Final Answer:
When the Distance Measuring Equipment (DME) Ground Station (DGNS) discovers an error through its built-in test equipment, the "FAULT" light illuminates.
Step-by-step explanation:
The "FAULT" light serves as an indicator on the Distance Measuring Equipment (DME) Ground Station (DGNS) to alert operators when the system has detected an error during its built-in test equipment (BITE) process. BITE is a self-diagnostic feature incorporated into avionic systems, including DME, to monitor the health and functionality of the equipment.
When an anomaly or error is detected, the "FAULT" light illuminates, signaling to operators that there is a potential issue that requires attention. This visual indication helps aviation personnel identify and address problems promptly, ensuring the reliability and accuracy of distance measuring information provided by the DGNS.