Simulation results for an encounter situation with multiple obstacle vessels based on MPC-PSO. (a) Full trajectory plot of the ASV and each obstacle vessel. (b) Trajectory plot up to the point where the distance between the ASV and each obstacle vessel are the closest. (c) The course offset angle χ_ca and the propulsion P of the ASV for collision avoidance. (d) Actual course command χ of the ASV.