Modern DME UNITS
Folks, pretty much all modern DME units are multiple channels ( typically 3 ). This means that the unit tunes and locks to 3 DME stations at the same time. Typically, channel 1 is paired with the onside VHF nav, while channel 2 and 3 are tunned automatically by the onside FMS. Modern FMSs will tune the nearest 2 DME stations based on entered, last know position, or GNSS position if received , so the FMS can come up with an Rnav solution. The geometry of the tuned stations is taken into account ( i.e elevation vs height of airplane.....often it's not used on ground). This Rnav solution depending on the maker of the FMS, will either be blent with other nav sources with each one having its own weight ( aka importance ), or just kept as a back up when GNSS quits. All Rockwell collins FMS work this way, the older ones would blend nav sensors, while the newer ones will use a priority based selection skeme, with GNSS first, DME/DME second, VOR/DME third, and IRS last. Those FMS (Rockwell )that are designed to shoot RNP AR approaches, will jump from GNSS to IRS directly, and will not use DME/DME or VOR/DME for position. Consult the Faa Advisory circular for RNP AR, and it's all in there. Some avionics might use DME/DME for backup during an RNP-AR, but not Vor/ Dme.
Important Note : even before the advent of RNP AR ops, with FMS that would blend several nav solutions ( GPS, Dme/Dme, Vor/ Dme, and IRS), VOR//Dme updating needed to be turned off to comply with US RNAV dep and arrivals procedures.
Hope that helps
Last edited by vickers vanguard; 9th Jan 2018 at 03:27.