After reading about Localiser Type Directional Aid (LDA) and Instrument Guidance System (IGS) on SKYbrary, my understanding of classifying these approaches is this:
|Course Offset||With GP||Without GP|
|up to 5°:||ILS approach||LOC approach|
|more than 5°:||IGS approach||LDA approach|
(In the US (TERPS), the limit is only 3° course offset, but ICAO Doc 8168 allows up to 5°)
However, the LOC R Rwy 26 approach at Innsbruck (LOWI) is classified as LOC, even though it has a GP (and a course offset of 3°). AFAIK, the approach chart does not indicate at all that a GP is available (correct me, if I'm wrong):
But when digging into the AIP, one finds the GP information:
The GP frequency of 331.7 MHz is paired to the LOC frequency of 111.1 MHz (example source). So this LOC approach has a paired GP. Then, why is it not classified as ILS?
I suspect that it might have something to do with the LOC antenna location, which is not on the runway centerline:
However, if this offset no longer satisfies the conditions for an ILS, then shouldn't the approach be classified as IGS instead (because it has a GP)?