GBAS

Ibra wrote:

Ignoring GBAS as I have no clue how it works on SBAS GPS when flying LPV, RAIM is not required as long as SBAS is available, the final segment is coded to have continuous self monitoring to keep “LPV annunciation”:
- It can degrade to LNAV before 1000ft or FAF, then you fly LNAV MDA
- It can degrade to LNAV after 1000ft or FAF, then you fly MAP on RNAV1
- It can degrade to LOI near DA, then you fly MAP on DR

The SBAS systems alert the pilot 1 minute before the FAF that the approach is being downgraded to LNAV. You can always fly to the LNAV minimum if the GPS is annunciating LPV or LNAV, but only to the LNAV minimums if LNAV is annunciated. So a downgrade to LNAV occurs before crossing the FAF and any LOI after that point would require flying the MAP procedure or using heading to climb and switch to other navigation means, such as VOR or NDB. LPV has a very tight VAL of either 50 meters (LPV) or 35 Meters (LPV200), but the HAL which is 40 meters will grow to 556 meters in a downgrade and over 1800 meters on the missed approach, so in the vast majority of circumstances, losing LNAV is very rare. The LPV annunciation will be amber rather than green when either the LPV VPL or HPL exceeds the VAL or HAL before the FAF, so this is an advance warning of a likely downgrade to LNAV. Also if you look at VFOM on the satellite view, any value above about 60 feet for LPV and 40 feet for LPV200, the approach is likely to be downgraded. A failure of VPL inside the FAF will flag the vertical guidance.

Ibra wrote:

The SBAS correction is less accurate (this is relative) but you should always receive it if EGNOS satellite is above in the z-axis, maybe I am lucky with GPS-W accuracy:Paris Observatoire 34th RIMS station is 15nm away from runway (I sleep 1100m away from it ), I guess the reliability & quality of the correction depends on the location of runway threshold & ranging station?

What if RIMS station in on the airfield, any technical reason why that will not make SBAS as close to GBAS on the airfield?

The GBAS is able to be more accurate than SBAS because of the very limited area that the GBAS covers, only the immediate vicinity of the airport. SBAS covers a wide area, there are only around three dozen of ground stations which are used throughout all of WAAS covering Canada, US, Mexico, and Caribbean, which is a vast area. The ground stations are just as precisely surveyed, but since the corrections have to apply to a very large area, limits are reached. EGNOS covers all of Europe out to Iceland and down to North Africa. The EGNOS and WAAS satellites are geo stationary and always overhead very high in the sky in their service volumes, unlike the GPS satellites which circle the earth twice a day.

The ground stations are just as precisely surveyed, but since the corrections have to apply to a very large area, limits are reached

So error in SBAS corrections are mostly interpolation errors over wide area but near the RIMS station the correction should be very accurate…

On integrity, would you get more “LPV downgrade” when operating far away from ground stations than say “LPV downgrade” when operating near ground stations? or integrity is unrelated to SBAS correction accuracy?

Ibra wrote:

So error in SBAS corrections are mostly interpolation errors over wide area but near the RIMS station the correction should be very accurate…

Yes and no.

SBAS is not a simple “difference broadcaster” along the lines of “at point x, the position is off by 20 metres east an 5 metres south”, but it provides corrections for satellite clock and position errors (which increases position across the whole SBAS service volume, pretty much equally everywhere), as well as ionospheric delay corrections (which vary over the area).

The ionospheric delay is arranged in a grid and between grid points the receiver interpolates, and presumably the grid points closest to the ground station will be more accurate, but not necessarily so.

Yes I know EGNOS has own SBAS correction + extra GNSS position but with enough satellites on z-axis that would not make much difference

So the only technical difference between GBAS and SBAS is “ionospheric delay”, which has a historical average of “5m error” on RIMS stations (there is also satellites position error as they move up/down from their flight path but that should be corrected same way between SBAS as GBAS)

Ibra wrote:

So the only technical difference between GBAS and SBAS is “ionospheric delay”, which has a historical average of “5m error” on RIMS stations (there is also satellites position error as they move up/down from their flight path but that should be corrected same way between SBAS as GBAS)

Of course both GBAS and SBAS correct for ionospheric delay. The difference is that GBAS gives correction for a particular spot based on measurements on that spot so it doesn’t have to distinguish between different kinds of errors. Also the correction will be as good as it gets. SBAS gives corrections within a very large area based on measurements at various places in that area so the correction has to be estimated for most locations. When making that estimate various error sources (e.g. ionospheric delay) have to be considered separately.

NCYankee wrote:

RAIM being unavailable means the inputs to the algorithm are insufficient for the RAIM algorithm to check integrity. That is a different state from the RAIM algorithm determining an integrity fault. That is why if RAIM is still available at the FAF, the approach can continue, even if it is lost on the FAS.

@NCYankee many thanks and appreciate the other posts. Is my assumption (I think it may be stated in an FAA old GPS guide?) that if RAIM is unavailable past the FAF, and there is in effect Loss of Integrity, the GPS operates in ded reckoning mode? and it announces Loss of Integrity once you sequence to the waypoint after the missed approach point?

RobertL18C wrote:

Is my assumption (I think it may be stated in an FAA old GPS guide?) that if RAIM is unavailable past the FAF, and there is in effect Loss of Integrity, the GPS operates in ded reckoning mode? and it announces Loss of Integrity once you sequence to the waypoint after the missed approach point?

Robert,

If RAIM is unavailable, it means that integrity can’t be determined. If RAIM was still available at the FAF, you can continue along the FAS, even if RAIM is not available. In this case, the RAIM integrity can’t be determined, but since integrity was still valid at the FAF, the integrity is assumed to not deteriorate for the remaining flight to the MAP.. That is different than the case where RAIM is available and the integrity is less than required for the FAS.

An example, 4 satellites are needed for a position. A fifth satellite is needed to calculate RAIM. The position will be based on the 4 satellites that offer the best geometry. The question is can you trust them to provide a suitable accuracy during the approach. The fifth satellite is substituted for each satellite, one at a time to produce another 4 positions. The differences in the 4 additional positions are used to determine how trustworthy the original 4 satellites are in producing a position. At the FAF, the four satellites producing a position are found to be producing a trustworthy position. Then after the FAF, the fifth satellite is no longer being received, so now there is no way to determine the 4 additional positions to perform the integrity check. As long as the 4 satellites that are used to provide the position remain available, it is highly unlikely that their position accuracy will change of the remaining 2 minutes of the flight to the MAP. That would be RAIM unavailable. On the other hand, if all 5 satellites remained usable, the RAIM algorithm could still be available and if at any point on the FAS, the RAIM calculation indicated that the 4 satellites providing the position were not providing sufficient integrity, then using the GPS to continue the approach is not warranted. The integrity may still be good enough to execute the missed approach. If the integrity is not sufficient for that purpose, then one must revert to alternate means of navigation not using the GPS.

Ibra wrote:

So error in SBAS corrections are mostly interpolation errors over wide area but near the RIMS station the correction should be very accurate…

On integrity, would you get more “LPV downgrade” when operating far away from ground stations than say “LPV downgrade” when operating near ground stations? or integrity is unrelated to SBAS correction accuracy?

As best I can determine, SBAS provides 16 coded values (0 to 15) that specify the integrity of each individual satellite and it applies to the entire service volume. A value of 15 is “Do not use” and 14 is “Not Monitored”. Codes 0 thru 13 are used to specify a UDREIi of 0.75 meters to 150 meters. The GPS uses its position and the integrity of each satellite to determine an overall integrity HPL or VPL for the aircraft position. It does not appear to be based on the locations of the ground monitoring stations.

For Europe RAIM checks, I think they depend on the box, the flight leg and if it’s planning/flight

TSO129 GPS-NW non-SBAS box

During planning: before staring the aircraft one has to check RAIM for RNAV1 terminal legs (departure SID, arrival STAR) and RNP0.3 legs
Here,
https://augur.eurocontrol.int/tool/

During flight: by actually pressing Navigator menu and run the check themselves before starting RNAV1/RNP0.3

TSO146 GPS-NW SBAS box

During planning: no RAIM check is required but one need to read the “GPS NOTAMS” for SBAS availability
During flight: if SBAS is lost, GPS reverts to RAIM checks and vertical guidance is lost

- GPS “only” needs 4 stats to fly RNAV1 for the missed during an LOI
- TSO129 GPS-NW would run RAIM with fault detection on 5 sats
- TSO146 GPS-W would run RAIM with fault detection & exclusion on 6 sats

If GPS sats drop to 4 sats, it show “LOSS OF INTEGITY”, the 3D position is still very correct but GPS has no internal way to check it
If GPS sats drop to 3 sats, it show “NO GPS POSITION” on screen fly “DEAD REC”

NCYankee wrote:

It does not appear to be based on the locations of the ground monitoring stations.

Thanks, so being near RIMS station will not change much the parameters for any of the integrity tests but definitely help to reduce position accuracy errors…in other words, it cannot be used for “practical purposes”