Kefuddle..
That doesn't sound right. Which country are you talking about? Edit: I see you're in London.
For example, loading up an ILS approach that has the same flight path as the VOR/DME procedure when the VOR/DME approach is not available in the FMC. When using an overlay approach, the actual raw data is the primary reference and must be servicable.
Why would you fly an ILS on top of a VOR/DME and monitor the VOR/DME? Why not just fly the ILS?
This is not an overlay approach.
GPS overlay approaches are traditional VOR- or NDB-based approaches that have been approved to be flown using the guidance of IFR approach-certified GNSS avionics. Because of approach design criteria, LOC-based approaches cannot be overlaid.
That definition is from the Canadian documents. If you're ICAO you can read the same thing here:
http://www.icao.int/Meetings/PBN-Sym...s/9849_cons_en[1].pdf
See paragraph 2.2.6.
In some States, pilots are authorized to fly suitable VHF omnidirectional radio range (VOR), VOR/distance measuring equipment (DME), non-directional beacon (NDB) and NDB/DME non-precision approach (NPA) procedures using GPS guidance. These are termed “GPS overlay” approaches and allow operators to benefit from better accuracy and situational awareness without the need for the service provider to design a new approach.
Here's yet another definition of an overlay approach:
http://www.developpement-durable.gou...NSS__ENGv3.pdf
See paragraph 7.2.
B. Overlay approaches: publication example   (mainly found in the United States)
The primary navigation means is either the VOR DME or the GNSS.
To perform them with the RNAV/GNSS system, the operator must comply with the requirements of EASA AMC 20-27
Based on these definitions, it seems ICAO, Canada, the United States, and France all agree on what an overlay approach is. Where are you flying that this is not the case? RNP 0.3 is virtually identical to the specifications for GPS overlay approaches so I'm not surprised regarding Australia.
Elephant..
Do you have a reference for overlay approaches not being allowed in the UK? I'd be interested to read it.