▁ensure ▁the ▁aircraft ▁is ▁following ▁the ▁gli de ▁path ▁of ▁approximately ▁ 3 ° ▁above ▁horizontal ▁( ground ▁level ) ▁to ▁remain ▁above ▁ob stru ctions ▁and ▁reach ▁the ▁run way ▁at ▁the ▁proper ▁touch down ▁point ▁( i . e . ▁it ▁provides ▁vertical ▁guidance ). ▁ ▁Lim itations ▁Due ▁to ▁the ▁complexity ▁of ▁I LS ▁local izer ▁and ▁gli de ▁slope ▁systems , ▁there ▁are ▁some ▁limitations . ▁Local izer ▁systems ▁are ▁sensitive ▁to ▁ob stru ctions ▁in ▁the ▁signal ▁broadcast ▁area , ▁such ▁as ▁large ▁buildings ▁or ▁hang ars . ▁G lide ▁slope ▁systems ▁are ▁also ▁limited ▁by ▁the ▁terrain ▁in ▁front ▁of ▁the ▁gli de ▁slope ▁anten nas . ▁If ▁terrain ▁is ▁s lop ing ▁or ▁une ven , ▁ref lections ▁can ▁create ▁an ▁une ven ▁gli de path , ▁causing ▁unw anted ▁need le ▁def lections . ▁Additionally , ▁since ▁the ▁I LS ▁signals ▁are ▁pointed ▁in ▁one ▁direction ▁by ▁the ▁position ing ▁of ▁the ▁arrays , ▁gli de ▁slope ▁supports ▁only ▁straight - line ▁approaches ▁with ▁a ▁constant ▁angle ▁of ▁descent . ▁Install ation ▁of ▁an ▁I LS ▁can ▁be ▁cost ly ▁because ▁of ▁sit ing ▁criteria ▁and ▁the ▁complexity ▁of ▁the ▁anten na ▁system . ▁ ▁I LS ▁critical ▁areas ▁and ▁I LS ▁sensitive ▁areas ▁are ▁established ▁to ▁avoid ▁ha z ard ous ▁ref lections ▁that ▁would ▁affect ▁the ▁radi ated ▁signal . ▁The ▁location ▁of ▁these ▁critical ▁areas ▁can ▁prevent ▁aircraft ▁from ▁using ▁certain ▁tax i ways ▁leading ▁to ▁del ays ▁in ▁take offs ,