With Unmanned Aircraft System (UAS) being developed and deployed for an increasing number of applications, it is essential to meet demanding separation assurance and navigation performance requirements, especially considering the current evolution of the UAS Traffic Management (UTM) research framework. However, in dense urban environments characterized by tall buildings and complex man-made structures, Global Navigation Satellite System (GNSS) is prone to data degradations or complete loss of signal due to multipath effects, interference or antenna obscuration. Furthermore, there is always a risk of jamming and spoofing of GNSS signals, with low cost civilian GNSS receivers being more vulnerable to a spoof attack. Therefore, a number of Signals of Opportunity (SoOP) techniques are being explored to improve the navigation performance when UAS are employed in urban canyons. Electromagnetic signals found in urban environments including analogue/digital radio, analogue/digital television, WiFi, Global System for Mobile Communications (GSM) and Code Division Multiple Access (CDMA) based signals are considered to model the system performance parameters. Implementation methods for exploiting SoOP such as Angle of Arrival (AOA), Time of Arrival (TOA), Received Signal Strength (RSS) and Time Difference of Arrival (TDOA) are introduced and compared. Integration of SoOP techniques in novel low-cost Navigation and Guidance Systems (NGS) is also investigated. As SoOP are not natively intended to be used for navigation purposes, no single source of SoOP for navigation can work in all environments and hence a SoOP source has to be selected based on specific requirements in the considered urban environment. Constraints of power and weight on the Unmanned Aerial Vehicle (UAV) besides hardware and software costs are also factors that are considered when selecting appropriate SoOP signal sources. Therefore, there is a clear opportunity for considerable savings in both infrastructure
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