Overview of TeV gamma ray observations

TeV emission has been observed with high signal-to-noise from: two pulsar-driven supernova remnants, the Crab Nebula and PSR B1706-44 and a blazar class AGN, Markarian 421. Other sources, observed without the benefit of major background reduction techniques, are some of the compact object binaries. These discoveries plus the discoveries of EGRET of more than 130 sources at lower energies point to a growing number of scientific questions capable of being addressed by this field (particularly for distant sources via intergalactic absorption processes). Rapid further development will come as new instruments employing a variety of background reduction techniques come on-line.     

Design by extrapolation: an evaluation of fault tolerant avionics

Over the past 30 years, safety-critical avionics systems such as Fly-By-Wire (FBW) flight controls, full-authority digital engine controls, and other systems have been introduced on many commercial and military airplanes and spacecraft. Early FBW systems, such as on the F-16 and Airbus A320, were considered revolutionary and introduced with extreme caution. These early systems and their successors all make use of redundant and fault-tolerant avionics to provide the required dependability and safety, but have used significantly different architectures. This paper examines the different levels of criticality and fault tolerance required by different types of avionics systems, establishes architectural categories of fault-tolerant architectures, and identifies the discriminating features of the varied approaches. Examples of discriminators include the level of redundancy, methods of engaging backup systems, protection from software errors, and the use of dissimilar hardware and software. The strengths and weaknesses of the approaches will be identified. The paper concludes with some speculation on trends for future systems based on this evaluation of previous systems     

Principal components, covariance matrix tapers, and the subspaceleakage problem

A new class of robust space-time adaptive beamforming techniques is introduced to address a broad range of subspace leakage phenomena that arise in many sensor array applications. When present, these leakage phenomena can significantly increase the effective rank of the dominant colored noise interference spectrum, thereby reducing the appeal of techniques that exploit low-rank dominant interference (such as principal components (PC) or diagonal loading) to reduce sample support (training) requirements. By combining the covariance matrix taper (CMT) approach with either PC or diagonal loading, the minimal sample support properties of these techniques can be preserved     

Novel sensor technology for monitoring and control of critical plant nutrient parameters.

A novel dielectric sensor technology has been developed for monitoring and control of plant nutrient delivery systems as part of NASA's Controlled Ecological Life Support System (CELSS) program. A unique measurement phenomenon was discovered in which the electrostatic field is shunted to a third terminal of the sensor, resulting in a much greater sensitivity to changes in the complex dielectric properties of the nutrient solution. Based on this phenomenon, a small, flexible, thin-film shunting dielectric sensor (SDS) was designed to provide low-frequency, non-invasive measurement of both the thickness and nutrient concentration of the layer of solution on a plant growth surface. Test results indicate a sensitivity of +/- 0.05mm in layer thickness while characterization of the ability to measure nutrient concentration continues. The development plan for this sensor is presented and other applications are discussed.     

Development of a Digital Array Radar (DAR)

Twenty-first century littoral and open-sea missions present US Navy (USN) shipboard-radar systems with the challenge of detecting small targets in severe clutter and against multiple sources of interference. In Fiscal Year 2000 (FY00), the Office of Naval Research (ONR) sponsored a program to develop an active array radar that includes a digital beamforming (DBF) architecture. The DBF radar system has the potential for improved time-energy management, improved signal-to-clutter (S/C) ratios, improved reliability and reduced life-cycle costs. This paper summarizes the latest developments of the program during FY00     

System identification of dynamic closed-loop control of total peripheral resistance by arterial and cardiopulmonary baroreceptors.

Prolonged exposure to microgravity in space flight missions (days) impairs the mechanisms responsible for defense of arterial blood pressure (ABP) and cardiac output (CO) against orthostatic stress in the post-flight period. The mechanisms responsible for the observed orthostatic intolerance are not yet completely understood. Additionally, effective counter measures to attenuate this pathophysiological response are not available. The aim of this study was to investigate the ability of our proposed system identification method to predict closed-loop dynamic changes in TPR induced by changes in mean arterial pressure (MAP) and right atrial pressure (RAP). For this purpose we designed and employed a novel experimental animal model for the examination of arterial and cardiopulmonary baroreceptors in the dynamic closed-loop control of total peripheral resistance (TPR), and applied system identification to the analysis of beat-to-beat fluctuations in the measured signals. Grant numbers: NAG5-4989.     

Initial closed operation of the CELSS Test Facility Engineering Development Unit.

As part of the NASA Advanced Life Support Flight Program, a Controlled Ecological Life Support System (CELSS) Test Facility Engineering Development Unit has been constructed and is undergoing initial operational testing at NASA Ames Research Center. The Engineering Development Unit (EDU) is a tightly closed, stringently controlled, ground-based testbed which provides a broad range of environmental conditions under which a variety of CELSS higher plant crops can be grown. Although the EDU was developed primarily to provide near-term engineering data and a realistic determination of the subsystem and system requirements necessary for the fabrication of a comparable flight unit, the EDU has also provided a means to evaluate plant crop productivity and physiology under controlled conditions. This paper describes the initial closed operational testing of the EDU, with emphasis on the hardware performance capabilities. Measured performance data during a 28-day closed operation period are compared with the specified functional requirements, and an example of inferring crop growth parameters from the test data is presented. Plans for future science and technology testing are also discussed.     

Coronal Holes and the High-Speed Solar Wind

Coronal holes are the lowest density plasma components of the Sun's outer atmosphere, and are associated with rapidly expanding magnetic fields and the acceleration of the high-speed solar wind. Spectroscopic and polarimetric observations of the extended corona, coupled with interplanetary particle and radio sounding measurements going back several decades, have put strong constraints on possible explanations for how the plasma in coronal holes receives its extreme kinetic properties. The Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft has revealed surprisingly large temperatures, outflow speeds, and velocity distribution anisotropies for positive ions in coronal holes. We review recent observations, modeling techniques, and proposed heating and acceleration processes for protons, electrons, and heavy ions. We emphasize that an understanding of the acceleration region of the wind (in the nearly collisionless extended corona) is indispensable for building a complete picture of the physics of coronal holes.     

The Low-Latitude Boundary Layer at the Flanks of the Magnetopause

This is a brief overview on what we know and do not know about the low-latitude boundary layer (LLBL) at the flanks of the magnetotail. On the basis of recent observations, simulations and theories we conclude that reconnection is the dominant process in generating the LLBL and its structure probably even under northward IMF conditions. Part of the LLBL always seems to be on open field lines. Possibly the LLBL possesses a double structure with its outer part open and inner part closed. Anomalous diffusive processes cannot sustain the LLBL but provide sufficient diffusivity for reconnection. Strong diffusion is only expected in narrow localized regions and can make the transition to superdiffusion. Kelvin-Helmholtz instability (KHI) is favoured for northward IMF, producing vortices at the tail flanks. Its contribution to efficient mass transport still remains questionable. Coupling of the LLBL to the ionosphere can strongly affect the internal structure of the LLBL, causing turbulent eddies and detachments of plasma blobs as also field-aligned currents and electron heating. The structure and dynamics of the LLBL are affected by field-aligned electric potentials that decouple the LLBL from the ionosphere. Non-ideal coupling simulations suggest that the dusk flank is decoupled, favouring KHI, while the dawn flank is dominated by currents and turbulence.