DME only on LOC Approaches
 

Can anybody provide any insight as to why at least in Australia you can only use a DME for a LOC approach? I've heard that apparently you cannot use GNSS to reference the position of the ILS but can't find any reference or material to support this one way or another.

Edit: Should have been clearer, some approaches give you the option of referencing a VOR using the GNSS, but no approaches that I've found allow you to use the GNSS to reference the ILS itself e.g. putting IAV into your flight plan

Gotta legal references to support this?

Don’t believe it’s correct, to my knowledge GNSS can be used if it’s IFR approved and the correct waypoint is referenced with regards to navigation of the aircraft.

It's due to the fact that a LOC approach is a 2D NPA which uses a Minimum Descent Altitude that you can hold till the Missed Approach Point. On ILS LOC approches that are linked to the ILS DME then you must have DME to define this position. GNSS receivers don't have the waypoint of the ILS DME in their database. (comes about with the demise of most marker beacons in Australia, where generally the MM was the MAPT.

The only Navaids generally contained in the GPS database are VORs and NDBs, so unless the LOC/ILS approach references the distance to one of these types of aids you will need DME or an alternate approach. LOC/ILS approaches using VOR/DME are usually LOC/ILS-Y, whereas when they use the DME from the ILS they are usually LOC/ILS-Z.

Check out the ILS Y & ILS Z at AV, read the notes.

A quick look at three plates shows the note: GPS permitted in lieu of DME.

I should clarify no plate that I can find says that you can use the GPS in lieu of the DME when you are referencing distance from the ILS. Using the example of Avalon, you can reference the point AV but not the point IAV using GPS, and every other plate that I've looked at in Aus has the same thing.

Maybe because the DME associated wth ILS installations is not included in the Nav Database?

As has previously been noted, DME are not azimuth aids and so they are not available in a navigation database as a navigable point. On a typical NDB/DME or VOR/DME installation the DME is very close to the NAVAID. In this case there are provisions made that these locations are equivalent and so GNSS in permitted in lieu of DME provided you are using the aid as the reference waypoint.



For a typical ILS installation the DME is located at or near the GP antenna, which is a significant distance away from the localiser array. The centre point of the localiser array is what is stored as the reference point for an ILS system, and so if you selected to navigate by that point there would be significant difference between DME reading and nav system distance. You will note on these charts that GNSS is permitted in lieu of DME but there is no reference waypoint given. This is the case where the Final Approach Fix has been named and there is reference to the threshold position. This has been done to enable navigation on an ILS using a coded ILS approach, where you can use the distance in the coded waypoints as a DME equivalent.

I hope this explains it....

Alpha

This could be due to the fact there can be a significant distance between the ILS/LOC reference point and the “co-located” DME reference point (e.g. ICN at YBCS shown below).


https://cimg2.ibsrv.net/gimg/pprune....50bc3a3a5.jpeg

I don’t have access to a GNSS at the moment (and like a lot of others, not sure when I will again) but I don’t know if you input the waypoint ICN if it gives you the option to select between the ICN/LOC or the ICN/DME waypoints like it does when you type input something like CS and it gives choices of NDB, VOR.