Verifying collision avoidance behaviours for unmanned surface vehicles using probabilistic model checking

Collision avoidance is an essential safety requirement for unmanned surface vehicles (USVs). Normally, its practical verification is non-trivial, due to the stochastic behaviours of both the USVs and the intruders. This paper presents the probabilistic timed automata (PTAs) based formalism for three collision avoidance behaviours of USVs in uncertain dynamic environments, which are associated with the crossing situation in COLREGs. Steering right, acceleration, and deceleration are considered potential evasive manoeuvres. The state-of-the-art prism model checker is applied to analyse the underlying models. This work provides a framework and practical application of the probabilistic model checking for decision making in collision avoidance for USVs

The role of the atmosphere on Northern Hemisphere ice sheet evolution during the late Pleistocene

During the late Pleistocene, the Northern Hemisphere ice sheets waxed and waned with a periodicity of around 100 kyr. They are among the largest topographic features that can amplify, pace or drive global climate change on different time scales. Studying ice sheet-climate feedback through numerical modelling is necessary for understanding the physical mechanisms of the Earth system. As mainly land-based ice sheets, the role of the atmosphere on Northern Hemisphere ice sheet evolution during the late Pleistocene is investigated. The evolution of Northern Hemisphere ice sheets through the last glacial cycle is simulated with the glacial index method by using the climate forcing from a general circulation model, COSMOS. By comparing the simulated results to geological reconstructions, we first show that the modelled climate is capable of capturing the main features of the ice-sheet evolution. However, large deviations exist, likely due to the absence of nonlinear interactions between ice sheet and other climate components. The model uncertainties of the climate forcing are examined using the output from nine climate models from the Paleoclimate Modelling Intercomparison Project Phase III. The results show a large variability in simulated ice sheets between the different models. We find that the ice sheet extent pattern resembles summer surface air temperature pattern at the Last Glacial Maximum, confirming the dominant role of surface ablation process for high-latitude Northern Hemisphere ice sheets. This study shows the importance of the upper boundary condition for ice sheet modelling, and implies that careful constraints on climate output is essential for simulating realistic glacial Northern Hemisphere ice sheets. Evidence from proxy records indicates that millenniala scale abrupt climate shifts, called Dansgaard-Oeschger events, happened during past glacial cycles. We show that the Dansgaard-Oeschger events can regulate the mean state of the Northern Hemisphere ice sheets. Sensitivity experiments show that the simulated mean state is influenced by the amplitude of the climatic noise. The most likely cause of this phenomenon is the nonlinear response of the surface mass balance to temperature. It could also cause the retreat processes to be faster than the buildup processes within a glacial cycle. We propose that the climate variability hindered ice sheet development and prevented the Earth system from entering a full glacial state from Marine Isotope Stage 4 to Marine Isotope Stage 3 about 60,000 years ago. Antarctic ice core and deep ocean sediment core records imply that the interglacial climate during Marine Isotope Stage 13 is relatively cold, and ice sheets were likely larger. From perspective of equilibrium simulations, we modelled the MIS 13 climate with a coupled climate-ice sheet model AWI-CM-PISM under different orbital configurations at 495, 506 and 517 kyr BP. Summer insolation at 65 $circ$N at 495 kyr BP is similar to the preindustrial, but with lower greenhouse gas values. It leads to more ice sheet buildup than present-day. Boreal summer at perihelion at 506 kyr BP causes a warmer summer over Northern Hemisphere continents. This could inhibit the development of Northern Hemisphere ice sheets. Lower obliquity induces cooling over the polar regions and is favorable for the ice sheet buildup. Besides polar regions, mountains with high elevations are also favourable for ice sheet buildup. The Cordilleran Ice Sheet is likely more sensitive and has faster response to boreal summer insolation change than the other large scale Northern Hemisphere ice sheets

Energy Spectrum Extraction and Optimal Imaging via Dual-Energy Material Decomposition

Inferior soft-tissue contrast resolution is a major limitation of current CT scanners. The aim of the study is to improve the contrast resolution of CT scanners using dual-energy acquisition. Based on dual-energy material decomposition, the proposed method starts with extracting the outgoing energy spectrum by polychromatic forward projecting the material-selective images. The extracted spectrum is then reweighted to boost the soft-tissue contrast. A simulated water cylinder phantom with inserts that contain a series of six solutions of varying iodine concentration (range, 0-20 mg/mL) is used to evaluate the proposed method. Results show the root mean square error (RMSE) and mean energy difference between the extracted energy spectrum and the spectrum acquired using an energy-resolved photon counting detector(PCD), are 0.044 and 0.01 keV, respectively. Compared to the method using the standard energy-integrating detectors, dose normalized contrast-to-noise ratio (CNRD) for the proposed method are improved from 1 to 2.15 and from 1 to 1.88 for the 8 mg/mL and 16 mg/mL iodine concentration inserts, respectively. The results show CT image reconstructed using the proposed method is superior to the image reconstructed using the standard method that using an energy-integrating detector.Comment: 4 pages, 4 figures in The 2015 IEEE Nuclear Science Symposium and Medical Imaging Conference Recor