Cooperative sensing and monitoring event is one of the important applications of sensor networks. Node deployment and duty cycle configuration of event detection in a large wireless sensor networks is an enormous challenge. In this paper, through in-depth analysis we find that there is a tradeoff between node duty cycle and event quality monitoring. That is, adopting larger duty cycle enables network to deploy fewer nodes. Or deploying more nodes can reduce their duty cycle. Both of them can reach the event detecting quality requirement of application. Based on the above findings, a novel subtraction deployment strategy (SDS) combined with the unequal node duty cycle in the network is presented. This strategy improves the duty cycle of nodes in non-hotspots area when reduces the number, thereby minimizing the deployment cost on the premise of meeting the detecting quality of application. We introduce a formal model to indicate the tradeoff between deployment cost and the quality of event detection. Subsequently, we present a heuristic algorithm to find near-optimal deployment solutions for practical scenarios. Both theoretical and experimental simulation results show that the proposed scheme SDS for event detection can reduce the deployed number of nodes by 11.49% and do no harm to detecting quality. In the deployment of the same node number, the weighted delay of detection can be reduced as much as 47%, and the weighted detection percentage can be increased by 1.56 times without reducing the network lifetime compared with those in uniform deployment scheme.

Electronic compass is often used to provide the absolute heading reference for tracking the user's head and hands in virtual reality (VR) and augmented reality (AR), especially for outdoor AR applications. However, compass is vulnerable to environment magnetism disturbance. Existing compass calibration methods require complex steps and true heading reference which is often impossible to be obtained in outdoor AR applications, and is useful only when compass is in horizontal plane. An autocalibration method without the need of heading reference and redundant sensors is proposed in This work. First the compass error model based on physical principle is presented, then the algorithm to calculate the compensation coefficients with a set of sample measurements of the sensors in the compass is described. Because the influence of the environmental disturbance has been effectively compensated, the calibrated compass can provided accurate heading even when it is under large tilt attitude.