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 an ILS, the DME is often sited midway between the reciprocal runway thresholds with the internal 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 GNSS system (an RNAV system meeting TSO-C129/ -C145/-C146),[11] to begin the approach.