ISSUES ON APV/SBAS (GNSS) APPROACH PROCEDURE DESIGN STANDARDS

Jesús Pérez Ramos

Jorge Blanco Monge
Luis Pérez Sanz
Eva María García Moreno
INECO , Airspace Organization, Structuration and Design Department. Av. Partenón 4, 28042 Madrid, Spain

1. BIOGRAPHY During a NPA, aircraft approaching is only

provided with lateral guidance, being itself who
INECO Airspace Department is mainly involved in decide its vertical profile to a Minimum Decision
activities related to Instrument Flight Procedures Altitude/ Height (MDA/H), from which the pilot
Design, and also involved in other different decides whether the landing continues or the
activities concerning airspace organization and approach must be aborted for whatever reason
management, giving technical support to Aena, (lack of visibility, objects on the runway…)
Spanish Air Navigation Service Provider, with
which it works in close collaboration. It has During a PA, aircraft descending from the Final
recently participated in the design of LPV Approach Point (FAP) receives lateral and vertical
(Localizer Performance with Vertical guidance) guidance to follow a straight glide path to reach the
experimental procedures for several Spanish Decision Altitude/Height (DA/H). Flying crew is
airports, in the frame of Aena's research effort to given proportional angular deviation indication
assess potential benefits from a future from that nominal path, both lateral and vertical.
implementation of LPV approaches. He has had Signal in Space (SIS) for this guidance is broadcast
also participation in the design of LPV from the ground by the Instrument Landing
experimental approaches in the frame of GIANT System (ILS), composed of two ground
(EGNOS Introduction in the Aviation Sector) and subsystems, that is, Localizer (LOC) for lateral
OPTIMAL (Optimized Procedures and Techniques guidance, located on the runway axis beyond
for IMprovement of Approach and Landing) runway end, and Glide Path (GP) for vertical
projects. guidance, located aside the runway threshold.
Basically, LOC and GP signals define two planes,
the intersection of which is the nominal descent
2. INTRODUCTION path.

Typically, aircraft transition from route to the

airport consists, firstly, on a Standard Arrival
procedure (STAR) which makes aircraft descent
from cruising altitude to initiate the approach
procedure.

In turn, approaches have usually three phases:

Initial Approach, where the main descent is
performed; Intermediate Approach, where level
flight is preferred, in order to allow the crew to set Figure 1. ILS subsystems location
the adequate speed and configuration, as well as to
perform the proper checks for the final descent to
the runway; and Final Approach to the runway
threshold (THR)

According to the nature of that final descent, the

traditional classification has distinguished between
Non-Precision Approach (NPA) and Precision
Approach (PA).
 as SBAS Signal in Space (SIS) requirements for
APV are less demanding than those for PA,
according to ICAO Annex 10. Consequently,
minimum DH for APV is intended to be higher
than PA DH. While APV approaches can be
carried down to a 250ft DH at the least, ILS CAT I
operations can be carried down to 200ft.

Type of Horizontal Vertical

Operation Accuracy Accuracy
16.0m 20m
APV-I
Figure 2. ILS approach path (52ft) (66ft)
16.0m 8.0m
APV-II
GNSS Navigation has recently brought new types (52ft) (26ft)
of approaches into the scene. In particular, since 6.0m to
2007, ICAO (International Civil Aviation 16.0m 4.0m
Cat-I
Organization) PANS-OPS, Vol. II, Section 3, (52ft) (20ft to
Chapter 5, include criteria for the design of 13ft)
Approaches Procedures with Vertical Guidance
(APV) based on GNSS plus SBAS (Satellite Based Horizontal Vertical
Augmentation System). Two levels of Operation
Alert Limit Alert Limit
performance, APV I and APV II are defined. APV-I 40m (130ft) 50m (164ft)
APV-II 40m (130ft) 20m (66ft)
APV/SBAS flight path is analogue to an ILS PA. 15m to 10m
It consists on a straight descent from the CAT I 40m (130ft) (50ft to
Intermediate Approach minimum altitude to the 33ft)
runway. This descent path is typically aligned with
the runway axis (although a small offset up to 5º is However, it is worth to mention that there are also
permitted). Nominal Glide Slope is 3º. ongoing efforts in order to standardize APV
approaches down to 200ft in the future.
But, contrary to ILS, in the APV/SBAS case the
final descent path geometry is preloaded in the In summary, APV/SBAS approach procedures
airborne data base, instead of being broadcast from bring geometric lateral and vertical guidance
ground facilities and constructed in space by means (angular) during final approach operations, without
of radiation diagrams. Then GNSS/SBAS avionics the need for a ground based radio navigation
calculates aircraft position; that position is facility like the Instrument Landing System (ILS).
compared with the stored nominal path to be Basically, current ICAO philosophy and standards
followed, and deviation indication is given to the for the design and obstacle assessment of
crew. APV/SBAS approaches are quite similar to those
for ILS.
Final Approach Segment (FAS) Data Block
corresponding to the desired approach is selected APV/SBAS approaches are intended to be
by the crew from the airborne data base, in a way implemented mainly in runways where an ILS
similar to the conventional ILS frequency tuning. system is not suitable for whatever reason (space,
Deviation indication is also in an ILS “look-alike” money…). They are also considered a proper back-
manner. In fact, the whole operation is meant to be up for ILS Precision Approaches (PAs) in case of
quite similar to ILS, in order to ease a smooth and an ILS outage, potentially allowing better (lower)
friendly introduction of this kind of approaches. operational minima than conventional Non-
Precision Approaches (NPAs).
However, it is important to notice that, although
SBAS receiver mode for the APV approach is
designed ‘PA Mode’, APV is NOT a PA approach,

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However, according to the experience gained by
INECO from some experimental designs Nom
ina l
performed in several Spanish airports, the G lide
Path
Y Z
W
expectable minima improvement compared with
conventional NPA due to the introduction of
AXIS
vertical guidance, could be compromised in the

TH
case of some obstacle rich environments, because

R
of certain limitations in current APV/SBAS design Figure 4. ILS OAS around Final Approach - 3D view
standards.
Coefficients depend mainly on the distance LOC-
THR, aircraft categories, glide path angle, or
3. OBSTACLE ASSESSMENT minimum Missed Approach climb gradient.

ICAO PANS-OPS Obstacle Assessment Surfaces OAS system was developed by means of a
(OAS) and obstacle assessment methodology for statistics method based on gathered actual data.
APV/SBAS approaches, are both based and They contain the isoprobability curves which
derived directly from the ILS OAS method. correspond to a 1E-7 operational safety target (one
accident every 1E7 approaches). They protect not
OAS are a set of surfaces defined around the only the final approach, but also an initial dead
runway, which are used for obstacle assessment, in reckoning missed approach climb during missed
order to obtain an Obstacle Clearance approach.
Altitude/Height (OCA/H) for the intended
approach. DA/H will be never lower than this Only obstacles penetrating OAS have to be
OCA/H value. considered to establish the procedure minima due
to obstacles, OCA/H.
3.1. ILS
Therefore, the lower OAS are above terrain, the
ILS CAT I OAS are composed of six sloped plane more restrictive (higher) minima may be obtained,
surfaces, symmetrically displayed around the because more obstacles can potentially penetrate
runway (called W, X, Y, and Z), plus the the surfaces.
horizontal plane containing the runway threshold.
OCA/H is set adding a Height Loss (HL) value to
Planes are defined by the A, B, and C coefficients the highest obstacle penetrating the OAS. That HL
in the plane formula: z=Ax+By+C, considering a value takes into account that aircraft aborting the
coordinate system centered in the runway approach, takes time and descent from the moment
threshold, with the ‘x’ axis parallel to the runway, that missed approach is decided, to the moment
positive against the approach. that configuration has changed and climb is
initiated. HL values depend on aircraft category.
E”

Operational minima for this kind of approach are

D”
Y charted under the designation ‘LPV’.
C” X C D E Z
X
W
Y 3.2. APV OAS DERIVATION
E”

Figure 3. ILS OAS template – Top view

Given ILS OAS, ICAO PANS-OPS derives APV
OAS as follows.

Taking into account that Vertical Alert Limits

(VAL) for SBAS SIS serving an APV approach are
more conservative that those for Cat I, original ILS
X OAS planes are displaced vertically according to
that VAL difference, in order to keep the safety

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target. For this process, a nominal 12m VAL is any guidance, whereas there is RNAV guidance to
considered for ILS CAT I (compared with the initiate the APV/SBAS Missed Approach.
50m/20m for APVI/APVII respectively).
Displacement is hence equal to ‘VAL minus 12m’.

Figure 8. APV vs ILS OAS

Figure 5. APV OAS derivation 3.3. APV OAS ISSUE

Regarding the surfaces on the runway center line, Both for ILS and APV/SBAS surfaces around
APV OAS include W’ and W. final descent (W and X) rise to reach Intermediate
Approach minimum altitude minus the MOC
W’ is obtained considering a lowered Vertical (Minimum Obstacle Clearance) set for the
Path Angle (VPA) corresponding to a full scale intermediate phase. OAS extension enter
deflection (0.75VPA) flight path, lowered by the Intermediate Approach to meet intermediate
VAL. protection areas. Intermediate protection areas
take into account only Lateral Navigation
Further from the runway, where ILS W is lower (LNAV). Obstacle assessment philosophy before
than this W’, ILS W is considered. the APV segment is completely different.
Basically, it grants a MOC for the minimum flight
altitude over all the obstacles in the Intermediate
Approach area.

Figure 9. OAS extend into Intermediate App

Figure 6. APV OAS W’ plane

INTERM.
APP.
APV OAS (LNAV)
APV
SEGMENT
(LPV)

FAP
Figure 7. APV OAS Template

OAS also protect initial Missed Approach climb.

ILS CAT I OAS contour extends to reach 300m
over threshold level, widening laterally. However,
APV OAS extend laterally to a certain constant Figure 10. Intermediate Approach. Areas interface
width. This difference comes from the fact that in
the ILS, that very first climb is performed without

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This extension of OAS into the previous segment, current standards for APV/SBAS procedure design
try to protect the maneuver in case of an early start do not cover this same option. ICAO PANS-OPS
of descent, below the nominal Glide Path. Volume II, Part III, Section 3, Chapter 5.4.2
estates that “For navigation database coding
ILS and APV OAS falling into the Intermediate purposes, the waypoint located at the FAP shall
Approach are lower than the minimum not be considered as a descent fix. The APV OAS
intermediate height minus the MOC (see Figure surfaces extend into the intermediate approach
9). That means that some high obstacles in the segment but not beyond this segment”.
Intermediate Approach, although cleared for that Given this, many obstacles under the intermediate
previous phase, could penetrate OAS, hence being segment, while overcome in an ILS OAS study,
critical for the APV procedure minima. should be taken into consideration in the case of
APV/SBAS obstacle assessment, potentially
ICAO PANS-OPS allows to overcome this bringing much higher minima to the approach than
situation in the case of ILS. Provided that the point expected.
where Final Approach starts (FAP) can be
perfectly identified, aircraft can be kept from an This situation can cause severe penalties when
early descent bellow intermediate minimum before designing APV/SBAS approaches for complex
that point. According to that, design criteria for and mountainous scenarios, where terrain under
ILS permits to define a Step Down Fix (SDF) at the approach path keeps high before the final
the FAP location, curtailing OAS extension into descent.
de Intermediate Approach. A 15% slope plane
from the FAP cut the OAS then. 3.4. EXAMPLES

ILS FAP definition implies a tolerance, both INECO has been collaborating with Aena (Spanish
lateral and “along-track” (ATT), which depends air navigation service provider) in a research effort
on the navigation aid used to set its position. For regarding NPA-APV comparison in terms of
instance, it can be defined by a DME indication minima, for several Spanish airports.
(currently many ILS facilities include a Distance During this work, it was detected that, for certain
Measure Equipment indicating distance to scenarios, the problem explained above
threshold), or by a radio beacon to be overflown compromised expectable minima improvement by
during the approach. APV/SBAS implementation. In some of them, like
San Sebastian Airport, great expectations were put
FAP tolerance is taken into account when defining in the potential APV contribution.
the 15% slope plane.
3.4.1. San Sebastián RWY 04

San Sebastián airport is a very mountainous

scenario, also suffering severe bad weather
constrains frequently.

Current instrument approach to RWY 04 is an

extremely complex NPA VOR procedure,
implying a high workload for flying crews. It has
also very high published OCA/H, 3500ft/3490ft.

Figure 11. APV OAS issue ILS is not an option for this scenario due to the
lack of terrain available around the runway.
Obstacles before and below that 15% plane can be
ignored in the OAS evaluation.

In the case of an APV/SBAS approach, despite the

fact that FAP location can be perfectly identified,

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 4. CONCLUSIONS

According to the experience gained by INECO

from the experimental designs performed in
several Spanish scenarios, the expectable minima
improvement compared with conventional NPA
due to the introduction of APV/SBAS approaches,
could be compromised in the case of some
obstacle rich environments. Limitations in current
APV/SBAS design standards may put these
approaches further from ILS than expected.
Figure 12. San Sebastian RWY 04
Even if APV/SBAS approaches are standardized
Therefore, APV appears as a possible option to down to a 200ft minimum DH in the future (hence
improve instrument approach conditions there. equaling ILS CAT I approaches), this issue can
make results for APV design quite different than
Considering an APV approach which meets those for ILS in some cases.
current standards, estimated OCA/H would be
around 2700/2689ft and 2727/2716ft for CAT A It would be advisable that current data base
and CAT C aircraft respectively. That means that codification constraints avoiding OAS curtailment
minima keep very high and penalizing, despite the during APV/SBAS procedure design are studied
fact that vertical guidance is introduced in the and eliminated.
scene.

However, if a SDF was considered in the FAP and 5. REFERENCES

OAS were curtailed, minima could be reduced to
1600/1589ft, which means a great improvement • ICAO Obstacle Clearance Panel OCP-14
from current values. meeting material, March 2005
• ICAO Procedures for Air Navigation
3.4.2. Granada RWY 27 Services – Aircraft Operations (PANS-
OPS), 5th Edition
Also in the case of Granada RWY 27 terrain is • ICAO Annex 10, Aeronautical
very irregular and mountainous bellow approach Telecommunication, 6th Edition
path.

While the Granada RWY 09 has both NPA and

PA available for landing, there are not instrument
approaches published for RWY 27 in the Spanish
Aeronautical Information Publication (AIP) yet.
This means a great lack of flexibility in the airport
configuration.

In this scenario, OAS curtailment would allow

optimum APV minima: OCA/H would be
2115/255ft for CAT A aircraft, and 2146/286ft for
CAT D aircraft. However, if current standards are
met and OAS are extended into the Intermediate
Approach, APV approach simply is not an option
for this scenario.