Many airliners use DME-DME RNAV (or DME-DME-IRU RNAV). A NOTAM about a DME station being “on test, do not use” made me think. How does the RNAV system know that a DME station is on test? Is the crew supposed to enter the IDs of all unsuitable DME stations before flight…? That could be quite a chore.
The same issue of course if a DME station is NOTAMed as unreliable.
I always thought that the system that identifies a DME listens to the ID audio and if it is TST, then it is unusable..
01
Many airliners use DME-DME RNAV (or DME-DME-IRU RNAV).
I have no idea, but do they really? At least as a required element for their enroute navigation performance? I mean still, in 2021? Or is it just something being told from one pilot generation to the other?
I guess “being on test” means they want to limit the number of users? DME receivers usually can handle about 100 aircraft interrogations but no more than that, when you hit the max stack it will start swapping them to keep up with interrogations (just like max secondary radar targets) that may cause it to ignore a calibration flight in progress or interrupt their data collection? for a normal flights, when you get kicked out of “DME queue” all it takes is ON/OFF to kick someone else 
I have no idea how various DME-DME RNAV cope with DME saturation or reject? but if they have any extra means of RNP (3rd DME or IRU/GPS fix), they should cope very well with that NOTAM, I doubt there are airlines or heavy metal flying these days without proper RNP1 maybe few Hercules? or Antonovs? or Islanders? that no one cares about 
Yes, DME-DME is used for fixing up INS errors.
Modern airliners, even if they cannot fly GPS approaches, have GPS too for this purpose, and there is some order of preference (don’t know the details; maybe an airline pilot can chip in).
The system obviously contains a database of DMEs.
I would speculate that the system does the morse code ident, which is very easy to do in software.
Concorde, according to one pilot I spoke to in the cockpit, after one of the last flights (in 2003), used a single DME, which is (perhaps surprisingly) possible if the object is moving. Mentioned here.
boscomantico wrote:
I have no idea, but do they really? At least as a required element for their enroute navigation performance? I mean still, in 2021? Or is it just something being told from one pilot generation to the other?
Sweden has more than a dozen DME stations for no other purpose than DME-DME RNAV. I doubt they would keep them around if they weren’t actually used.
A DME-DME sensor is available for most air carrier aircraft for enroute navigation over land. INS handles short duration gaps. DME-DME is available as a backup to GPS and in the US can be approved for RNAV 1 and RNAV 2 operations. The FAA is reducing the number of VOR systems by about 1/3, but the collocated DME for the most part are being retained to provide adequate coverage for DME-DME.
The DME- DME fixing on a modern airliner is a totally automatic function of the FMC , the DME units are auto tuned to get the best geometry, off the top of my head I think the B738 could look at 8 DME stations simultaneously, just like GPS DME-DME is used to back up the IRS.
Most companies mandate turning off the FMC DME position update if a GPS approach is being performed.
A_and_C wrote:
The DME- DME fixing on a modern airliner is a totally automatic function of the FMC , the DME units are auto tuned to get the best geometry, off the top of my head I think the B738 could look at 8 DME stations simultaneously, just like GPS DME-DME is used to back up the IRS.
That was my point. What happens if a DME station is NOTAMed as unreliable or “do not use”. Is the assumption that the FMC will be able to exclude that station automatically if it’s reading doesn’t tally with the other DME distances? Or do you simply accept that a bad reading will be sufficiently “diluted” by the good ones?
