Frequency-controlled series-resonant converter with synchronous rectifier

This paper presents an analysis and experimental results for a frequency-controlled series-resonant dc-dc converter that consists of a Class-D zero-voltage-switching (ZVS) series-resonant inverter and a center-tapped synchronous rectifier. If the dc output voltage is low, the efficiency of the converter is dominated by the efficiency of the rectifier. Low on-resistance metal-oxide-semiconductor field-effect transistors (MOSFETs) are used in the rectifier instead of diodes because the forward voltage drop across the rectifying device is low, resulting in a high efficiency. The dc output voltage is regulated against variations in the load resistance and the dc input voltage by varying the operating frequency. Experimental results are presented for a converter with a dc input voltage of 150 V, an output voltage of 5 V, and a dc load resistance ranging from 0.5 to 5.5 R. The measured efficiency was 86% for a 50 W output and 89% for a 25 W output. The theoretical results were in good agreement with the measured results.     

Global and local geospace modeling in ISTP

The objective of the University of Maryland ISTP theory project is the development of the analytical and computational tools, which, combined with the data collected by the space and ground-based ISTP sensors, will lead to the construction of the first causal and predictive global geospace model. To attain this objective a research project composed of four complementary parts is conducted. First the global interaction of the solar wind-magnetosphe re system is studied using three-dimensional MHD simulations. Appropriate results of these simulations are made available to other ISTP investigators through the Central Data Handling Facility (CDHF) in a format suitable for comparison with the observations from the ISTP spacecrafts and ground instruments. Second, simulations of local processes are performed using a variety of non-MHD codes (hybrid, particle and multifluid) to study critical magnetospheric boundary layers, such as the magnetopause and the magnetotail. Third, a strong analytic effort using recently developed methods of nonlinear dynamics is conducted, to provide a complementary semi-empirical understanding of the nonlinear response of the magnetosphere and its parts to the solar wind input. The fourth part will be conducted during and following the data retrieval and its objective is to utilize the data base in conjunction with the above models to produce the next generation of global and local magnetospheric models. Special emphasis is paid to the development of advanced visualization packages that allow for interactive real time comparison of the experimental and computational data. Examples of the computational tools and of the ongoing investigations are presented.     

Kelvin–Helmholtz Instability: Lessons Learned and Ways Forward

The Kelvin–Helmholtz instability (KHI) is a ubiquitous phenomenon across the Universe, observed from 500 m deep in the oceans on Earth to the Orion molecular cloud. Over the past two decades, several space missions have enabled a leap forward in our understanding of this phenomenon at the Earth’s magnetopause. Key results obtained by these missions are first presented, with a special emphasis on Cluster and THEMIS. In particular, as an ideal instability, the KHI was not expected to produce mass transport. Simulations, later confirmed by spacecraft observations, indicate that plasma transport in Kelvin–Helmholtz (KH) vortices can arise during non-linear stage of its development via secondary process. In addition to plasma transport, spacecraft observations have revealed that KHI can also lead to significant ion heating due to enhanced ion-scale wave activity driven by the KHI. Finally, we describe what are the upcoming observational opportunities in 2018–2020, thanks to a unique constellation of multi-spacecraft missions including: MMS, Cluster, THEMIS, Van Allen Probes and Swarm.     

Bayesian gamma mixture model approach to radar target recognition

This paper develops a Bayesian gamma mixture model approach to automatic target recognition (ATR). The specific problem considered is the classification of radar range profiles (RRPs) of military ships. However, the approach developed is relevant to the generic discrimination problem. We model the radar returns (data measurements) from each target as a gamma mixture distribution. Several different motivations for the use of mixture models are put forward, with gamma components being chosen through a physical consideration of radar returns. Bayesian formalism is adopted and we obtain posterior distributions for the parameters of our mixture models. The distributions obtained are too complicated for direct analytical use in a classifier, so Markov chain Monte Carlo (MCMC) techniques are used to provide samples from the distributions. The classification results on the ship data compare favorably with those obtained from two previously published techniques, namely a self-organizing map and a maximum likelihood gamma mixture model classifier.     

Closed-loop, estimator-based model of human posture following reduced gravity exposure

A computational and experimental method is employed to provide an understanding of a critical human space flight problem, posture control following reduced gravity exposure. In the case of an emergency egress, astronauts' postural stability could be life saving. It is hypothesized that muscular gains are lowered during reduced gravity exposure, causing a feeling of heavy legs, or a perceived feeling of muscular weakness, upon return to Earth's 1 g environment. We developed an estimator-based model that is verified by replicating spatial and temporal characteristics of human posture and incorporates an inverted pendulum plant in series with a Hill-type muscle model, two feedback pathways, a central nervous system estimator, and variable gains. Results obtained by lowering the variable muscle gain in the model support the hypothesis. Experimentally, subjects were exposed to partial gravity (3/8 g) simulation on a suspension apparatus, then performed exercises postulated to expedite recovery and alleviate the heavy legs phenomenon. Results show that the rms position of the center of pressure increases significantly after reduced gravity exposure. Closed-loop system behavior is revealed, and posture is divided into a short-term period that exhibits higher stochastic activity and persistent trends and a long-term period that shows relatively low stochastic activity and antipersistent trends.     

Visual analysis in a deployable antenna experiment [satellite antenna]

To establish a large deployable antenna, monitoring and collimation are essential for reliable and precise deployment. We have developed an analysis method to detect shifts in several images, in which the combination of cross-correlations between images and approximations at subpixel precision enables us to detect shifts in images with a precision of up to 0.01 pixels. The LDREX mission; which was a preliminary experiment for a large deployable antenna, ETS-VIII, was performed in December 2000. During this experiment, anomalies occurred in the antenna, and deployment was aborted. To understand the cause of the anomalies, we used our visual analysis method. Using this analysis, we detected vibrating features in the antenna, which were useful for explaining the anomalies. We outline our visual analysis method and discuss its application in monitoring the deployable antenna.     

Switched Ethernet testing for avionics applications

Switched Ethernet is being implemented as an avionics communication architecture. A commercial standard (ARINC-664) and an aircraft vendor-specific implementation known as avionics full duplex switched Ethernet (AFDX) have been developed that defines the topology and use of switched Ethernet in an avionics application. In avionics applications, the movement of data between devices must take place in a deterministic fashion and must be delivered reliably. All aircraft flight hardware must be tested to be sure that it will communicate information properly in the switched Ethernet network. The airframe manufacture must test the integrated network to verify that all flight hardware is communicating properly. Testing and maintenance testing is required to perform data communication level testing of switched Ethernet architectures for avionics applications to insure that all communication is deterministic and reliable. This paper provides an overview of a switched Ethernet avionics network and identifies the testing challenges associated with a switched Ethernet avionics application. A practical implementation performing the required tests is discussed.     

The plasma Environment of Mars

Space Science Reviews -     

Radar: The Cassini Titan Radar Mapper

The Cassini RADAR instrument is a multimode 13.8 GHz multiple-beam sensor that can operate as a synthetic-aperture radar (SAR) imager, altimeter, scatterometer, and radiometer. The principal objective of the RADAR is to map the surface of Titan. This will be done in the imaging, scatterometer, and radiometer modes. The RADAR altimeter data will provide information on relative elevations in selected areas. Surfaces of the Saturn’s icy satellites will be explored utilizing the RADAR radiometer and scatterometer modes. Saturn’s atmosphere and rings will be probed in the radiometer mode only. The instrument is a joint development by JPL/NASA and ASI. The RADAR design features significant autonomy and data compression capabilities. It is expected that the instrument will detect surfaces with backscatter coefficient as low as −40 dB.RADAR Team LeaderThis revised version was published online in July 2005 with a corrected cover date.     

On the Role of Interchange Reconnection in the Generation of the Slow Solar Wind

The heating of the solar corona and therefore the generation of the solar wind, remain an active area of solar and heliophysics research. Several decades of in situ solar wind plasma observations have revealed a rich bimodal solar wind structure, well correlated with coronal magnetic field activity. Therefore, the reconnection processes associated with the large-scale dynamics of the corona likely play a major role in the generation of the slow solar wind flow regime. In order to elucidate the relationship between reconnection-driven coronal magnetic field structure and dynamics and the generation of the slow solar wind, this paper reviews the observations and phenomenology of the solar wind and coronal magnetic field structure. The geometry and topology of nested flux systems, and the (interchange) reconnection process, in the context of coronal physics is then explained. Once these foundations are laid out, the paper summarizes several fully dynamic, 3D MHD calculations of the global coronal system. Finally, the results of these calculations justify a number of important implications and conclusions on the role of reconnection in the structural dynamics of the coronal magnetic field and the generation of the solar wind.