There are several different types of VGSI(s). The primary systems covered in this chapter are visual approach slope indicators (VASI), precision approach path indicators (PAPI), pulsating visual approach slope indicators (PVASI), T-VASI, three-color VASI, and helicopter approach path indicator (HAPI). Each of these systems presents a different type of visual indication to the pilot and requires different in-flight interpretation
Keep in mind that they can all emit erroneous light signals due to light beam refraction in weather conditions of low mist especially when the dew bulb and dry bulb temperatures are close together.