Editing Instrument landing system (section)

===DME substitution===
{{Main article|Distance measuring equipment}}
[[Distance measuring equipment]] (DME) provides pilots with a [[slant range]] measurement of distance to the runway. DMEs are augmenting or replacing markers in many installations. The DME provides more accurate and continuous monitoring of correct progress on the ILS glide slope to the pilot, and does not require an installation outside the airport boundary. When used in conjunction with a dual runway approach ILS, the DME is often sited midway between the reciprocal runway thresholds with the internal [[distance measuring equipment#Timing|delay]] modified so that one unit can provide distance information to either runway threshold. For approaches where a DME is specified in lieu of marker beacons, ''DME required'' is noted on the instrument approach procedure and the aircraft must have at least one operating DME unit, or an IFR-approved system using a GNSS (an [[RNAV]] system meeting TSO-C129/ -C145/-C146),<ref>{{cite web |url=https://www.faa.gov/documentLibrary/media/Advisory_Circular/90-108.pdf |title=AC90-108 |access-date=2020-10-27 |url-status=live |archive-url=https://web.archive.org/web/20170211075542/http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/248edc4663973b148625785500458012/$FILE/90-108_Chg1_incorporated.pdf |archive-date=2017-02-11 }}</ref> to begin the approach.