Replacing GNS530/430W to take advantage of VNAV in my Jetprop

LPV, LNAV/VNAV, and LNAV+V all use a VAL of 50 meters, so if the vertical flags or there is a downgrade, vertical guidance is gone.

I thought +V doesn’t need SBAS, so how likely is this situation in reality?

RobertL18C wrote:

Doesn’t IFR GPS suppress LOI messages, and may even use internal deduced reckoning, once you are past the FAF? Or is this only on LNAV?

I don’t think so. If you lose lateral integrity after the FAF, you need to abandon the approach.

Peter wrote:

But you can re-engage the VNAV to +V, surely? Might have to go missed if it happens on the glideslope.

LPV, LNAV/VNAV, and LNAV+V all use a VAL of 50 meters, so if the vertical flags or there is a downgrade, vertical guidance is gone. LPV200 has a VAL of 35 meters, but there is only downgrade to LNAV which occurs before the FAF. After the FAF, if you exceed VAL, the vertical flag will be displayed or the GP will be removed. If you exceed HAL, the approach must be abandoned.

Peter wrote:

But you can re-engage the VNAV to +V, surely? Might have to go missed if it happens on the glideslope.

It’s no different than getting “GS flag” on ILS on final, you go for LOC minima or go missed?
However, unlike ILS, it won’t re-engage back to L/VNAV or +V if the signal is back to VPL specs, it just LNAV

I guess it depends on how one flies the glide slope and weather, in stable air you can go with ROD(ground speed) for slope by hand or autopilot down to LNAV MDH? in turbulence maybe with tiny bit of chasing with stick and autopilot or go-missed? so the answer probably depends on conditions

I am not sure if what is the individual AP behavior though when ILS goes LOC or LPV goes LNAV? but one can always revert to HDG/VS when they don’t like it, at least you know what is going on and have some control…

For LPV, it downgrades straight to LNAV and skips L/VNAV and LNAV+V

But you can re-engage the VNAV to +V, surely? Might have to go missed if it happens on the glideslope.

eal wrote:

Presumably if the VPL exceeded 50 meters, then vertical guidance would be withdrawn, and the approach downgraded to lateral LNAV only?

For LPV, it downgrades straight to LNAV and skips L/VNAV and LNAV+V
For L/VNAV, it downgrades straight to LNAV and skips LNAV+V

You only need to stay within HPL specs to get LNAV which likely to relate to “quality of core signal without SBAS component”
I doubt you it ever exceeds HPL> 556 meters (RNP 0.3) if no RAIM is predicted at the time of the arrival but if it does it’s LOI

Doesn’t IFR GPS suppress LOI messages, and may even use internal deduced reckoning, once you are past the FAF? Or is this only on LNAV?

From Google: “GAGAN has two Indigenous Geostationary satellites broadcasting the SBAS signals. The satellites are identified as GSAT-8 with PRN 127 and GSAT-10 with PRN 128. Another GEO satellite GSAT-15 (PRN 132)is in orbit and integration with the satellite is in under progress.”

So from your screenshot, it appears you are receiving the GAGAN satellites. More ground stations provide overall better correction, probably mostly for the ionospheric corrections, but are not required to be everywhere in the service volume. The US WAAS system provides for LPV 200 outside of the area covered by ground stations. A VPL of 43 meters (128 feet) is not far from the limit of 50 meters that is required for LNAV/VNAV, but still within tolerance for LNAV/VNAV or even LPV other than LPV 200.