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According to ICAO's classification, LPV SBAS approach is in the APV group, because it doesn't meet some specifications for precision approaches. But what exactly does it not meet?


Glossary:

  • LPV: Localizer performance with vertical guidance
  • SBAS: Space-based augmentation system
  • APV: Approach with vertical guidance
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    $\begingroup$ I am slightly surprised about LPV SBAS not being classified as a Precision Approach in ICAO, but the topic is relatively new to me. Do you have any references? $\endgroup$ – DeltaLima Mar 9 '16 at 8:54
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    $\begingroup$ not a duplicate: that one asks IF this one asks WHY $\endgroup$ – Federico Jun 15 '16 at 18:44
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    $\begingroup$ Agreed. Not a duplicate. $\endgroup$ – Ryan Mortensen Jun 15 '16 at 18:51
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Precision Approaches (PA), as you mention, have specific performance requirements, one of which is how "good" the vertical position measurement must be during an approach. In the case of a PA, ground systems either directly measure the aircraft's vertical position on the glide path (Precision Approach Radar in the case of parallel approaches), or provide a means to follow a precise vertical glide slope via a gauge in the cockpit.

For Approaches with Vertical Guidance (APV), there is vertical guidance, but the aircraft's vertical position is measured onboard the aircraft by either a GPS position source or a barometric sensor. Either of those sensors are subject to measurement errors:

  • Barometric Measurement Errors: ICAO Annex 10 section 4.4.2.4 defines the error model that is used for barometric altitude reporting systems. The 95% bound of the worst case is +/-282ft.
  • Barometric Setting Errors: Additional errors are introduced depending on what barometric MSL pressure is selected on the altimeter and how that compares to current (actual) atmospheric pressures.
  • GPS Measurement Errors: While GPS position sensors provide very good horizontal position, GPS vertical position is generally poor. According to the FAA, the 95% vertical error of GPS systems used in civilian contexts is on the order of 300ft and according to this Advisory Circular (section 5-7), geometric/GPS altitude does not meet requirements for use in air traffic control internationally.

Combining these errors with flight technical errors while flying the approach, the total error you can expect can add up quickly. Compare that to the fact that in the vertical guidance of an ILS improves the closer you get to the ground, you can quickly see why the alternatives of GPS or altimetry don't quite cut it...

So specifically to your question: For an approach to be a precision approach, you need precise vertical position measurements, which an SBAS cannot provide.

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    $\begingroup$ I'm a bit confused by this answer. If the vertical error is up to 300ft then LPV approaches couldn't exist. The LPV DA at my local airport is 285ft AGL, there's no way that's possible with 300ft of error. In fact, the FAA page you linked to quotes a position error of "less than 10 meters"; perhaps you meant 30ft, not 300ft? It also says "augmented by WAAS and GBAS, GPS will be able to supply a precision approach capability (CAT-I with WAAS [...])" And your final sentence says that SBAS can't provide precise vertical position, but that's exactly what an LPV approach does provide. $\endgroup$ – Pondlife Apr 4 '16 at 13:42
  • $\begingroup$ @Pondlife - You are correct. What I was referring to was the general FAA (as well as other regulators) skepticism of state measurement exclusively onboard aircraft. Many of the model assumptions are outdated and artificially inflate worst case modeling, which contributes to slow adoption of new capability in aviation. Observed performance, in position accuracy is much better than what worst-case modeled performance could be. Therefore, system integrity (maximum undetected error) becomes the driving parameter. But, the reclassification in ymb1 answer is evidence that it might be changing $\endgroup$ – nodapic Feb 24 '19 at 1:21
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The ICAO classifications have changed:

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(eurocontrol.int, 2017)

ICAO has been reworking the approach classifications since c. 2012, because of the confusion they were causing in the PBN environment. Good news is, LPV SBAS Cat I is now (since at least 2013) a precision approach.

Approaches now are two types, A and B. The approach minima are ≥250 feet and <250 feet respectively. Another new classification is 2D and 3D. 3D approaches are those with vertical guidance.

Any 3D Type B approach, such as the LPV Cat I, is now considered a precision approach.


Sources and further reading:

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  • $\begingroup$ I'm a bit confused. I can´t find any reference to LPV in ICAO Annex 10, vol. 1. Is LPV a term recognized by ICAO or is it only used by other organizations? $\endgroup$ – jinawee Jun 13 '19 at 15:02
  • $\begingroup$ @jinawee: This screenshot is from Annex 10 Vol 1 (2018). As you can see, they haven't yet adapted the new Annex 6 terms (that's why they included that comparison table). The older terms used by Annex 10 you can find on page ATT D-14 (para 6.2.1.1), which are APV-I and APV-II. $\endgroup$ – ymb1 Jun 13 '19 at 19:32
  • $\begingroup$ Uhm, them Annex 6 doesn´t mention LPV either (2016). I'm under the impression that LPV isn´t used by ICAO at all. I found mentions by the FAA, European Commision regulations, RTCA DO-229D, etc. $\endgroup$ – jinawee Jun 14 '19 at 9:07
  • $\begingroup$ @jinawee: I haven't checked the older Annex 6, but the slide in the question does show LPV. I think before the new classifications, ICAO simply used [SBAS] APV. $\endgroup$ – ymb1 Jun 14 '19 at 11:42
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There is a substantial distance between actual GPS/SBAS performance AND theoretical error modelling done to bound SBAS performance. In the real world, HPL/VPL (horizontal/vertical protection levels) are in the 15 meter levels while measured errors are in the 2-3 meters level, rarely getting to 10 meters. Actual errors are always 5-10 meters better than HPL/LPV levels. The whole thing was modelled using theoretical worst case calculations that might never ever happen in the real world.

Essentially the FAA/EASA is just being way over zealous in what performance they allow LPV to be used for. LPV200 is only allowed to be used up to minimums, even in visual conditions, while ILS is allowed to be used for autoland below minimums if visual conditions are present. Eventually FAA/EASA should allow for coupled LPV autopilot at least until 100 radar altitude.

Plus for now we're only using single frequency, GPS network only SBAS receivers. The next generation of SBAS will feature GPS+Galileo+Glonass+Compass+regional augmentation with dual frequency processing at the end user (aereal) receivers.

This will yield: 1 - Global LPV200 performance 2 - Currently we have a few minutes a day where LPV200 performance isn't available in the California coast and hours a day where LPV200 performance isn't available in northern Canada and Western Alaska. That should pretty much be gone. There could still be a few weak spots worldwide where LPV200 could be unavailable for a few minutes a day 3 - The ACTUAL performance of dual frequency/multi constellation SBAS should be good enough for CAT II approaches in the real world, but since mathematical models are used instead of real world performance, we might never get SBAS CAT II or it could take many decades.

The keys are: With dual frequency service iono corrections are now calculated in real time at the end receiver. With 4 constellations+regional we go from normal 9 GPS ranging sources to over 20 and sometimes 30 ranging sources. Actual typical accuracy should sub meter 99.99% of the time or CAT IIIc performance. But this is meaningless for aviation safety authorities, what matters to them is their ultra super duper pessimistic mathematical models that still predict errors that don't even allow for CAT II !

In conclusion, LPV200 is a precision approach. Even LPV250 is a precision approach too. That's in practice. But FAA/EASA don't like it.

There's also a conflict of interest in GBAS vs SBAS features. GBAS requires a per metro area installation and barely provides for full CAT I approaches today (including coupled landings) and eventually will provide CAT IIIa approaches. If SBAS could provide CAT IIIa, then GBAS is dead, and there are billions invested on that ! So people don't want SBAS features to invade their precious future GBAS performance envelope !

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    $\begingroup$ HPL/VPL are integrity indicators, not accuracy indicators so it does not make much sense to compare them to typical actual errors. Integrity is crucial for the safety case, because even though a 99.99% accuracy of 1 meter sounds great, you want to still to be safe if the system is in some degraded mode, and the accuracy cannot be met. That is what integrity monitoring is for and HPL/VPL is a metric for it in GNSS. FAA and EASA understand the difference between accuracy and integrity and prevent people who don't from making expensive mistakes. $\endgroup$ – DeltaLima Dec 10 '17 at 20:50

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