What does a red light on the SGSI indicate to a pilot?

A red light on the Standardized Glide Slope Indicator (SGSI) indicates that the aircraft is below the proper glide slope. This visual cue is crucial for pilots, especially during landing approaches, as it informs them that they need to increase altitude to align with the designated glide slope. The glide slope is an essential part of the Instrument Landing System (ILS), providing pilots with vertical guidance during the final approach to landing. When the red light is illuminated, it serves as an urgent reminder for pilots to make necessary adjustments to ensure a safe landing trajectory. Understanding this signal helps improve situational awareness and enhances safety during flight operations.