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Dual-station cross localization method is another promising technical route to realize cooperative localization in GNSSdenied environment, requiring only two anchor node UAVs instead of at least three nodes if inter-node distance measurement based localization method is used. Traditional Angle-of Arrival (AOA) estimation techniques cannot obtain accurate and highly available angle measurements used for position calculation, for example, optical measuring equipment can provide precise angle measurements but is restricted by weather condition, array antenna can be used in all situations but its measurement error would be too large. In order to acquire precise azimuth and pitch estimations, an optical phase scanning based AOA estimation method is utilized in this work. At the remote antenna unit (RAU), one received microwave signal is applied to a phase modulator (PM), and another received microwave signal coupled with a low-frequency large-voltage sawtooth-wave signal is applied to another PM, this sawtooth-wave signal is utilized to scan the phase of the optical sideband from 0 to 2π. By transmission through a segment of fiber link, the AOA value can be measured by processing the obtained low-frequency electrical signals at the central office (CO). Experimental results demonstrate that the AOA estimation precision could be less than 2.27° when the distance between array elements is half-wavelength of the microwave (d = λ/2 0.015 @10GHz), and smaller than 0.017° if the array elements spacing is bigger than 2m for medium-sized or large-sized UAV. Then the experimental result is applied into dual station cross localization simulation structure of UAV swarm, then localization precision distribution is evaluated in different scenarios, corresponding outcomes indicate that optical phase scanning based high precision AOA estimations are beneficial to cooperative localization, and in order to acquire more accurate cooperative localization results, the positions of anchor node UAVs need to be properly adjusted.
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