What is a VGSI?
From Wikipedia, i hope this might help:
Visual Glide Slope Indicator or Visual Glideslope Indicator (VGSI) is a ground device that uses lights to assist a pilot in landing an airplane at an airport. The lights define a vertical approach path during the final approach to a runway and can help the pilot determine if the airplane is too high or too low for an optimum landing.
There are several different types of VGSI's:
Visual Approach Slope Indicator (VASI)
Precision Approach Path Indicator (PAPI)
Pulsating Visual Approach Slope Indicator (PVASI)
Three-color Visual Approach Slope Indicator (T-VASI)
Helicopter Approach Path Indicator (HAPI)