Determination of ILS Critical and Sensitive
Areas: A Comparison of Flight Measurement
versus Simulation Techniques
S. Dale Courtney
Senior Navigation Engineer
US FAA, Air Traffic Organization
Washington, DC, USA
Voice: +1 202 493 5172
E-mail: dale.courtney@faa.gov
ABSTRACT
The quality of the Instrument Landing System (ILS)
signal-in-space can be degraded by reflectors or sources
of multipath. Static multipath is produced by signal
reflection from fixed objects such as aircraft hangers or
fencing on the aerodrome.
Dynamic multipath is
produced by signal reflection from moveable objects such
as aircraft taxing around the runway environment. ILS
critical and sensitive areas are implemented around
Localizer and Glide Path facilities to protect aircraft on
approach from dynamic multipath that could cause the
ILS signal-in-space to exceed allowable tolerances for
alignment and roughness. Aircraft operation in critical
and sensitive areas are either prohibited or operationally
controlled. ILS critical and sensitive area sizes have been
determined by both flight measurements, while aircraft
are staged around the aerodrome, and more recently by
computer simulation and modeling. The Navigation
Systems Panel of the International Civil Aviation
Organization (ICAO) is currently producing guidance
material on the size of these areas given todays larger
aircraft. This paper will compare flight measurement
versus computer simulation techniques in determining
critical and sensitive area sizes. This paper will also
present critical and sensitive area size recommendations
determined by work of the ICAO Navigation Systems
Panel.
INTRODUCTION
Reflecting objects within the ILS radiated signal coverage
volume, whether fixed objects or vehicles, can cause
degradation of the signals-in-space beyond applicable
tolerances, through signal blockage and/or multipath
interference. The amount of degradation is a function of
the location, size, and orientation of the reflecting
surfaces, and the ILS antenna characteristics. It is
convenient to consider disturbances caused by aircraft and
vehicles separately from disturbances caused by fixed
objects such as buildings and terrain. The analysis of
disturbances caused by aircraft and vehicles to the ILS
signal-in-space performance results in necessary areas of
protection around the localizer and glide path facilities
referred to as critical and sensitive areas.
Dimensions of critical and sensitive areas have
historically been determined by direct flight measurement
as well as varying levels of mathematical modeling. In
direct flight measurement, a reflector such as an airplane
is positioned in an area of concern and the resulting
disturbance on the ILS signal-in-space is measured with a
flight inspection aircraft. Mathematical modeling predicts
the amount of disturbance on the ILS signal-in-space due
to reflectors using techniques from simple ray tracing to
complex simulations made possible with computer
models.
Once necessary dimensions for critical and sensitive areas
are determined about localizer and glide path facilities,
the areas can be restricted or operationally managed so
that aircraft or vehicles do not cause out of tolerance
course and alignment changes and structure roughness to
aircraft conducting an ILS approach. The International
Civil Aviation Organization (ICAO) recognizes the
importance of critical and sensitive areas on ILS
operations and publishes dimensions of these areas in
Attachment C, Information and Material for Guidance in
the Application of the Standards and Recommended
Practices for ILS, VOR, PAR, 75 MHz Marker Beacons
(En-Route), NDB and DME to Annex 10, Volume 1,
Standards and Recommended Practices for Aeronautical
Telecommunications, Radio Navigation Aids.
BACKGROUND AND DEFINITIONS
ICAOs Navigation Systems Panel recognized the
importance to consider the completeness of the current
material on critical and sensitive areas considering
introduction of larger aircraft such as the Airbus A380. In
March 2007, the Navigation Systems Panel commissioned
an Ad-Hoc group to consider updates of ILS guidance
material in the area of critical and sensitive area sizes
necessary to assure integrity of ILS operations. The AdHoc groups work was completed and accepted by the
panel in April 2008.
The Ad-Hoc group first aligned the definitions of critical
and sensitive areas to be consistent and more flexible with
how ILS operations are managed in the operational
environment. The new definitions are:
Critical Area: The ILS critical area is an area of
defined dimensions about the localizer or glide path
antenna, such that aircraft and other vehicles within the
area cause out-of-tolerance disturbances to the ILS
signals-in-space from the limit of the coverage to a
distance of 3.7 km (2NM) from the landing threshold.
Sensitive Area: The ILS sensitive area is an area
of defined dimensions about the localizer or glide path
antenna, such that aircraft and other vehicles within the
area cause out-of-tolerance disturbances to the ILS
signals-in-space from a distance of 3.7 km (2NM) from
the landing threshold to the point at which the ILS signal
is no longer required for the intended operation.
Next, the Ad-Hoc group defined the criteria used to
determine the dimensions of critical and sensitive areas.
The group understands total distortion is the combination
of fixed and mobile sources. The group recognizes that
the root sum square (RSS) combination of the
disturbances due to fixed and mobile objects gives a
statistically valid representation of the total disturbance as
compared to that of an algebraic sum. For example, a
limit of plus or minus 5 A for localizer course structure
would be respected with plus or minus 3 A of
disturbance due to fixed objects and an allowance of plus
or minus 4 A for mobile objects:
Critical and sensitive areas were developed for several
configurations of localizer and glide path antenna types
matrixed with aircraft representing four different aircraft
height groups. One quickly sees that using the direct
flight measurement technique of evaluating different
aircraft positioned in various locations around the
aerodrome becomes both time and cost prohibitive.
Further analysis reveals aircraft positioning and
orientation can be critical in determining an out of
tolerance distortion. Mathematical model simulations can
determine distortion produced from many positions and
orientations of aircraft with a much smaller investment of
time and resources when compared to direct flight
measurement.
Several European Air Navigation Service Providers
(ANSP) recently completed a series of tests to validate
mathematical models with direct flight measurement.
These tests were conducted at the Frankfurt, Toulouse,
and Heathrow aerodromes. Locations of interest were
determined using mathematical simulation. Boeing B747
and Airbus A380 aircraft were positioned as determined
in the simulation and then ground and flight
measurements were made of the resulting ILS distortion.
Similar tests were conducted in the United States at the
Dallas-Ft. Worth aerodrome in 1982. These tests were
used to validate mathematical simulation models.
Samples of the validation work conducted at the DallasFt. Worth aerodrome are presented in Figures 1 and 2.
3A2 + 4 A2 = 5A
Fixed sources are considered to consume 60% of
allowable signal-in-space tolerance leaving 80% of
allowable tolerance to be consumed by mobile sources.
Critical and sensitive area determination is then based on
distortion from mobile objects consuming 80% or more of
allowable tolerance, considering an RSS model.
Finally, the group determined critical and sensitive area
dimensions using simulations from complex computer
mathematical models. The mathematical models used
were well validated by direct comparison with ground and
flight measurements for a variety of specific situations
and environments.
COMPARISON OF FLIGHT
VERSUS SIMULATION
MEASUREMENT
Figure 1. Localizer Approach with B747 Positioned
1000 Feet from Array and Center of Aircraft 250 Feet
Off Centerline with Tail Toward Centerline
Figure 2. Glide Path Approach with B747 Positioned
448 Feet from Threshold and 376 Feet from Centerline
Figure 4. Critical Area Simulation of B747 with
Medium Aperture Localizer Array
SIMULATION RESULTS
The simulation results presented illustrate aircraft
positions (colored points) that cause distortion to the ILS
signal-in-space and consume 80% or more of the
allowable tolerance.
Simulation results presented first show the reduction in
critical area sizes when choosing larger aperture localizer
arrays. The aircraft chosen for this comparison is the
Boeing 747.
Figure 5. Critical Area Simulation of B747 with Large
Aperture Localizer Array
Simulation results presented next show the reduction in
critical area sizes when choosing an M-Array glide path
in lieu of a null reference configuration. The aircraft
chosen for this comparison is the Boeing 747.
Figure 3. Critical Area Simulation of B747 with Small
Aperture Localizer Array
Figure 6. Critical Area Simulation of B747 with Null
Reference Glide Path
Figure 8. Localizer Critical Area Simulation of A380
Aircraft
Figure 7. Critical Area Simulation of B747 with MArray Glide Path
Figure 9. Localizer Critical Area Simulation of B747
Aircraft
Simulation results presented below show the reduction in
localizer critical area sizes with smaller height group
aircraft. The localizer array chosen for this comparison is
the medium aperture array.
Figure 10. Localizer Critical Area Simulation of B757
Aircraft
Figure 11. Localizer Critical Area Simulation of
GulfStream V Aircraft
Figure 13. Glide Path Critical Area Simulation of
B747 Aircraft
Final simulation results presented show the reduction in
glide path critical area sizes with smaller height group
aircraft. The glide path array chosen for this comparison
is the M-Array.
Figure 14. Glide Path Critical Area Simulation of
B757 Aircraft
Figure 12. Glide Path Critical Area Simulation of
A380 Aircraft
Figure 15. Glide Path Critical Area Simulation of
GulfStream V Aircraft
SUMMARY OF SIMULATIONS
Computer model simulations of critical and sensitive
areas were completed for aircraft representing the most
demanding for four separate height groups, three localizer
antenna configurations, two glide path antenna
configurations, and tolerances considered for both
Category I and Category III. The aircraft modeled
included the Airbus A380, the Boeing B747, the Boeing
B757, and the GulfStream V. Small (8 Element),
Medium (13 Element), and Large (16 Element) localizer
arrays were modeled. M-Array and null reference glide
path configurations were modeled. The simulations
considered the most demanding orientation of the aircraft
including parallel to the runway, orthogonal to the
runway, and a 45 degree orientation to represent an
aircraft in a turn.
With the simulations completed, critical and sensitive area
diagrams were constructed to contain the dimensions
realized from simulation. Tables following the diagrams
present the actual dimensions of critical and sensitive
areas.
Critical Area
Rear Sensitive
Sensitive Area
Additional rear Sensitive Area for
orthogonal to runway orientation
Additional Sensitive Area for non-orthogonal, non-parallel to runway orientation
ZS1
ZS2
ZC
Y2
Y1
120 m
Y3
YC
runway
XC
1500 m
300 m
XTH
XS
Not to scale
Dimensions in meters
Note - Y3 and XTH are determined by a site-specific study. Where no specific study has been
undertaken, the default value for XTH is zero.
Figure 16. Diagram of Localizer Critical and Sensitive Areas
H 6 m (Note 1)
6 m < H 14 m
14 m < H 20 m
20 m < H 25 m
e.g. Large Ground Vehicle
e.g. B757, A320
e.g. B747SP
e.g. A380, AN124
Aircraft/Vehicle Height
Small
Medium
Large
Small
Medium
Large
Small
Medium
Large
Small
Medium
Large
180m
140m
100m
360m
290m
205m
670m
600m
470m
1040m
860m
790m
ZC
10m
10m
10m
35m
35m
35m
50m
50m
50m
60m
60m
60m
YC
50m
50m
50m
110m
80m
70m
160m
130m
100m
200m
160m
110m
Note5
210m
130m
Note5
420m
320m
Note5
850m
710m
Note5
1330m
1070m
ZC
Note5
10m
10m
Note5
35m
35m
Note5
50m
50m
Note5
60m
60m
YC
Note5
60m
50m
Note5
100m
75m
Note5
150m
110m
Note5
190m
130m
200m
300m
300m
500m
300m
300m
1100m
600m
600m
2000m
1500m
950m
Y1
40m
60m
60m
90m
60m
60m
150m
60m
60m
200m
135m
60m
Y2
40m
60m
60m
90m
60m
60m
150m
60m
60m
230m
135m
60m
ZS1
15m
15m
15m
35m
35m
35m
50m
50m
50m
60m
60m
60m
ZS2
15m
15m
15m
35m
35m
35m
50m
50m
50m
60m
60m
60m
Note5
300m
300m
Note5
300m
300m
Note5
LLZ to
Threshold
distance
LLZ to
Threshold
distance
Note5
LLZ to
Threshold
distance +
200m
LLZ to
Threshold
distance
Y1
Note5
60m
60m
Note5
60m
60m
Note5
150mK
120mK
Note 5
205mK
145mK
Y2
Note5
60m
60m
Note5
60m
60m
Note5
175mK
125mK
Note 5
225mK
150mK
ZS1
Note5
15m
15m
Note5
35m
35m
Note5
60m
60m
Note 5
70m
70m
ZS2
Note5
15m
15m
Note5
45m
45m
Note5
160m
160m
250m
250m
250m
XS
Note5
300m
300m
Note5
300m
300m
Note5
LLZ to
Threshold
distance +
100m
LLZ to
Threshold
distance +
50m
Note 5
LLZ to
Threshold
distance +
200m
LLZ to
Threshold
distance +
200m
Y1
Note5
60m
60m
Note5
60m
60m
Note5
160mK
130mK
Note 5
210mK
145mK
Y2
Note5
60m
60m
Note5
60m
60m
Note5
250mK
185mK
Note 5
350mK
225mK
ZS1
Note5
15m
15m
Note5
35m
35m
Note5
60m
60m
Note 5
70m
70m
ZS2
Note5
15m
15m
Note5
45m
45m
Note5
160m
160m
Note5
250m
250m
Antenna Aperture
Note 3
Critical Area CAT I
XC
Critical Area CAT II/III XC
Sensitive Area CAT I XS
Note 6
Sensitive Area CAT II XS
Note 6
Sensitive Area CAT III
Note 6
Table 1. Typical Localizer Critical and Sensitive Area Dimensions
Runway Threshold
Not to scale
Ws
Runway
YC [o,p]
50m
Glide Path
Antenna
YS [o,p]
45
ZC
XC [o,p]
Figure 17. Diagram of Glide Path Critical and Sensitive Areas
Aircraft/Vehicle Height
GP Antenna Type
H 6 m (Note 1)
6 m < H 14 m
14 m < H 20 m
20 m < H 25 m
e.g. Large Ground Vehicle
e.g. B757, A320
e.g. B747SP
e.g. A380, AN124
N-Ref
M-Array
N-Ref
M-Array
N-Ref
M-Array
N-Ref
M-Array
Critical Area CAT I
Xc,o
510m
310m
830m
340m
860m
500m
1400m
600m
Critical Area CAT I
Xc,p
230m
220m
320m
310m
500m
400m
380m
400m
Critical Area CAT I
Yc,o
15m
15m
10m
10m
10m
10m
10m
10m
Critical Area CAT I
Yc,p
25m
15m
55m
25m
75m
45m
75m
45m
Critical Area CAT I
Zc
0m
0m
0m
0m
45m
45m
45m
45m
Note 2
Critical Area CAT II/III
Xc,o
580m
340m
1100m
540m
1100m
610m
1700m
790m
Critical Area CAT II/III
Xc,p
300m
260m
420m
360m
550m
420m
600m
460m
Critical Area CAT II/III
Yc,o
15m
15m
10m
10m
20m
10m
10m
20m
Critical Area CAT II/III
Yc,p
55m
25m
75m
45m
75m
55m
75m
65m
Critical Area CAT II/III
Zc
0m
0m
0m
0m
45m
45m
45m
45m
Note 2
Sensitive Area CAT I
Xs,o
500m
290m
770m
480m
1120m
500m
1290m
710m
Sensitive Area CAT I
Xs,p
235m
220m
410m
220m
525m
345m
520m
365m
Sensitive Area CAT I
Ys,o
5m
5m
5m
5m
10m
15m
10m
10m
Sensitive Area CAT I
Ys,p
45m
25m
85m
35m
115m
75m
135m
135m
Sensitive Area CAT I
Zs
0m
0m
50m
50m
75m
75m
50m
50m
Note 2
Sensitive Area CAT II/III
Xs,o
680m
350m
980m
530m
1430m
650m
1580m
790m
Sensitive Area CAT II/III
Xs,p
320m
250m
460m
335m
600m
400m
650m
465m
Sensitive Area CAT II/III
Ys,o
10m
15m
5m
5m
20m
15m
10m
10m
Sensitive Area CAT II/III
Ys,p
85m
35m
155m
45m
175m
105m
205m
175m
Sensitive Area CAT II/III
Zs
30m
30m
60m
60m
125m
125m
100m
100m
Note 2
Table 2. Typical 3 Glide Path Critical and Sensitive Area Dimensions
CONCLUSIONS
b.
Critical and sensitive area dimensions are
heavily influenced by the type of ILS antenna
array in actual use.
c.
Validated mathematical computer simulation can
be used to more completely define critical and
The following conclusions are reached:
a.
Critical and sensitive area dimensions are
heavily influenced by aircraft tail height and
size.
sensitive areas as compared to direct flight
measurement techniques.
d.
Care should be exercised when using direct flight
measurement to quantify effects of a mobile
reflector on ILS signal-in-space performance as
small change to reflector location and orientation
can result in varying levels of measured
distortion.
ACKNOWLEDGMENTS
The author wishes to acknowledge the work of the ICAO
Navigation Systems Panels Ad-Hoc group on critical and
sensitive areas for their dedicated work that facilitated the
preparation of this material.
Dale Courtney is a senior navigation engineer for the United States Federal Aviation
Administration in the Air Traffic Organizations Technical Operations Service. He is
currently working at the Federal Aviation Administrations headquarters office in
Washington, DC. Before his assignment at the headquarters office, he worked at the
Federal Aviation Administrations Southern Regional office in Atlanta, Georgia.
Dale Courtney received a Bachelor of Electrical Engineering from Auburn University in
1991. He joined the Federal Aviation Administration after graduation in 1991.
As a senior engineer for navigation programs, Dale Courtney works in the areas of
conventional navigation, lighted visual aids, global navigation satellite system, satellite
based augmentation systems, and ground based augmentation systems providing
implementation engineering, operations engineering, systems engineering, technical
support, and policy development. Dale serves as a technical advisor to the United States
member of the International Civil Aviation Organizations Navigation Systems Panel and
also serves as the United States lead to the Navigation Systems Panels Conventional
Navigation and Testing Subgroup.
Dale Courtney is a former broadcast engineer, an amateur radio operator, a certified
Project Management Professional, and an instrument-rated private pilot. Dale resides in
Arlington, Virginia.
