The fast growing popularity of smartphones and tablets enables us to use various intelligent mobile applications. As many of those applications require position information, smart mobile devices provide positioning methods such as Global Positioning System (GPS), WiFi-based positioning system (WPS), or Cell-ID-based positioning service. However, those positioning methods have different characteristics of energy-efficiency, accuracy, and service availability. In this paper, we present an Energy-Efficient Collaborative and Opportunistic Positioning System (ECOPS) for heterogeneous mobile devices. ECOPS facilitates a collaborative environment where many mobile devices can opportunistically receive position information over energy-efficient and prevalent WiFi, broadcasted from a few other devices in the communication range. The position-broadcasting devices in ECOPS have sufficient battery power and up-to-date location information obtained from accurate but energy-inefficient GPS. A position receiver in ECOPS estimates its location using a combination of methods including received signal strength indicators and 2D trilateration. Our field experiments show that ECOPS significantly reduces the total energy consumption of devices while achieving an acceptable level of location accuracy. ECOPS can be especially useful for unique resource scarce, infrastructureless, and mission critical scenarios such as battlefields, border patrol, mountaineering expeditions, and disaster area assistance. 1. Introduction Smart mobile devices such as smartphones and tablets are rapidly becoming prevalent in our lives. They have spurred a paradigm shift from traditional restricted phone applications to intelligent mobile applications such as location-based, context-aware, and situation-aware services. For example, a social-network-based traffic information system [1] allows each mobile user to report and use real-time traffic information, in addition to the archived traffic information from the US Department of Transportation. As many of those application services require position information, smart mobile devices provide various positioning services via Global Positioning System (GPS) [2], WiFi-based positioning system (WPS) [3], or Cell-ID Positioning [4]. Being dedicated equipment for positioning, GPS becomes available for many smart devices as an additional feature and is considered to be an accurate and preferred method for location-based services (LBSs) [5, 6]. However, its high energy consumption, due to the Time To First Fix (TTFF), becomes a significant drawback. WPS
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