The primary precision approaches are ILS, GLS, and MLS. Approaches other than these require special authorizations, and include TLS (Transponder Landing System), SCAT-I DGPS, and special procedures.
AIM 5-4-5 breaks down approaches as Precision, Approach with Vertical Guidance (APV), and Non Precision.
7. TERPS criteria are provided for the following types of instrument approach procedures: (a) Precision Approach (PA). An instrument approach based on a navigation system that provides course and glidepath deviation information meeting the precision standards of ICAO Annex 10. For example, PAR, ILS, and GLS are precision approaches. (b) Approach with Vertical Guidance (APV). An instrument approach based on a navigation system that is not required to meet the precision approach standards of ICAO Annex 10 but provides course and glidepath deviation information. For example, Baro-VNAV, LDA with glidepath, LNAV/VNAV and LPV are APV approaches. (c) Nonprecision Approach (NPA). An instrument approach based on a navigation system which provides course deviation information, but no glidepath deviation information. For example, VOR, NDB and LNAV. As noted in subparagraph i, Vertical Descent Angle (VDA) on Nonprecision Approaches, some approach procedures may provide a Vertical Descent Angle as an aid in flying a stabilized approach, without requiring its use in order to fly the procedure. This does not make the approach an APV procedure, since it must still be flown to an MDA and has not been evaluated with a glidepath.
I have no problem accepting the fact that GPS (LPV) is not classified as a precision approach. Actually other than criteria for selecting an alternate it has little effect. I am a little puzzled about GLS however. My understanding is that the acronym means Global Navigation Satellite System Landing System. The AIM indicates that this is a precision approach dependant on WAAS and LAAS. Are there any in existence in the US, in the World? Another question, is this the approach of the future, and just exactly when will these LAAS installations be started, completed?