High-level modeling and analysis of the traffic alert and collisionavoidance system (TCAS)

We demonstrate a high-level approach to modeling, analyzing, and verifying complex safety-critical systems through a case study on the traffic alert and collision avoidance system (TCAS); an avionics system that detects and resolves aircraft collision threats. Due to the complexity of the TCAS software and the hybrid nature of the closed-loop system, the traditional testing technique of exhaustive simulation does not constitute a viable verification approach. Moreover, the detailed specification of the system software employed to date as a means toward analysis and verification neither helps in intuitively understanding the behavior of the system nor enables the analysis of the closed-loop system behavior. We advocate defining high-level hybrid system models that capture the behavior not only of the software but also of the airplanes, sensors, pilots, etc. In particular, we show how the core components of TCAS can be captured by relatively simple hybrid I/O automata, which are amenable to format analysis. We then outline a methodology for establishing conditions under which TCAS guarantees sufficient separation in altitude for aircraft involved in collision threats. The contributions of the paper are the high-level models of the closed-loop TCAS system and the demonstration of the usefulness of high-level modeling, analysis, and verification techniques     

Bone Health in Patients with Dyslipidemias: An Underestimated Aspect

Beyond being aging-related diseases, atherosclerosis and osteoporosis share common pathogenetic pathways implicated in bone and vascular mineralization. However, the contributory role of dyslipidemia in this interplay is less documented. The purpose of this narrative review is to provide epidemiological evidence regarding the prevalence of bone disease (osteoporosis, fracture risk) in patients with dyslipidemias and to discuss potential common pathophysiological mechanisms linking osteoporosis and atherosclerosis. The effect of hypolipidemic therapy on bone metabolism is also discussed. Despite the high data heterogeneity and the variable quality of studies, dyslipidemia, mainly elevated total and low-density lipoprotein cholesterol concentrations, is associated with low bone mass and increased fracture risk. This effect may be mediated directly by the increased oxidative stress and systemic inflammation associated with dyslipidemia, leading to increased osteoclastic activity and reduced bone formation. Moreover, factors such as estrogen, vitamin D and K deficiency, and increased concentrations of parathyroid hormone, homocysteine and lipid oxidation products, can also contribute. Regarding the effect of hypolipidemic medications on bone metabolism, statins may slightly increase BMD and reduce fracture risk, although the evidence is not robust, as it is for omega-3 fatty acids. No evidence exists for the effects of ezetimibe, fibrates, and niacin. In any case, more prospective studies are needed further to elucidate the association between lipids and bone strength.     

A parallel algorithm for surface-based object reconstruction

This paper presents a parallel algorithm that approximates the surface of an object from a collection of its planar contours. Such a reconstruction has wide applications in such diverse fields as biological research, medical diagnosis and therapy, architecture, automobile and ship design, and solid modeling. The surface reconstruction problem is transformed into the problem of finding a minimum-cost acceptable path on a toroidal grid graph, where each horizontal and each vertical edge have the same orientation. An acceptable path is closed path that makes a complete horizontal and vertical circuit. We exploit the structure of this graph to develop efficient parallel algorithms for a message-passing computer. Givenp processors and anm byn toroidal graph, our algorithm will find the minimum cost acceptable path inO(mn log(m)/p) steps, ifp =O(mn/((m + n) log(mn/(m + n)))), which is an optimal speedup. We also show that the algorithm will sendO(p ²(m + n)) messages. The algorithm has a linear topology, so it is easy to embed the algorithm in common multiprocessor architectures.     

FDGE: A forward-difference grid engine and its application in rendering algebraic surfaces in parallel

The display of an implicitly defined surface is obtained by its projection onto the viewing plane. Ray casting is a technique that accomplishes this projection byfiring a mapping ray through each pixel of the screen into the world space. The intersection points of this ray with the surface are found; these points are further tested to determine which one is visible and is within the viewing volume. Finally, if a point that satisfies the above conditions is found, then the point is further processed for the determination of its shading value.Much progress in rendering algebraic surfaces has been made recently. However, most of the proposed solutions are based on subdivision methods. This paper focuses on a method that performsdirect rendering of such a surface that will minimize the number of operations; this method will also replace almost all required multiplications with additions. Furthermore, the VLSI-oriented algorithm lends itself to parallelism. In addition, how this method can be used in the calculation of the shading values is described and a VLSI architecture is briefly discussed. Finally, a by-product of this method is that it can be used to efficiently calculate the values of a bivariate polynomial in a rectangular grid and in parallel.