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LPV fallback procedure if EGNOS not working

I don't have LPV capability currently (and frankly, for the UK especially, it is of close to zero operational relevance) but I have a yoke-mounted Garmin 496 which is supposed to pick up EGNOS.

However, much of the time, it doesn't seem to.

The antenna (the original Garmin one) is stuck on top of the instrument panel so it should have a very good view of the sky.

Unless there is something wrong with the installation, or with the unit (the firmware was last upgraded maybe 3 years ago), this doesn't sound too good for LPV availability.

And the G496's GPS is outstanding. It is way better than any other GPS I have ever used, miles better than the GPS in any handheld "computing device" (especially the Ipad2 whose GPS is marginal in the TB20), so I would think it should work well in this scenario.

The antenna wiring is done with a high quality 50 ohm coax and the GPS satellite signal levels in the config menu are showing as good.

Presumably pilots with the GNS "W" or GTN boxes can see if EGNOS is being received, even if they cannot fly any LPV approaches other than the Alderney one.

Any feedback on how often it works?

My understanding is that if EGNOS is not available, the unit falls back to a normal nonprecision approach, and then RAIM must be available prior to the FAF.

Would it ever just fall back to the "advisory glideslope" mode - this is the mode where you get the "continuous descent" glideslope profile published on the Jepp plates, which can also be autopilot coupled. Or does this mode disappear totally, at the same time as LPV disappears?

At which point is LPV inhibited? Presumably EGNOS is absolutely mandatory for every part of the "glideslope" so an outage means an instant go-around (as with an ILS GS failure), whereas with a normal GPS/RNAV approach you will get continued lateral guidance if RAIM was valid prior to the FAF even if RAIM fails after the FAF ... is that correct?

Administrator
Shoreham EGKA, United Kingdom

My GNS430W always has EGNOS and my GPSMAP695 (yoke mounted, internal antenna) always shows the EGNOS satellite in its sat overview page. I have never observed a loss of the EGNOS signal. However, I have had numerous GPS signal losses caused by my DME.

The iPad with external Bluetooth GPS on the glareshield is equally reliable. EGNOS is a strong signal from a geostationary satellite, it should not be difficult for a device to maintain reception.

Achima,

I have never seen a DME interfere with GPS. Many Com frequencies will, particularly in the vicinity of 121.2XX and 131.2XX. I have a KY197A and it loves to clobber the GPS in the 121 frequency range. I have been able to tame it so that I don't lose position or integrity. Many of the Bendix-King and Narco units generate harmonics in the GPS frequency range.

KUZA, United States

Peter,

Here in the US, with the current WAAS system, most of North America is covered by three Geo satellites with the WAAS signal. It takes all three of them to go down to lose the WAAS signal. There are at least two up link stations. The entire system has only gone down once when software was updated and that was for a few hours. Before the third GEO was added, the west most GEO was affected by a strong CME and the ground lost control over it. The transponder was still on and could not be switched off, so it kept being used to broadcast the WAAS signal. However, it slowly drifted east and crossed the paths of other GEO commercial satellites which had to be moved in their boxes to avoid the Zombie GEO. As they would get close, the GEOs would be shut down to avoid transmission interference and some services were temporarily lost from time to time. The drift eastward continued for more than 6 months. Finally the Zombie satellite lost orientation to the sun and powered itself down and restarted. When it finished the restart, ground control was able to reestablish control, updated its software so it couldn't happen again, and over a several month period moved it back to its station. Even with all this going on, the WAAS system continued to work on a single GEO without interruption.

As long as at least one GEO satellite is providing the SBAS data, RAIM is not required to be used. We need to have a WAAS Unavailable NOTAM in effect to end up defaulting back to RAIM and the standard GPS service, in which case only LNAV minimums are available.

The GEO satellite WAAS signal is correction and integrity data and not a ranging signal, which means that it can't itself be used as if it were an additional GPS viewable in the sky. However, two of the GEOs have stable enough orbits to permit them to also used as if they are an additional GPS with sufficient accuracy to be included in the WAAS precision approach (AKA LPV) service. The third GEO flys a "8" station orbital pattern and can only be used to provide a GPS NPA signal.

KUZA, United States

With the EGNOS system operating, it provides the WAAS GPS with the integrity. As long as this is true, regardless what the integrity data indicates, RAIM is not used. In order to provide LPV service, the lateral integrity must be less than 40 meters (HPL) and the vertical integrity (VPL) must be at or below 50 meters or 35 meters, with the lower value required if the DH is below 250 feet. The same value of 50 meters is required for LNAV/VNAV use or when advisory vertical guidance is provided on a LNAV procedure. Although both criteria must be met for vertical guidance, HPL is almost always lower than VPL. This means that VPL will fail first. The fail down mode is always to LNAV without any vertical guidance. The HPL/VPL are checked at one minute from the FAF. If below tolerance, the approach fails down to LNAV. Here in the CONUS, this is a very rare occurrence and rarely lasts for more than a few minutes. In my flying, I have never encountered it during the last 6 years. I suspect that you will experience more outages in the UK because of geography as you only have Ireland to the west of you. Most of our outages are on the extremes of the coverage area as there are few places to locate a ground sensor site in the Pacific to the west of California and in the gulf of Mexico.

KUZA, United States

HPL and VPL are not directly displayed on most WAAS GPS units, but you can display the HFOM and VFOM values on the GNS430W/530W, G1000 and the GTN navigators. These are figure of merit values and are the two sigma position uncertainty whereas the HPL/VPL values are 5 sigma values. You can relate the two most of the time. A VPL of 50 meters will display as a VFOM value of approximately 60 feet and a VPL of 35 meters displays as a VFOM of 40 feet. Here in the CONUS, most of the time VFOM is in the low 20's of feet. To those who have WASS GPS units, what values of VFOM do you typically see?

KUZA, United States

I see nobody answered this question, from years ago. It would be interesting to see what people get in Europe.

Administrator
Shoreham EGKA, United Kingdom

Peter,

I don’t know what you are looking for, but EGNOS or any SBAS is only required for LPV or LP procedures. Fall back from LPV service is only to LNAV service without vertical guidance including +V. Fall back occurs at 1 minute before the PFAF if it is going to occur. If EGNOS is not working, LPV is unavailable, but the standard GPS service is unaffected by EGNOS. An EGNOS failure is highly unlikely as there are redundancies. The US WAAS system has only experienced 4 hours of failure in a single incident since 2003, so that is pretty good reliability. There was also an incident of one of the WAAS satellites going rogue which eliminated redundancy and service to parts of Alaska. The rogue satellite still provided WAAS but it no longer maintained station and it drifted eastward without any control from the ground. It finally lost the ability to keep the solar panels pointed at the sun and reset itself. After the reset, the ground was able to reestablish control and eventually move it back west to where it belonged, but not before a new WAAS satellite was launched. There are now three WAAS satellites and most of the CONUS is in the footprint of all three. Only one needs to be working to get LPV service.

KUZA, United States

Don’t you just fly the RNAV or ILS or VOR or NDB approach instead?

EGNOS not available is a different issue than the service level that EGNOS is providing does not meet a specific level of service such as LPV, LPV200, LNAV/VNAV, or LNAV. If EGNOS is in service and being received, then the integrity it provides determines the level of service, good, bad or ugly. When EGNOS is not being received, then the standard GPS RAIM determines the integrity.

KUZA, United States
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