What is the future of DME?
Answered By AOPA
Although DME-required approaches still exist, they are increasingly rare.  Avionics vendors have quit selling new DME transponders in some cases.  At what point in time should a Part 91, mostly-VFR certificated aircraft owner consider deleting DME from the avionics suite?
4 Replies
AOPA Staff Answer
There is a comprehensive description of the FAA's NEXGEN strategy covering the 2016-2030 time period available at https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/transition_programs/nextgen_dme/ . With respect to DME, the strategy document states an objective to maintain safety and an acceptable level of service when a system fails or facility is degraded, and to prevent or mitigate impact to air traffic operations. As such, the FAA will retain and expand the Distance Measuring Equipment (DME) infrastructure as necessary to support continued PBN operations in the event of GNSS service disruptions. Non-PBN operations will be supported through a rationalized network of other ground-based navigation equipment to ensure safety for all NAS users.
Specific requirements still exist for DME for certain terminal procedures and although GPS is an approved alternative, it seems the FAA is not yet committed to eliminating them. As stated above this is due to potential GPS system failures and probably the recognition that there are still several aircraft that continue to rely on the DME system.
That being said, in today's environment, Part 91, mostly-VFR certificated aircraft operators could probably survive without their ground based DME equipment if they have a mostly reliable GPS source on-board. 
FAA is retaining many DMEs to support DME-DME RNAV, which is a feature of the flight management systems on many business jets and transport aircraft. See, for example, the FAA Strategic Plan, FY2019-2022, which notes:
FAA has established the following multi-year strategies to facilitate expanded infrastructure development, modernization, and construction:

• Install NextGen Distance Measuring Equipment (DME), in addition to Very High-Frequency Omni-directional Range (VOR) Minimum Operational Network (MON), to enhance infrastructure resiliency of Performance-Based Navigation (PBN) operations during Global Positioning System (GPS) outages.

The FAA Navigation Programs Strategy also addresses DME:
5.4.1 NextGen DME Program
The overall objective for the NextGen DME Program is for DME/DME to provide a backup to enable aircraft to continue PBN operations during a GNSS disruption. Most commercial aircraft are equipped with GNSS and DME/DME avionics that can be used to continue flying RNAV procedures to their destinations and to an ILS or VOR approach (during a GNSS disruption). Under the Figure 11: Non Directional Beacon (NDB) NextGen DME Program, the HPDME network will be enhanced to fill coverage gaps and eliminate single-point failures....
(See, e.g., this PDF: https://www.gps.gov/cgsic/meetings/2019/lawrence.pdf)

You can use an IFR-approved GNSS (i.e., GPS-based "suitable RNAV system" as described in the AIM and other references) to substitute for DME.

For example, the January 30, 2020 udpate to the AIM includes a revision that clarifies that you can use GPS to substitute for marker beacons associated with ILS approaches (see AIM 1−1−9 and 1−1−10).

FAA is also charting stand-alone DMEs to support RNAV. See this item at my blog: https://bruceair.wordpress.com/2017/02/14/stand-alone-dmes-on-charts/.

Bruce Williams:
You can use an IFR-approved GNSS (i.e., GPS-based "suitable RNAV system" as described in the AIM and other references) to substitute for DME.

...with one exception -- you cannot use GPS as a sub for DME for lateral guidance on the final segment of an instrument approach procedure, such as is done on the KMTN VOR/DME or TACAN Z RWY 15 and KWAL VOR/DME or TACAN RWY 10 approaches.  Fortunately, those are, AFAIK, the only two such approaches in the USA using DME for lateral guidance on the final segment, so it's not a big deal in reality, but nice to know if you're on Jeopardy and have to take Instrument Approach Procedures for $500.

When our flying club upgraded the avionics of our C-177B for ADS-B about a year ago, we deleted the DME receiver along with the ADF (the display was inop anyway) and haven't missed it since. Here's the new panel: