quired ▁is ▁noted ▁on ▁the ▁Inst rument ▁Appro ach ▁Pro cedure ▁and ▁the ▁aircraft ▁must ▁have ▁at ▁least ▁one ▁operating ▁D ME ▁unit , ▁or ▁an ▁IF R ▁approved ▁G PS ▁system ▁( an ▁R NA V ▁system ▁meeting ▁T SO - C 1 2 9 / ▁- C 1 4 5 /- C 1 4 6 ), ▁to ▁begin ▁the ▁approach . ▁ ▁Appro ach ▁light ing ▁ ▁Some ▁install ations ▁include ▁medium - ▁or ▁high - int ens ity ▁approach ▁light ▁systems ▁( ab bre vi ated ▁A LS ). ▁Most ▁often , ▁these ▁are ▁at ▁larger ▁air ports ▁but ▁many ▁small ▁general ▁av iation ▁air ports ▁in ▁the ▁U . S . ▁have ▁approach ▁lights ▁to ▁support ▁their ▁I LS ▁install ations ▁and ▁obtain ▁low - visibility ▁minim ums . ▁The ▁A LS ▁ass ists ▁the ▁pilot ▁in ▁transition ing ▁from ▁instrument ▁to ▁visual ▁flight , ▁and ▁to ▁align ▁the ▁aircraft ▁vis ually ▁with ▁the ▁run way ▁center line . ▁Pil ot ▁observation ▁of ▁the ▁approach ▁light ing ▁system ▁at ▁the ▁Dec ision ▁Alt itude ▁allows ▁the ▁pilot ▁to ▁continue ▁desc ending ▁towards ▁the ▁run way , ▁even ▁if ▁the ▁run way ▁or ▁run way ▁lights ▁cannot ▁be ▁seen , ▁since ▁the ▁A LS ▁counts ▁as ▁run way ▁end ▁environment . ▁In ▁the ▁U . S ., ▁an ▁I LS ▁without ▁approach ▁lights ▁may ▁have ▁C AT ▁I ▁I LS ▁visibility ▁minim ums ▁as ▁low ▁as ▁ 3 / 4 ▁mile ▁( run way ▁visual ▁range ▁of ▁ 4 ,