Con-pilot -
DME/DME is more accurate than GPS, really? That's a bit of a surprise, what type and brand of FMS manual stated that? I'll admit it has been five years since I have retired, but it was my understanding that the GPS was the most accurate navigation system one could have.
The equipment was a Universal UNS-1A, with inputs from laser inertial gyro, GPS, and multi scanning DME/DME/DME. To quote from the manual
The FMS's unique auto-scanning DME-DME-DME positioning
function uses the internal navigation data base to continually map
the stations surrounding the aircraft (VORTACs, DMEs, ILS-DMEs,
LOC-DMEs, TACANs). The FMS then automatically tunes and
reads these, one every four seconds, utilizing the blind channel of an
external digital DME or Universal's Radio Reference Sensor (VOR,
DME, TACAN). Range information to all responding DMEs within
approximately 250 nautical miles of the FMS position are corrected
for slant range error and are then compared with those computed
from the nav database to verify reasonableness and to ensure DME
position integrity is maintained.
Flights, which never leave areas of multiple DME coverage, can
expect exceptional position accuracies.
The manual makes it clear that the multiple DME position solution is considered the most accurate, when receiving sufficient DME signals. It seems to make sense to me, as DME and GPS are both calculating position by timing the radio signal, but with GPS, both transmitter and receiver are moving in space, whereas with DME one half of the equipment is in a fixed accurately known position