Explanation: RADAR systems transmit highly focused beams of powerful radio frequency (RF) energy. The primary and most immediate concern before activating such a system is the safety of personnel. High-power RF exposure, particularly to the eyes and other sensitive tissues, can cause harmful heating effects. Therefore, ensuring no one is in the direct path of the antenna's beam is a critical first step to prevent injury, aligning with fundamental RF safety regulations and best practices across all radio services. While checking grounds (A) is important for equipment protection, assuring the display (B) is for functionality, and informing others (C) is for operational coordination, none of these directly address the immediate physical danger posed by the powerful RF emissions to nearby personnel. Personnel safety from RF exposure is paramount.