See: > The FAA's network of Distance Measuring Equipment (DME) NAVAIDs will provide a PBN-capable† backup to GPS; however, for aircraft without scanning DME re

See: > The FAA's network of Distance Measuring Equipment (DME) NAVAIDs will provide a PBN-capable† backup to GPS; however, for aircraft without scanning DME receivers (DD) or DD with Inertial Reference Unit aiding (DDI) equipment, the FAA will provide a conventional navigation backup service based on the VOR MON. The VOR MON is designed to enable aircraft, having lost GPS service, to revert to conventional navigation procedures. * https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gbng/vormon/ https://www.faa.gov/about/office_org/headquarters_offices/at... * https://www.federalregister.gov/documents/2016/07/26/2016-17579/provision-of-navigation-services-for-the-next-generation-air-transportation-system-nextgen https://www.federalregister.gov/documents/2016/07/26/2016-17... † Performance-Based Navigation