in an optimizer-driven multiple-shooting approach. Significant computational efficiency gains are realized through the utilization of the modular approach to unified derivatives. Further computational efficiency is achieved by capitalizing on the sparsity of the constraint Jacobian matrix. This work demonstrates that a vectorized multiple-shooting approach for propagating a state-time history is superior in terms of computational efficiency as the number of segments in the state-propagation is increased

Application specific asynchronous microengines for efficient high-level control

technical reportDespite the growing interest in asynchronous circuits programmable asynchronous controllers based on the idea of microprogramming have not been actively pursued Since programmable control is widely used in many commercial ASICs to allow late correction of design errors to easily upgrade product families to meet the time to market and even efficient run time modications to control in adaptive systems we consider it crucial that self timed techniques support efficient programmable control This is especially true given that asynchronous (self-timed) circuits are well suited for realizing reactive and control intensive designs We offer a practical solution to programmable asynchronous control in the form of application-speciffic microprogrammed asynchronous controllers (or microengines). The features of our solution include a modular and easily extensible datapath structure support for two main styles of hand shaking (namely two phase and four phase), and many efficiency measures based on exploiting concurrency between operations and employing efficient circuit structures Our results demonstrate that the proposed microengine can yield high performance-in fact performance close to that offered by automated high level synthesis tools targeting custom hard wired burstmode machines

Application specific asynchronous microgengines for efficient high-level control

technical reportDespite the growing interest in asynchronous circuits, programmable asynchronous controllers based on the idea of microprogramming have not been actively pursued. Since programmable control is widely used in many commercial ASICs to allow late correction of design errors, to easily upgrade product families, to meet the time to market, and even effect run-time modifications to control in adaptive systems, we consider it crucial that self-timed techniques support efficient programmable control. This is especially true given that asynchronous (self-timed) circuits are well suited for realizing reactive and control-intensive designs. We offer a practical solution to programmable asynchronous control in the form of application-specific micro-programmed asynchronous controllers (or microengines). The features of our solution include a modular and easily extensible datapath structure, support for two main styles of handshaking (namely two-phase and four-phase), and many efficiency measures based on exploiting concurrency between operations and employing efficient circuit structures. Our results demonstrate that the proposed microengine can yield high performance?in fact performance close to that offered by automated high-level synthesis tools targeting custom hard-wired burstmode machines

Anytime planning for agent behaviour

For an agent to act successfully in a complex and dynamic environment (such as a computer game)it must have a method of generating future behaviour that meets the demands of its environment. One such method is anytime planning. This paper discusses the problems and benefits associated with making a planning system work under the anytime paradigm, and introduces Anytime-UMCP (A-UMCP), an anytime version of the UMCP hierarchical task network (HTN) planner [Erol, 1995]. It also covers the necessary abilities an agent must have in order to execute plans produced by an anytime hierarchical task network planner

Hindering data theft with encrypted data trees

Data theft is a major threat for modern organizations with potentially large economic consequences. Although these attacks may well originate outside an organization’s information systems, the attacker—or else an insider—must even-tually make contact with the system where the information resides and extract it. In this work, we propose a scheme that hinders unauthorized data extraction by modifying the basic file system primitives used to access files. Intuitively, our proposal emulates the chains used to protect valuable items in certain clothing shopping centers, where shoplifting is prevented by forcing the thief to steal the whole rack of items. We achieve this by encrypting sensitive files using nonces (i.e., pseudorandom numbers used only once) as keys. Such nonces are available, also in encrypted form, in other objects of the file system. The system globally resembles a distributed Merkle hash tree, in such a way that getting access to a file requires previous access to a number of other files. This forces any potential attacker to extract not only the targeted sensitive information, but also all the files chained to it that are necessary to compute the associated key. Further-more, our scheme incorporates a probabilistic rekeying mechanism to limit the damage that might be caused by patient extractors. We report experimental results measuring the time overhead introduced by our proposal and compare it with the effort an attacker would need to successfully extract information from the system. Our results show that the scheme increases substantially the effort required by an insider, while the introduced overhead is feasible for standard computing platforms