Peter wrote:
Baro input is normally installed on IFR GPSs, in the form of the 10 gray code wires being brought in (not the static pipe itself). With RAIM, it allows a RAIM check to pass with 1 less satellite. Lots of old threads here.
But as denopa said the GPS doesn’t know the QNH so this would not be useful. In GA, the QNH knob of an encoding altimeter is a 10 turn pot which outputs a voltage; no IFR GPS I know of has an input for that, and it would have no use for it. OTOH, how exactly is BARO-VNAV implemented unless the box knows the QNH?
In the Proline 21 “visual” approach it needs you to have set QNH correctly, so it definitely uses that info.
That is quite an integrated system though.
Which GA navigators (GTN/IFD) do BARO-VNAV and offer +V or LPV? I can look in the IM to see if there is a connection for the QNH pot.
If the GTN is installed with a G500 there is an ethernet connection between the boxes. The protocol is proprietary and I don’t know what the devices talk about but it would be trivial to provide this information to the GTN, of course.
First of all, advisory vertical guidance is only “A” means of descending from the FAF to the MDA(H). It is not official vertical guidance and is not required to fly the procedure, therefore does not need to be that accurate, as almost any means of descending from the FAF to the MDA is OK, including the Dive and Drive method.
Advisory Vertical is optional in the TSO and as it had been originally implemented, it required the vertical VPL to be <= 50 meters, the same as used for a vertically guided approach with LNAV/VNAV minimums or LPV with a DH 250 feet or greater. Then Avidyne added support for vertical guidance for their GPS systems for use outside of the SBAS service volume. So in places like Hawaii where the VPL does not support >=50 meters and it is outside the service volume for vertically guided approaches, they now enjoy the advisory vertical guidance function on LNAV procedures. The argument was “who cares”, it is only advisory, not official. Garmin then added the same capability in software version 6.41, see this item listed in the software update notification: “Added support for advisory vertical guidance on LNAV approaches in regions without SBAS coverage.” Pilots are not supposed to use advisory vertical guidance below the MDA and those that do are at risk of being removed from the gene pool anyway.
Timothy wrote:
Well, we know it works, right? And we know that GPS without SBAS is barely up to the job (which is why 129 receivers don’t do glidepaths) so there must be an algorithm in there. I’ll try to investigate.
I don’t agree, at least with respect to advisory vertical guidance (only valid to be used for descent to an MDA) or a visual approach guidance (which is not intended to be used in less than visual conditions). There is no need for baro corrected altitude for a visual maneuver or for descent to the MDA using advisory vertical guidance,
I’ve had a look through a GTN750 IM and cannot see anything referring to it accepting a baro setting etc.
It may have that capability over ethernet or ARINC429 but there aren’t many encoding altimeters which have ethernet or ARINC429, especially in the GA sphere.
As NCyankee shows, non-SBAS GPS altitude is accurate enough for an advisory descent. Even my old KLN94 is within 10-20ft on the altitude most of the time.
Do we have a definitive view on what happens to LPV and L/VNAV approaches if SBAS is switched off or absent? Do they annunciate as LNAV or LNAV+V and do they have a GP?
Timothy wrote:
Do we have a definitive view on what happens to LPV and L/VNAV approaches if SBAS is switched off or absent? Do they annunciate as LNAV or LNAV+V and do they have a GP?
Next time I fly, I will let you know, but my expectation is that the annunciation will be LNAV+V and there will be an Advisory GP.
It may have that capability over ethernet or ARINC429 but there aren’t many encoding altimeters which have ethernet or ARINC429, especially in the GA sphere.
As the G5 becomes more and more popular this changes. The G5 provides baro corrected altitude via the GAD29 to the GTNs.
