Markley F.L.,NASA |
Reynolds R.G.,Millennium Space Systems Inc. |
Liu F.X.,Stinger Ghaffarian Technologies Inc. |
Lebsock K.L.,Orbital Sciences Corp
Journal of Guidance, Control, and Dynamics | Year: 2010
Spacecraft reaction-wheel maneuvers are limited by the maximum torque and/or angular momentum that the wheels can provide. The torque or momentum envelope for an n- wheel configuration can be obtained by projecting the (n-dimensional hypercube, representing the domain boundary of individual wheel torques or momenta, into three-dimensional space via the 3 x n matrix of wheel axes. This paper elucidates the properties of the projected hypercube and presents algorithms for determining this maximal torque or momentum envelope for general wheel configurations and for distributing a prescribed torque or momentum among the n wheels. We show that these algorithms provide 22, 27, and 33% more capability than the more conventional pseudoinverse algorithm for configurations of four, five, and six wheels, respectively. Analysis of a representative slew using six wheels shows that these algorithms can provide either a 25% reduction in maximum wheel momentum or a 30% reduction in slew time when compared with the pseudoinverse algorithm.
Mazarico E.,NASA |
Mazarico E.,Massachusetts Institute of Technology |
Neumann G.A.,NASA |
Smith D.E.,NASA |
And 4 more authors.
Icarus | Year: 2011
We use high-resolution altimetry data obtained by the Lunar Orbiter Laser Altimeter instrument onboard the Lunar Reconnaissance Orbiter to characterize present illumination conditions in the polar regions of the Moon. Compared to previous studies, both the spatial and temporal extent of the simulations are increased significantly, as well as the coverage (fill ratio) of the topographic maps used, thanks to the 28Hz firing rate of the five-beam instrument. We determine the horizon elevation in a number of directions based on 240m-resolution polar digital elevation models reaching down to ∼75° latitude. The illumination of both polar regions extending to ∼80° can be calculated for any geometry from those horizon longitudinal profiles. We validated our modeling with recent Lunar Reconnaissance Orbiter Wide-Angle Camera images. We assessed the extent of permanently shadowed regions (PSRs, defined as areas that never receive direct solar illumination), and obtained total areas generally larger than previous studies (12,866 and 16,055km2, in the north and south respectively). We extended our direct illumination model to account for singly-scattered light, and found that every PSR does receive some amount of scattered light during the year. We conducted simulations over long periods (several 18.6-years lunar precession cycles) with a high temporal resolution (6h), and identified the most illuminated locations in the vicinity of both poles. Because of the importance of those sites for exploration and engineering considerations, we characterized their illumination more precisely over the near future. Every year, a location near the Shackleton crater rim in the south polar region is sunlit continuously for 240days, and its longest continuous period in total darkness is about 1.5days. For some locations small height gains (∼10m) can dramatically improve their average illumination and reduce the night duration, rendering some of those particularly attractive energy-wise as possible sites for near-continuous sources of solar power. © 2010 Elsevier Inc.
Perry III R.L.,Stinger Ghaffarian Technologies Inc.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010
Previous studies have correlated the particle fallout rates within cleanrooms to MIL-STD-1246 cleanliness levels. Unfortunately "cleanliness levels" are not linear and do not lead to easily understood increases with respect to either cleanroom class or time. Additionally, cleanroom "class" is rarely static but varies throughout the processing flow in accordance with the activity levels. A numerical evaluation of the particle fallout normalized to area coverage demonstrates a correlation that is directly proportional to both cleanroom class and exposure time, yielding a simple Class-Hour formulation. Application of this formulation allows for dynamic monitoring of the projected fallout rates using a standard air particle counter. The theoretical results compare favorably with historical data and recent studies. © 2010 SPIE.
Sampath A.,Purdue University |
Sampath A.,Stinger Ghaffarian Technologies Inc. |
Shan J.,Purdue University
IEEE Transactions on Geoscience and Remote Sensing | Year: 2010
This paper presents a solution framework for the segmentation and reconstruction of polyhedral building roofs from aerial LIght Detection And Ranging (lidar) point clouds. The eigenanalysis is first carried out for each roof point of a building within its Voronoi neighborhood. Such analysis not only yields the surface normal for each lidar point but also separates the lidar points into planar and nonplanar ones. In the second step, the surface normals of all planar points are clustered with the fuzzy k-means method. To optimize this clustering process, a potential-based approach is used to estimate the number of clusters, while considering both geometry and topology for the cluster similarity. The final step of segmentation separates the parallel and coplanar segments based on their distances and connectivity, respectively. Building reconstruction starts with forming an adjacency matrix that represents the connectivity of the segmented planar segments. A roof interior vertex is determined by intersecting all planar segments that meet at one point, whereas constraints in the form of vertical walls or boundary are applied to determine the vertices on the building outline. Finally, an extended boundary regularization approach is developed based on multiple parallel and perpendicular line pairs to achieve topologically consistent and geometrically correct building models. This paper describes the detail principles and implementation steps for the aforementioned solution framework. Results of a number of buildings with diverse roof complexities are presented and evaluated. © 2009 IEEE.
Woronowicz M.S.,Stinger Ghaffarian Technologies Inc.
AIP Conference Proceedings | Year: 2011
The Lunar Atmosphere Dust Environment Explorer (LADEE) spacecraft is being designed for a mission featuring low altitude orbits of the Moon to take relevant ambient measurements before that environment becomes altered by future exploration activities. Instruments include a neutral mass spectrometer capable of measuring ambient species density levels below 100 molecules/cm3. Coincidentally, with a favorable combination of spacecraft orientations, it is also possible to measure plume gases from LADEE attitude control system thruster operations as they are reflected from the daytime lunar surface and subsequently intercepted by the spacecraft as it orbits overhead. Under such circumstances, it may be possible to test a variety of properties and assumptions associated with various transient plume models or to infer certain aspects regarding lunar surface properties. © 2011 American Institute of Physics.