Safety arguments for next generation location aware computing

Concerns over the accuracy, availability, integrity and continuity of Global Navigation Satellite Systems (GNSS) have limited the integration of GPS and GLONASS for safety-critical applications. More recent augmentation systems, such as the European Geostationary Navigation Overlay Service (EGNOS) and the North American Wide Area Augmentation System (WAAS) have begun to address these concerns. Augmentation architectures build on the existing GPS/GLONASS infrastructures to support locationbased services in Safety of Life (SoL) applications. Much of the technical development has been directed by air traffic management requirements, in anticipation of the more extensive support to be offered by GPS III and Galileo. WAAS has already been approved to provide vertical guidance against ICAO safety performance criteria for aviation applications. During the next twelve months, we will see the full certification of EGNOS for SoL applications. This paper identifies strong similarities between the safety assessment techniques used in Europe and North America. Both have relied on hazard analysis techniques to derive estimates of the Probability of Hazardously Misleading Information (PHMI). Later sections identify significant differences between the approaches adopted in application development. Integrated fault trees have been developed by regulatory and commercial organisations to consider both infrastructure hazards and their impact on non-precision RNAV/VNAV approaches using WAAS. In contrast, EUROCONTROL and the European Space Agency have developed a more modular approach to safety-case development for EGNOS. It remains to be seen whether the European or North American strategy offers the greatest support as satellite based augmentation systems are used within a growing range of SoL applications from railway signalling through to Unmanned Airborne Systems. The key contribution of this paper is to focus attention on the safety arguments that might support this wider class of location based services

The Role of Trust and Interaction in GPS Related Accidents: A Human Factors Safety Assessment of the Global Positioning System (GPS)

The Global Positioning System (GPS) uses a network of orbiting and geostationary satellites to calculate the position of a receiver over time. This technology has revolutionised a wide range of safety-critical industries and leisure applications ranging from commercial fisheries through to mountain running. These systems provide diverse benefits; supplementing the users existing navigation skills and reducing the uncertainty that often characterises many route planning tasks. GPS applications can also help to reduce workload by automating tasks that would otherwise require finite cognitive and perceptual resources. However, the operation of these systems has been identified as a contributory factor in a range of recent accidents. Users often come to rely on GPS applications and, therefore, fail to notice when they develop faults or when errors occur in the other systems that use the data from these systems. Further accidents can stem from the ‘over confidence’ that arises when users assume automated warnings will be issued when they stray from an intended route. Unless greater attention is paid to the human factors of GPS applications then there is a danger that we will see an increasing number of these failures as positioning technologies are integrated into increasing numbers of application

Elimination of subharmonics in direct look-up table (DLT) sine wave reference generators for low-cost microprocessor-controlled inverters

This paper investigates distortion of an inverter reference waveform generated using a direct look-up (DLT) algorithm. The sources of various distortion components are identified and the implications for application to variable speed drives and grid connected inverters are described. Harmonic and subharmonic distortion mechanisms are analyzed, and compared with experimental results. Analytical methods are derived to determine the occurrence of subharmonics, their number, frequencies and maximum amplitudes. A relationship is established identifying a discrete set of synthesizable frequencies which avoid sub-harmonic distortion as a function of look-up table length and a practical method for calculation of the look-up table indices, based on finite length binary representation, is presented. Real time experimental results are presented to verify the analytical derivations

Effective thermal conductivity of porous solder layers

Microscopic voids in the die attachment solder layers of power semiconductor devices degrade their overall thermal transfer performance. This paper presents analytical results of the effect of spherical and spheroidal void geometries on the thermal conductivity of bulk media. Analytical results are compared with axially symmetric and three-dimensional thermal simulations of single and multiple cavity defects in planar structures. The effective thermal conductivity of the die to the case attachment solder layer of two commercial metal oxide semiconductor field effect transistor (MOSFET) devices is estimated using these results, with cavity dimensions and distributions obtained by electron microscopy

Low order harmonic cancellation in a grid connected multiple inverter system via current control parameter randomization

In grid connected multiple inverter systems, it is normal to synchronize the output current of each inverter to the common network voltage. Any current controller deficiencies, which result in low order harmonics, are also synchronized to the common network voltage. As a result the harmonics produced by individual converters show a high degree of correlation and tend to be additive. Each controller can be tuned to achieve a different harmonic profile so that harmonic cancellation can take place in the overall system, thus reducing the net current total harmonic distortion level. However, inter-inverter communication is required. This paper presents experimental results demonstrating an alternative approach, which is to arrange for the tuning within each inverter to be adjusted automatically with a random component. This results in a harmonic output spectrum that varies with time, but is uncorrelated with the harmonic spectrum of any other inverter in the system. The net harmonics from all the inverters undergo a degree of cancellation and the overall system yields a net improvement in power quality

Suppression of line voltage related distortion in current controlled grid connected inverters

The influence of selected control strategies on the level of low-order current harmonic distortion generated by an inverter connected to a distorted grid is investigated through a combination of theoretical and experimental studies. A detailed theoretical analysis, based on the concept of harmonic impedance, establishes the suitability of inductor current feedback versus output current feedback with respect to inverter power quality. Experimental results, obtained from a purpose-built 500-W, three-level, half-bridge inverter with an L-C-L output filter, verify the efficacy of inductor current as the feedback variable, yielding an output current total harmonic distortion (THD) some 29% lower than that achieved using output current feedback. A feed-forward grid voltage disturbance rejection scheme is proposed as a means to further reduce the level of low-order current harmonic distortion. Results obtained from an inverter with inductor current feedback and optimized feed-forward disturbance rejection show a THD of just 3% at full-load, representing an improvement of some 53% on the same inverter with output current feedback and no feed-forward compensation. Significant improvements in THD were also achieved across the entire load range. It is concluded that the use of inductor current feedback and feed-forward voltage disturbance rejection represent cost–effect mechanisms for achieving improved output current quality

Calibration of a novel microstructural damage model for wire bonds

In a previous paper, a new time-domain damage-based physics model was proposed for the lifetime prediction of wire bond interconnects in power electronic modules. Unlike cycle-dependent life prediction methodologies, this model innovatively incorporates temperature- and time-dependent properties so that rate-sensitive processes associated with the bond degradation can be accurately represented. This paper presents the work on the development and calibration of the damage model by linking its core parameter, i.e., “damage,” to the strain energy density, which is a physically quantifiable materials property. Isothermal uniaxial tensile data for unbonded pure aluminum wires (99.999%) have been used to develop constitutive functions, and the model has been calibrated by the derived values of the strain energy density

Comprehensive Solution to the Cosmological Constant, Zero-Point Energy, and Quantum Gravity Problems

We present a solution to the cosmological constant, the zero-point energy, and the quantum gravity problems within a single comprehensive framework. We show that in quantum theories of gravity in which the zero-point energy density of the gravitational field is well-defined, the cosmological constant and zero-point energy problems solve each other by mutual cancellation between the cosmological constant and the matter and gravitational field zero-point energy densities. Because of this cancellation, regulation of the matter field zero-point energy density is not needed, and thus does not cause any trace anomaly to arise. We exhibit our results in two theories of gravity that are well-defined quantum-mechanically. Both of these theories are locally conformal invariant, quantum Einstein gravity in two dimensions and Weyl-tensor-based quantum conformal gravity in four dimensions (a fourth-order derivative quantum theory of the type that Bender and Mannheim have recently shown to be ghost-free and unitary). Central to our approach is the requirement that any and all departures of the geometry from Minkowski are to be brought about by quantum mechanics alone. Consequently, there have to be no fundamental classical fields, and all mass scales have to be generated by dynamical condensates. In such a situation the trace of the matter field energy-momentum tensor is zero, a constraint that obliges its cosmological constant and zero-point contributions to cancel each other identically, no matter how large they might be. Quantization of the gravitational field is caused by its coupling to quantized matter fields, with the gravitational field not needing any independent quantization of its own. With there being no a priori classical curvature, one does not have to make it compatible with quantization.Comment: Final version, to appear in General Relativity and Gravitation (the final publication is available at http://www.springerlink.com). 58 pages, revtex4, some additions to text and some added reference

Low-carbon development for Colombia

Colombia is well-positioned to pursue a low-carbon development path given the country’s already large hydropower production, a model urban transport program, and significant potential to reduce emissions from agriculture, forestry, and other land-use (AFOLU). While reducing carbon emissions through these and other activities can demonstrate Colombia’s commitment to addressing global climate change, the primary driver of such activities should be that they are part of the country’s economic and sustainable development agenda. Policymakers need to ensure that public policies for climate mitigation support projects that are economic and achieve macroeconomic goals such as generating income and employment. Given the importance of hydropower and Colombia’s huge potential for expanding agriculture and forestry, another consideration is that mitigation measures should not increase the country’s vulnerability to climate change impacts—some could actually do that—but should also increase the country’s resilience to natural and manmade disasters. These findings are based on an analysis of Colombia’s climate mitigation options made jointly by the World Bank and the Department of National Planning (DNP)

Low-carbon development for Colombia

Colombia is well-positioned to pursue a low-carbon development path given the country’s already large hydropower production, a model urban transport program, and significant potential to reduce emissions from agriculture, forestry, and other land-use (AFOLU). While reducing carbon emissions through these and other activities can demonstrate Colombia’s commitment to addressing global climate change, the primary driver of such activities should be that they are part of the country’s economic and sustainable development agenda. Policymakers need to ensure that public policies for climate mitigation support projects that are economic and achieve macroeconomic goals such as generating income and employment. Given the importance of hydropower and Colombia’s huge potential for expanding agriculture and forestry, another consideration is that mitigation measures should not increase the country’s vulnerability to climate change impacts—some could actually do that—but should also increase the country’s resilience to natural and manmade disasters. These findings are based on an analysis of Colombia’s climate mitigation options made jointly by the World Bank and the Department of National Planning (DNP)