▁beyond ▁strict ▁limits ▁is ▁detected , ▁either ▁the ▁I LS ▁is ▁automatically ▁switched ▁off ▁or ▁the ▁navigation ▁and ▁identification ▁components ▁are ▁removed ▁from ▁the ▁car rier . ▁Either ▁of ▁these ▁actions ▁will ▁activ ate ▁an ▁indic ation ▁(' fail ure ▁flag ') ▁on ▁the ▁instruments ▁of ▁an ▁aircraft ▁using ▁the ▁I LS . ▁ ▁Local izer ▁back ▁course ▁Modern ▁local izer ▁anten nas ▁are ▁highly ▁direction al . ▁However , ▁usage ▁of ▁older , ▁less ▁direction al ▁anten nas ▁allows ▁a ▁run way ▁to ▁have ▁a ▁non - prec ision ▁approach ▁called ▁a ▁local izer ▁back ▁course . ▁This ▁lets ▁aircraft ▁land ▁using ▁the ▁signal ▁transm itted ▁from ▁the ▁back ▁of ▁the ▁local izer ▁array . ▁High ly ▁direction al ▁anten nas ▁do ▁not ▁provide ▁a ▁sufficient ▁signal ▁to ▁support ▁a ▁back ▁course . ▁In ▁the ▁United ▁States , ▁back ▁course ▁approaches ▁are ▁typically ▁associated ▁with ▁Category ▁I ▁systems ▁at ▁smaller ▁air ports ▁that ▁do ▁not ▁have ▁an ▁I LS ▁on ▁both ▁ends ▁of ▁the ▁primary ▁run way . ▁Pil ots ▁flying ▁a ▁back ▁course ▁should ▁dis reg ard ▁any ▁gli de ▁slope ▁indic ation . ▁ ▁Mark er ▁be a cons ▁ ▁On ▁some ▁install ations , ▁marker ▁be a cons ▁operating ▁at ▁a ▁car rier ▁frequency ▁of ▁ 7 5   M Hz ▁are ▁provided . ▁When ▁the ▁transmission ▁from ▁a ▁marker ▁be acon ▁is ▁received ▁it ▁activ ates ▁an ▁indicator ▁on ▁the ▁pilot ' s ▁instrument ▁panel ▁and ▁the ▁tone ▁of ▁the ▁be acon ▁is ▁aud ible ▁to ▁the ▁pilot