131. You have been monitoring your 3-cm radar screen and a series of 12 concentric circles suddenly appears centered on the screen. What is the most likely cause of this situation?

A series of concentric circles centered on the radar screen indicates activation of the vessel's Search and Rescue Transponder (SART). SARTs respond to X-band radar signals, producing this pattern when located near the radar antenna. This distinguishes it from a survival craft, radar fault, or EPIRB activation, which operate differently and would not generate this specific radar return.