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Do RNAV LPV approaches use GPS or Baro Alt? (and temperature compensation on approaches)

My thanks to you all for your comments. It seems clear that the vertical guidance uses GPS-derived Alt. I don’t know whether this applies to the source for the audio annunciation “minimumums”, however I will write to the training instructor.
kind regards
Bob

Bob
Flying since 1985, but still lots to ...
EGKA, United Kingdom

LPV – GPS based lateral and vertical navigation (glideslope), uses SBAS (WAAS in the US, EGNOS in Europe)

Flyamax wrote:

@Bob tells us that the GTN750 has this and you need to know how it is handled. Which one it uses on an approach: GPS-related or baro-VNAV since baro-VNAV stays temperature limited and RAIM has to be checked.?

What you write is incorrect. A GSN430W, GNS530W, GTN650 or GTN750 regardless if it has a barometric altitude input connected does not do baro-VNAV.

Flyamax wrote:

LNAV/VNAV approaches give lateral guidance only.

Incorrect. Approaces that have an LNAV/VNAV are designed with a glideslope. LNAV/VNAV are “originally” designed to have lateral navigation based on GPS and vertical navigation based on barometric altitude information. BUT. It is possible to generate the glideslope based on GPS altitude (GPS with SBAS). This mean that in some cases a GSN430W, GNS530W, GTN650 or GTN750 navigator will provide a glideslope when flying an approach that has LNAV/VNAV mimima (but not any LPV minima). The criteria for when that will happen is complicated.

ESTL

Bob wrote:

I don’t know whether this applies to the source for the audio annunciation “minimumums”, however I will write to the training instructor.

All minima are baro other than Cat II or III ILS which use RADALT. This does mean in cold temperature operations you should temperature adjust the minima even if flying on a GPS glidepath.

EGTK Oxford

All minima are baro other than Cat II or III ILS which use RADALT.

On the CJ4 Proline you can choose the “minima” annunciation to come from baro or RADALT.

Perhaps anybody may correct me if this is wrong: baro-aiding is used as an integrity augmentation which gives the possibility to use a GPS independant source to give guidance. Thus, it requires only 4 stallites + the static source to detect any anomaly. I reckon that this is the reason why the GPS uses primarily a baro-VNAV are used. I heard somewhere that LNAV/VNAV approaches existed before WAAS and at this time you needed a certified baro-VNAV system to fly those apparoaches to the minimums.

This is what the pre-W IFR certified GPSs have. It is slightly bizzare because as soon as there are 4 or more sats received, the baro data is disregarded, IIRC. So it looks like a lot of effort (wiring up the 11 gray code wires to the GPS, etc) for almost no gain because some 99.9% of the time you are getting way more than 4 satellites. I have never understood the reasoning behind this feature.

Administrator
Shoreham EGKA, United Kingdom

JasonC wrote:

All minima are baro other than Cat II or III ILS which use RADALT.

Let’s say: They should be baro. However, the minimum call of the aircraft I fly will always be derived from radar altitude (when in range: 0…2500ft) which can be misleading. Therefore we are not supposed to use it.

EDDS - Stuttgart

Peter wrote:

It is slightly bizzare because as soon as there are 4 or more sats received, the baro data is disregarded,

A GPS 3D fix requires 4 satellites.
RAIM Fault Detection (FD) requires 5 satellites (or 4 if the GNSS receiver is baro aided)
Some GNSS receiver can provide RAIM with Fault Detection and Exclusion (FDE). It requires 6 satellites (or 5 if the GNSS receiver is baro aided)

So basically the introduction of pressure altitude to the GPS means it needs one less satellite.

That may be what the instructor Bob refers to may have been talking about, though obviously the instructor may not have understood it.

Administrator
Shoreham EGKA, United Kingdom

Anders wrote:

LNAV/VNAV approaches give lateral guidance only.

Incorrect.

You are absolutely right. Le phrase should have been LNAV aporaches give lateral guidance only.

Anders wrote:

…GTN750 regardless if it has a barometric altitude input connected does not do baro-VNAV.

Ok, then I missed something here. I know that my GNS430W does not do baro-VNAV. So I never cared about the concept as yet for myself. But what Bob’s instructor is trying to tell him then? I learned that LPV can provide WAAS vertical guidance to 200 ft. AGL (no baro mentioned here).

Last Edited by Flyamax at 02 Dec 11:54
France

Surely baro-aiding is not relevant to a GNSS system using SBAS?

Flyamax wrote:

Ok, then I missed something here. I know that my GNS430W does not do baro-VNAV. So I never cared about the concept as yet for myself. But what Bob’s instructor is trying to tell him then?

Bob’s instructor is wrong. The GTN750 can’t provide any vertical guidance (in the sense of a glidepath) without SBAS. Baro has nothing to do with it.

bookworm wrote:

Surely baro-aiding is not relevant to a GNSS system using SBAS?

No, it is not.

EGTK Oxford
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