  A cooperative Sense and Avoid (SAA) algorithm for safe navigation of
small-sized UAVs within an airspace is proposed in this paper. The proposed
method relies upon cooperation between the UAV and the surrounding
transponder-equipped aviation obstacles. To do so, the aviation obstacles share
their altitude and identification code with the UAV by using a Mode S operation
of the Secondary Surveillance Radar (SSR) after interrogation. The proposed SAA
algorithm benefits from the estimate of the aviation obstacle's elevation angle
for ranging. This results in more accurate ranging compared to the round-trip
time-based ranging, which is currently used in existing SAA systems. We also
propose a low-complexity and accurate radial velocity estimator for the Mode S
operation of the SSR which is employed in the proposed SAA system. Furthermore,
by considering the Pulse-Position Modulation (PPM) of the transponder reply as
a waveform of pulse radar with random pulse repetition intervals, the maximum
unambiguous radial velocity is obtained. The proposed SAA is equipped with an
intruder identification method that determines the risk level ofthe surrounding
transponder-equipped aviation obstacles. Given the estimated parameters, the
intruder identification method classifies the aviation obstacles into high-,
medium-, and low-risk intruders. The output of the classifier enables the UAV
to plan its path or maneuver for safe navigation accordingly. The root mean
square error (RMSE) of the proposed estimators are analytically derived, and
the effectiveness of our SAA solution is confirmed through simulation
experiments.
