Long Solution Times or Low Solution Quality: On Trade-Offs in Choosing a Power Flow Formulation for the Optimal Power Shut-Off Problem

The Optimal Power Shutoff (OPS) problem is an optimization problem that makes power line de-energization decisions in order to reduce the risk of igniting a wildfire, while minimizing the load shed of customers. This problem, with DC linear power flow equations, has been used in many studies in recent years. However, using linear approximations for power flow when making decisions on the network topology is known to cause challenges with AC feasibility of the resulting network, as studied in the related contexts of optimal transmission switching or grid restoration planning. This paper explores the accuracy of the DC OPS formulation and the ability to recover an AC-feasible power flow solution after de-energization decisions are made. We also extend the OPS problem to include variants with the AC, Second-Order-Cone, and Network-Flow power flow equations, and compare them to the DC approximation with respect to solution quality and time. The results highlight that the DC approximation overestimates the amount of load that can be served, leading to poor de-energization decisions. The AC and SOC-based formulations are better, but prohibitively slow to solve for even modestly sized networks thus demonstrating the need for new solution methods with better trade-offs between computational time and solution quality

Development of arcjet and ion propulsion for spacecraft stationkeeping

Near term flight applications of arc jet and ion thruster satellite station-keeping systems as well as development activities in Europe, Japan, and the United States are reviewed. At least two arc jet and three ion propulsion flights are scheduled during the 1992-1995 period. Ground demonstration technology programs are focusing on the development of kW-class hydrazine and ammonia arc jets and xenon ion thrusters. Recent work at NASA LeRC on electric thruster and system integration technologies relating to satellite station keeping and repositioning will also be summarized

Detecting heterozygosity in shotgun genome assemblies: Lessons from obligately outcrossing nematodes

The majority of nematodes are gonochoristic (dioecious) with distinct male and female sexes, but the best-studied species, Caenorhabditis elegans, is a self-fertile hermaphrodite. The sequencing of the genomes of C. elegans and a second hermaphrodite, C. briggsae, was facilitated in part by the low amount of natural heterozygosity, which typifies selfing species. Ongoing genome projects for gonochoristic Caenorhabditis species seek to approximate this condition by intense inbreeding prior to sequencing. Here we show that despite this inbreeding, the heterozygous fraction of the whole genome shotgun assemblies of three gonochoristic Caenorhabditis species, C. brenneri, C. remanei, and C. japonica, is considerable. We first demonstrate experimentally that independently assembled sequence variants in C. remanei and C. brenneri are allelic. We then present gene-based approaches for recognizing heterozygous regions of WGS assemblies. We also develop a simple method for quantifying heterozygosity that can be applied to assemblies lacking gene annotations. Consistently we find that ∼10% and 30% of the C. remanei and C. brenneri genomes, respectively, are represented by two alleles in the assemblies. Heterozygosity is restricted to autosomes and its retention is accompanied by substantial inbreeding depression, suggesting that it is caused by multiple recessive deleterious alleles and not merely by chance. Both the overall amount and chromosomal distribution of heterozygous DNA is highly variable between assemblies of close relatives produced by identical methodologies, and allele frequencies have continued to change after strains were sequenced. Our results highlight the impact of mating systems on genome sequencing projects

A Performance Comparison of Pulsed Plasma Thruster Electrode Configurations

Pulsed plasma thrusters are currently planned on two small satellite missions and proposed for a third. In these missions, the pulsed plasma thruster's unique characteristics will be used variously to provide propulsive attitude control, orbit raising, translation, and precision positioning. Pulsed plasma thrusters are attractive for small satellite applications because they are essentially stand alone devices which eliminate the need for toxic and/or distributed propellant systems. Pulsed plasma thrusters also operate at low power and over a wide power range without loss of performance. As part of the technical development required for the noted missions, an experimental program to optimize performance with respect to electrode configuration was undertaken. One of the planned missions will use pulsed plasma thrusters for orbit raising requiring relatively high thrust and previously tested configurations did not provide this. Also, higher capacitor energies were tested than previously tried for this mission. Multiple configurations were tested and a final configuration was selected for flight hardware development. This paper describes the results of the electrode optimization in detail

Cognitive visual tracking and camera control

Cognitive visual tracking is the process of observing and understanding the behaviour of a moving person. This paper presents an efficient solution to extract, in real-time, high-level information from an observed scene, and generate the most appropriate commands for a set of pan-tilt-zoom (PTZ) cameras in a surveillance scenario. Such a high-level feedback control loop, which is the main novelty of our work, will serve to reduce uncertainties in the observed scene and to maximize the amount of information extracted from it. It is implemented with a distributed camera system using SQL tables as virtual communication channels, and Situation Graph Trees for knowledge representation, inference and high-level camera control. A set of experiments in a surveillance scenario show the effectiveness of our approach and its potential for real applications of cognitive vision

Performance Evaluation of A High Energy Pulsed Plasma Thruster II

Abstract: NASA Glenn Research Center (GRC) has evaluated the performance of a pulsed plasma thruster (PPT) at energy levels up to 700 J. Previous testing was performed and reported on a thruster configuration designated HEPPT1b. This paper presents test results for evaluating the performance of a different thruster configuration designated HEPPT2. The main difference between the two configurations is in the lower transmission line inductance of HEPPT2 when compared to HEPPT1b. HEPPT2 tests were conducted at capacitance values of 100 µF, 180 µF and 260 µF for energy levels ranging from 100 J to 700J. Discharge current waveforms, polytetrafluoroethylene (PTFE) mass loss, and impulsebit magnitudes were measured and the thrust efficiency and specific impulse were computed. Test results indicated that at a given operating condition (both capacitance and energy), HEPPT2 performance was only 3-5% higher than HEPPT1b. The highest specific impulse of 3940 sec and thrust efficiency of 36% were attained by HEPPT2 at a capacitance of 260 µF and discharge energy of 700 J. Nomenclature AR = Planar electrodes aspect rati

Screening for PTSD and functional impairment in trauma-exposed young children: evaluation of alternative CBCL-PTSD subscales

The Child Behavior Checklist (CBCL 1.5–5 years) posttraumatic stress disorder (PTSD) subscale by Dehon & Scheeringa (2006) as a screener for PTSD in trauma-exposed young children has yielded inconsistent results so far. Therefore, the aim of this study was to create and examine the validity of alternative CBCL-PTSD subscales and compare them to the existing CBCL-PTSD subscale based on the DSM-5 PTSD diagnostic criteria for children 6 years and younger. Further, the CBCL-PTSD subscales were examined regarding their usefulness in screening for posttraumatic stress-related functional impairment. The sample comprised 116 trauma-exposed young children (M$_{age}$ = 3.42 years, SD$_{age}$ = 1.21 years, female = 49.1%). The psychometric properties of the existing CBCL-PTSD subscale as well as the alternative subscales based on expert rating (CBCL-PTSD-17) and based on variable importance (CBCL-PTSD-6) were evaluated by means of receiver operating characteristic curves, sensitivity, specificity, positive predictive values, and negative predictive values. Area under the curves for all three investigated CBCL-PTSD subscales were good to excellent for PTSD and functional impairment. Further, all three CBCL-PTSD subscales showed high sensitivity for PTSD and functional impairment. Considering the length and the performance of the three investigated subscales, the CBCL-PTSD-6 appears to be a promising and clinically useful CBCL-PTSD subscale as a screener for PTSD and functional impairment due to the easiest and most practicable application. For purposes of discriminant validation of the CBCL-PTSD-6, young children without a history of trauma should be compared to young children with trauma history

Neural Action Fields for Optic Flow Based Navigation: A Simulation Study of the Fly Lobula Plate Network

Optic flow based navigation is a fundamental way of visual course control described in many different species including man. In the fly, an essential part of optic flow analysis is performed in the lobula plate, a retinotopic map of motion in the environment. There, the so-called lobula plate tangential cells possess large receptive fields with different preferred directions in different parts of the visual field. Previous studies demonstrated an extensive connectivity between different tangential cells, providing, in principle, the structural basis for their large and complex receptive fields. We present a network simulation of the tangential cells, comprising most of the neurons studied so far (22 on each hemisphere) with all the known connectivity between them. On their dendrite, model neurons receive input from a retinotopic array of Reichardt-type motion detectors. Model neurons exhibit receptive fields much like their natural counterparts, demonstrating that the connectivity between the lobula plate tangential cells indeed can account for their complex receptive field structure. We describe the tuning of a model neuron to particular types of ego-motion (rotation as well as translation around/along a given body axis) by its ‘action field’. As we show for model neurons of the vertical system (VS-cells), each of them displays a different type of action field, i.e., responds maximally when the fly is rotating around a particular body axis. However, the tuning width of the rotational action fields is relatively broad, comparable to the one with dendritic input only. The additional intra-lobula-plate connectivity mainly reduces their translational action field amplitude, i.e., their sensitivity to translational movements along any body axis of the fly