Instrument approach charts list the equipment required to fly them in their name. For this example let's say we have an ILS or LOC/DME 36R approach. This approach requires an ILS receiver for the ILS portion, or both a LOC and DME receiver for the localizer only portion.
Let's also say we're flying an airplane that has an inoperative glideslope receiver and is otherwise /G equipped with an IFR-certified and up-to-date GPS. Can the GPS be used in lieu of DME in this case?
Yes, the details are in FAA AC 90-108:
7. Usage of Suitable RNAV Systems. Subject to the operating requirements in this AC,operators may use a suitable RNAV system in the following ways.
(1) Determine aircraft position relative to or distance from a VOR (see first note in subparagraph 7b), TACAN, NDB, compass locator (see second note in subparagraph 7b), DME fix; or a named fix defined by a VOR radial, TACAN course, NDB bearing, or compass locator bearing intersecting a VOR or Localizer (LOC) course
And all fixes, procedures etc. must be loaded from the GPS database, you can't enter any fixes yourself based on lat/long or whatever (section 9(c)(1)).
The short answer is, "Yes, GPS can substitute for DME".
The longer answer, splitting some very fine hairs, is that they are not exactly the same thing, and you should at least be vaguely aware of the differences.
DME literally measures distance, by measuring how long a signal takes to make the round-trip.
GPS only knows the position of the receiver and the position of the fix in the database, and just calculates the distance between. That means that GPS is using trigonometry and math to calculate the distance between two points on an idealized sphere, while DME is also accounting for slant-distance.
IF the location of the target fix is affected by slant-distance or by some peculiarity of terrain, there may be a tiny discrepancy between DME's measured actual distance and GPS's calculated distance. But in practice this difference is always going to be extremely small.
