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A new Sense and Avoid (SAA) method for safe navigation of small-sized UAVswithin an airspace is proposed in this paper. The proposed method relies uponcooperation between the UAV and the surrounding transponder-equipped aviationobstacles. To do so, the aviation obstacles share their altitude and theiridentification code with the UAV by using a miniaturized Mode S operationSecondary surveillance radar (SSR) after interrogation. The proposed SAAalgorithm removes the need for a primary radar and a clock synchronizationsince it relies on the estimate of the aviation obstacle's elevation angle forranging. This results in more accurate ranging compared to the round-triptime-based ranging. We also propose a new radial velocity estimator for theMode S operation of the SSR which is employed in the proposed SAA system. Theroot-mean-square error (RMSE) of the proposed estimators are analyticallyderived. Moreover, by considering the pulse-position modulation (PPM) of thetransponder reply as a waveform of pulse radar with staggered multiple pulserepetition frequencies, the maximum unambiguous radial velocity is obtained.Given these estimated parameters, our proposed SAA method classifies theaviation obstacles into high-, medium-, and low-risk intruders. The output ofthe classifier enables the UAV to plan its path or maneuver for safe navigationaccordingly. The effectiveness of the proposed estimators and the SAA method isconfirmed through simulation experiments.

Cooperative Sense and Avoid for UAVs using Secondary Radar