221,543 research outputs found
The Case for Durative Actions: A Commentary on PDDL2.1
The addition of durative actions to PDDL2.1 sparked some controversy. Fox and
Long argued that actions should be considered as instantaneous, but can start
and stop processes. Ultimately, a limited notion of durative actions was
incorporated into the language. I argue that this notion is still impoverished,
and that the underlying philosophical position of regarding durative actions as
being a shorthand for a start action, process, and stop action ignores the
realities of modelling and execution for complex systems
Precision tracking systems of the immediate future: A discussion
The present status and future expectations of four satellite tracking systems, satellite-to-satellite tracking, lasers, very long baseline interferometry (VLBI) and geoceiver are briefly discussed
Combustion instability sustained by unsteady vortex combustion
The determination of an internal feedback mechanism which leads to combustion instability inside a small scale
laboratory combustor is presented in this paper. During combustion instability, the experimental findings show that a large vortical structure is formed at an acoustic resonant mode of the system. The subsequent unsteady burning, within the vortex as it is convected downstream,
feeds energy into the acoustic field and sustains the large resonant oscillations. These vortices are formed when the
acoustic velocity fluctuation at the flameholder is a large fraction of the mean flow velocity. The propagation of
these vortices is not a strong function of the mean flow speed and appears to be dependent upon the frequency of the
instability. Continued existence of large vortical structures which characterize unstable operation depends upon the fuel-air ratio, system acoustics, and fuel type
The report of the Gravity Field Workshop
A Gravity Field Workshop was convened to review the actions which could be taken prior to a GRAVSAT mission to improve the Earth's gravity field model. This review focused on the potential improvements in the Earth's gravity field which could be obtained using the current satellite and surface gravity data base. In particular, actions to improve the quality of the gravity field determination through refined measurement corrections, selected data augmentation and a more accurate reprocessing of the data were considered. In addition, recommendations were formulated which define actions which NASA should take to develop the necessary theoretical and computation techniques for gravity model determination and to use these approaches to improve the accuracy of the Earth's gravity model
Spaceborne ranging system
A spaceborne laser ranging system is described that could survey a large network of ground reflectors, and provide their relative locations to a precision of + or - 1 cm. This performance is believed realizable for networks covering up to 10 to 6th power square kilometers and from only a few days of observations. This system could be used to monitor crustal movements in many areas of the world and has the potential to provide an almost real-time system for detecting precursory ground motions before large earthquakes
Recent advances in computational techniques
The determination of very precise orbits and geodynamic parameters from laser tracking data requires the continual development and improvement of the software systems and computational techniques. Computational accuracies at the few centimeter level are presently required to match the performance of the present day laser ranging systems and altimeters and in the next few years the accuracies are expected to increase further. The major error sources in orbit determination are briefly discussed and the present and future modeling activities needed to meet the accuracy requirements of the next few years are described
Prospects for TLRS baseline accuracies in the western USA
One of the main goals of the LAGEOS satellite mission is the detection of regional geotectonic movements. A parametric study with the intention to obtain the optimal baseline precision from dynamic solutions of laser ranging to LAGEOS is presented. The varied parameters are: length of reduced arc, number of tracking stations, data noise and rate, biases, refraction errors, system efficiency, gravity model errors in the value of GM. The baseline precisions are 1-10 cm depending upon the set of parameters adopted. General principles obtained are also presented
Planning Graph Heuristics for Belief Space Search
Some recent works in conditional planning have proposed reachability
heuristics to improve planner scalability, but many lack a formal description
of the properties of their distance estimates. To place previous work in
context and extend work on heuristics for conditional planning, we provide a
formal basis for distance estimates between belief states. We give a definition
for the distance between belief states that relies on aggregating underlying
state distance measures. We give several techniques to aggregate state
distances and their associated properties. Many existing heuristics exhibit a
subset of the properties, but in order to provide a standardized comparison we
present several generalizations of planning graph heuristics that are used in a
single planner. We compliment our belief state distance estimate framework by
also investigating efficient planning graph data structures that incorporate
BDDs to compute the most effective heuristics.
We developed two planners to serve as test-beds for our investigation. The
first, CAltAlt, is a conformant regression planner that uses A* search. The
second, POND, is a conditional progression planner that uses AO* search. We
show the relative effectiveness of our heuristic techniques within these
planners. We also compare the performance of these planners with several state
of the art approaches in conditional planning
Experimental study and evaluation of radioprotective drugs
Experimental study evaluates radioprotective drugs administered before exposure either orally or intravenously. Specifically studied are the sources of radiation, choice of radiation dose, choice of animals, administration of drugs, the toxicity of protective agents and types of protective drug
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