I ran into a weird issue today with our G1000 WAAS-equipped aircraft. The aircraft wanted me to begin decent .9nm before the FAF on the KDVT 25L LPV approach. The LPV is considered a non-precision approach (Edit: it is not an NP approach, it is a APV which would have you follow GS when intercepted at 4700), so you should fly at 4700 feet to FEGEV and then go capture the glide slope, so this has the potential to screw up a student or anyone for that matter by following the glide slope almost a mile early and getting a potential Pilot Deviation.
I asked ATC what they showed our altitude at and was correct all three times.
I could not find the proper FAA source quickly, but Honeywell had this to say:
An LPV glidepath is calculated entirely using SBAS altitude. That is, the GPS constructs the glidepath using GPS altitude and height above earth ellipsoid to produce a path that unlike baro-VNAV, is not susceptible to pressure and temperature variations. This means that like an ILS, an LPV glidepath is essentially a fixed path in space being tracked using GPS altitude to a DA that the pilot reads from a baro-altimeter. This is a component to how an LPV can achieve minimums comparable to an ILS.
So if they are both fixed paths in space, why is one of them having me descend so early?
I didn't know if anyone else had run into this issue before. Is it due to a faulty GPS receiver? (both GPS 1 and 2 were showing the same)
Any insights are appreciated.
(Edit: Concern is how early it is having us capture the GS where someone on the LNAV for the same approach would be at 4700 feet for almost a mile after we begin descending on the LPV)
On ground GPS signal and altimeter altitude match:
KDVT 25L LPV approach on glide .9nm ahead of FAF:
KCGZ ILS is right on (.1nm off) at correct altitude for FAF:
Approach plate: