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EMCON, or Emission Control, means turning off or suppressing the system’s radiating equipment to avoid detection. Only systems that emit radio signals are affected, since they reveal the aircraft’s presence. The radar altimeter and TACAN are active emitters: the radar altimeter sends pulses to measure altitude, and TACAN transmits mode data for navigation. When EMCON is active, these transmissions are disabled, which is why this option is the best fit. The other systems—ADF, GPS, and inertial navigation—primarily involve receiving signals or relying on internal sensors rather than emitting, so they aren’t the focus of EMCON in the same way.