Despite the extraordinary advances in global navigation satellite systems (GNSS), the inherent limitation of the weakness of their space-based signals makes such signals easy to block intentionally or accidentally. This makes GNSS insufficient for reliable anytime, anywhere navigation, particularly in GNSS-challenged environments, such as indoors, deep urban canyons, and GNSS-denied environments experiencing intentional jamming [1]. Several approaches have been proposed to address this inherent limitation of GNSS-based navigation, most notably augmenting GNSS receivers with dead- reckoning systems. This approach typically fuses the outputs of a fixed number of well-modeled heterogeneous sensors, particularly, GNSS receivers, inertial navigation systems, and digital map databases, with specialized signal processing algorithms. Motivated by the plenitude of ambient radio frequency signals in GNSS-challenged environments, a new paradigm to overcome the limitations of GNSS-based navigation is proposed. This paradigm, termed opportunistic navigation (OpNav), aims to extract positioning and timing information from ambient radio frequency signals of opportunity (SOPs). OpNav radio receivers continuously search for opportune signals from which to draw navigation and timing information, employing on-the-fly signal characterization as necessary [2]. In collaborative opportunistic navigation (COpNav), multiple OpNav receivers share information to construct and continuously refine a global signal landscape.

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Z.M. Kassas, "Collaborative Opportunistic Navigation," IEEE Aerospace and Electronic Systems Magazine, 28(6):38-41, June 2013.

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