Research on operational protection area of ILS localizer

Abstract

Precise delineation of the instrument landing system (ILS) localizer operational protection area is essential for maintaining navigation signal integrity and ensuring safe aircraft approaches. This study investigates the definition and evolution of operational protection area by synthesizing national standards and ICAO Annex 10 requirements. By employing computational electromagnetic simulations with the A380-800 model, we analyze the multipath interference generated by the aircraft and assess the impact of varying aircraft attitudes on signal stability. The findings provide quantitative data that supplement ICAO Annex 10 guidelines and offer a practical engineering demonstration for complex airport environments. Additionally, the study proposes optimized methodologies for site protection and delineation, providing valuable references for airport planning and operational management.

Introduction

Since its introduction in 1930 and subsequent adoption by the International Civil Aviation Organization (ICAO) in 1948, the instrument landing system (ILS) has served as the standard guidance system for aircraft approaches. Specifically, the ILS localizer provides horizontal guidance during the precision approach phase. Nevertheless, multipath interference remains a critical challenge; signals from the localizer antenna array are often reflected and re-radiated by terrain and nearby obstacles. This interference distorts the radiation field, inducing roughness, scalloping, or bending that can jeopardize the safety of aircraft landings.

Given the spatial constraints inherent to airport environments, maintaining a localizer site completely devoid of static and dynamic obstacles is often impractical. Consequently, establishing a localizer operational protection area is critical to mitigate multipath interference and ensure signal integrity complies with Annex 10, Volume I (hereinafter ‘Annex 10’)¹. While this area protects the precision approach segment from disturbances caused by mobile surface traffic, its enforcement inevitably restricts runway capacity^2–12. Addressing this trade-off requires site-specific research into ILS protection configurations. Methodologically, the impact of obstacles is typically modeled using computational electromagnetics techniques such as Physical Optics (PO) and the Uniform Theory of Diffraction (UTD)^13–17. Empirically, conducting rigorous assessments for specific operational scenarios is vital to balance the dual objectives of maximizing airport throughput and guaranteeing navigation safety.

Concept evolution of localizer protectional areas

As the global regulatory body, ICAO coordinates international civil aviation under the Convention on International Civil Aviation. Of particular relevance is Annex 10, which mandates the performance specifications and protection requirements for radio navigation systems across all flight phases. Contracting States are required to transpose these technical provisions into national legislation to maintain global interoperability. To ensure compliance, ICAO administers the Universal Safety Oversight Audit Program (USOAP), utilizing it as a critical mechanism to assess national safety oversight capabilities and maximize adherence to ICAO Standards and Recommended Practices (SARPs).

The 2018 amendment to Annex 10 addressed the operational impact of New Large Aircraft (NLA) on the size of ILS critical and sensitive areas, providing updated criteria for the delineation of this area (see Fig. 1).

Fig. 1.

Localizer critical and sensitive area (an10_v1_8ed).

The dimensions shown in Fig. 1 are determined by factors such as the ILS operation category, the aperture size of the localizer antenna array, the height of the aircraft’s vertical stabilizer, and the localizer’s distance to the runway threshold. For very large aircraft operating in Category III, the specific dimensions are provided in Table 1.

Table 1.

Dimensions for very large aircraft (CAT Ⅲ).

Aircraft height		20 m< H ≤ 25 m
Antenna aperture		Medium	Big
Critical area	XC	750 m	675 m
	ZC	60 m	60 m
	YC	70 m	50 m
Sensitive area	XS	Localizer to threshold distance	Localizer to threshold distance
	Y1	180 m×K	150 m×K
	Y2	260 m×K	180 m×K
	ZS1	70 m	70 m
	ZS2	250 m	250 m

National civil aviation authorities are responsible for safeguarding the ILS localizer’s operational environment and maintaining navigation signal quality during aircraft approach phases^18,19. To fulfill this responsibility, the CAAC has categorized the station protection area into two distinct areas: the site protection area and the operational protection area. While protection requirements for the latter align with ICAO specifications, the variables Y₃ and X_TH in Fig. 1 typically necessitate site-specific analysis. To facilitate implementation, the CAAC recommends adopting a default value for X_TH, providing a practical engineering solution.

Simulation of operational protection area based on extra large aircraft

Protection logic for the localizer operational protection area

Ideally, mandatory protection should be maintained for the critical area throughout localizer operations, covering the approach path down to the Category I decision height. Signal degradation within this area typically compromises the guidance quality for all aircraft simultaneously. Consequently, this area are demarcated by rigid boundaries, with access strictly controlled where they intersect with the airport maneuvering area. Conversely, the sensitive area theoretically extends from the Category I decision height to the runway threshold. Protection of this area is generally activated only during low-visibility operations (Category II/III), as interference here is transient and affects single aircraft. Nevertheless, realizing this comprehensive protection scheme presents logistical challenges, often necessitating a combination of technical and operational mitigation strategies.

The delineation of ILS critical and sensitive areas is governed by multiple variables, including the ILS category, antenna siting, airport surface layout, and the geometric properties (position, size, and vertical profile) of transiting aircraft or vehicles. Generally, signal disturbances are categorized as either dynamic interference (from mobile obstacles) or static interference (from permanent infrastructure and terrain). The interference budget for dynamic sources is typically determined residual to the static interference baseline. Consequently, should empirical static interference measurements diverge significantly from simulation models, the interference budget allocation requires recalibration. For instance, Formula (1) specifies the maximum permissible thresholds for Category III operations: 60% (3µA) for static sources and 80% (4µA) for dynamic sources. Statistical analysis suggests that combining these components via the Root Sum of Squares (RSS) method yields a more accurate estimation of total signal degradation than algebraic summation.

1

The simulation of the localizer operational protection area should utilize the 3D model of the corresponding aircraft and an accurate ILS propagation equation algorithm. The approach velocity used in the simulation is 105 knots (an ICAO-recommended value), which ensures a more conservative result. The dimensional parameters of the simulation model are shown in Fig. 2. Other key simulation parameters are listed in Table 2.

Fig. 2.

aircraft model parameters (A380-800).

Table 2.

Key simulation Parameters.

Typical parameters	Values
Terrain features	flat
Glide slop angle	3°
Static multipath	60%
Dynamic multipath	80%
Filter parameter	2.1 rad/s

Theoretical analysis of the impact of obstacles on aviation radio signals

The interference caused by obstacles (e.g., aircraft) is fundamentally modeled as an electromagnetic scattering phenomenon driven by induced surface currents. Upon excitation by an incident wave, these currents generate a secondary scattered field. Mathematically, this field is obtained by determining the surface integral of the induced sources. The Stratton-Chu representation provides the rigorous link between the surface electric and magnetic currents and the scattered field. By employing the Physical Optics (PO) approximation—which assumes the obstacle acts as a perfect electrical conductor (PEC)—the formulation can be simplified, yielding the following expression for the aircraft-scattered field²⁰:

2

3

In the formula, H^PO(r) represents the scattered field; is the incident wave magnetic field; is the surface current on the conductor; is the free space Green’s function; and n is the outer normal unit vector of the conductor surface.

The wings and other surfaces of an aircraft are not perfectly planar; rather, they are complex shapes featuring sharp edges and tips. Due to edge diffraction effects on these irregular features, the accuracy of the Physical Optics (PO) algorithm diminishes and therefore requires correction. The total scattered field, accounting for these diffraction effects, can be expressed as:^21–23:

4

In the formula, , R is the position vector from the source point to the center of the apex edge, is the unit vector along the apex edge, and L is the length of the apex edge. When calculating the backscatter field, f and g are considered in three distinct cases:

When electromagnetic waves irradiate the upper surface of the object:

5

6

When electromagnetic waves irradiate the lower surface of the object:

7

8

When both sides are illuminated:

9

10

For formula (9) and formula(10):

11

12

In the formula, m is the normalized outer vertex angle of the spire; the relationship between the two is given by m = α/π. Additionally, Ф_i is the angle between the transverse component of the incident wave unit vector i and the upper surface.

The obstacle surface is discretized into triangular facets to facilitate numerical modeling. The scattered field contribution from each facet is computed relative to its centroid, incorporating the specific phase delays. These individual scattering components are vectorially superimposed onto the direct localizer navigation signal along the approach path. Finally, the composite signal is demodulated by the airborne receiver, where signal distortions manifest as deviations on the Course Deviation Indicator (CDI), thereby affecting guidance integrity.

Simulation of operational protection area for aircraft in different States

According to the heights of objects provided by the airport, moving obstacles, including aircraft, are classified as shown in Table 3.

Table 3.

Classification for obstacle on airport surface.

Height	H ≤ 6	6< H ≤ 14	14< H ≤ 20	20< H ≤ 25
Classification	Ground vehicle	Medium aircraft	Large aircraft	Very large aircraft

According to the number of elements in the localizer antenna array, the antenna aperture is categorized into three types (Table 4).

Table 4.

Classification antenna Aperture.

Array numbers	N ≤ 11	12 ≤ N ≤ 15	16 ≤ N ≤ 20
Antenna aperture	Small	Medium	Big

The delineation of the localizer operational protection area is contingent upon several critical parameters, including the design aircraft type, the antenna array configuration, and the distribution network. To investigate the interference characteristics of Code F aircraft, this study utilizes the A380-800 as a representative model. Detailed simulation parameters for this scenario are enumerated in Table 5.

Table 5.

Simulation parameters.

Simulation parameters	Values
Runway length	3800 m
Runway width	60 m
Localizer to runway end	280 m
Antenna aperture	Big,20 elements(NM 7220a)
Feeding system	NM7000b
Operation category	CAT Ⅲ
Simulation frequency	110.95MHz
Airborne receiver type	Typical
Single simulation scale	6603

Numerical simulations were executed via the Advanced Trainer on Localizer (ATOLL) framework, utilizing an NVIDIA GeForce RTX 4070 GPU for computational acceleration. By establishing the runway centerline (approach direction) as the 0° reference, signal degradation was evaluated across discrete spatial points within the study area. Figure 3 presents the resulting interference distribution, with the associated color gradient legend detailed in Table 6.

Fig. 3.

Operational protection area simulation result (A380,0°).

Table 6.

Simulation result color block interpretation.

Block color	Impact on approaching aircraft (ICAO tolerance)
Green	0%～25%
Pale green	[25%～50%)
Yellow	[50%～75%)
Orange	75%～100%
Red/crimson	More / far more than100%

Figure 3 delineates the ICAO-defined critical, sensitive, and additional sensitive areas, represented by cyan, pink, and blue regions, respectively. The simulation results indicate that for an aircraft maintaining a 0° orientation (parallel to the centerline), the designated operational protection area covers all area posing a potential risk to navigation integrity. To exemplify the signal degradation, Fig. 4 depicts the distorted course structure resulting from an A380-800 stationed at two typical positions (A and B). Crucially, the presence of the aircraft within the orange-shaded region generates interference levels that exceed acceptable tolerances for precision approach operations.

Fig. 4.

Course structure at representative location.

ICAO guidelines for critical and sensitive areas designed to accommodate diverse aircraft orientations, specifically oblique angles that are neither parallel nor perpendicular to the runway axis. Adopting the 0° orientation as the baseline, this study simulated the aircraft at 45° counterclockwise intervals relative to the centerline. The corresponding results are depicted in Figs. 5, 6, 7, 8, 9, 10 and 11.

Fig. 5.

Operational protection area simulation result (A380,45°).

Fig. 6.

Operational protection area simulation result (A380,90°).

Fig. 7.

Operational protection area simulation result (A380,135°).

Fig. 8.

Operational protection area simulation result (A380,180°).

Fig. 9.

Operational protection area simulation result (A380,225°).

Fig. 10.

Operational protection area simulation result (A380,270°).

Fig. 11.

Operational protection area simulation result (A380,315°).

Based on a comprehensive analysis of Figs. 3, 5, 6, 7, 8, 9, 10 and 11, we draw the following conclusions:

1. When the aircraft is positioned at oblique angles (neither perpendicular nor parallel to the runway), the resulting multipath interference is significantly more severe compared to the 0° orientation. Analysis of the 135°, 225°, and 315° cases reveals that the standard sensitive area boundary (pink) fails to contain the critical interference area (orange-red), where signal degradation reaches 75%–100% of the ICAO tolerance. This finding scientifically justifies the necessity of incorporating “additional sensitive areas” into the ILS operational protection framework.

2. Regarding the cardinal orientations (0°, 90°, 180°, and 270°) typically encountered in operations, simulation results indicate that the protection area mandated by ICAO SARPs is conservative compared to the physical interference limits. Consequently, this discrepancy provides a robust theoretical foundation for performing site-specific safety assessments to optimize (narrow) the protection boundaries. Such optimization offers significant practical benefits for enhancing runway throughput and overall airport efficiency.

3. As indicated by the green region along the horizontal axis of symmetry in Figs. 3 and 8, the aircraft’s geometric profile—particularly its longitudinal and vertical extent—exerts the most significant influence on the localizer signal. Analysis confirms that the vertical stabilizer acts as the dominant scattering source degrading signal integrity. Consequently, computational models for protection areas must be calibrated to the maximum vertical stabilizer height of the design aircraft authorized for the runway.

Simulation of operational protection area for specific aircraft operational scenarios

As detailed in the preceding section, the delineation of operational protection areas within ICAO SARPs is predicated on the interference caused by aircraft in varying orientations to the localizer guidance signal.

In real-world airport environments, characterized by complex runway-taxiway configurations and rapid exit taxiways, aircraft orientations rarely align perfectly with idealized theoretical models. Consequently, performing site-specific safety assessments is essential. This approach not only optimizes operational efficiency but also ensures full compliance with ICAO SARPs, alongside national and industry-specific standards.

To evaluate a representative airport environment, this section examines a typical runway-taxiway layout adjacent to a terminal. Simulation parameters are maintained consistent with Tables 2 and 5, specifically modeling an A380-800 under Category III conditions. The operational scenario postulates a nominal approach path flowing from right to left. Specific aircraft orientations were assigned based on taxiway geometry: 0° and 180° on the parallel taxiway, 330° on the Rapid Exit Taxiway (RET), and 90°/270° on the connecting taxiways. The resulting interference contours are visualized in Fig. 12.

Fig. 12.

Simulation results for typical operational scenario.

(Source: Google Earth)

From the simulation results, if a single A380-800 is positioned in the yellow area on the right parallel taxiway (Fig. 12), it will cause interference ranging from 50% to 75% of the ICAO tolerance. If a single A380-800 is located on the rapid exit taxiway, it will generate interference that exceeds the ICAO tolerance (an unacceptable impact). Taking point C on the rapid exit taxiway (Fig. 12) as an example, the resulting influence curve at the localizer position is shown in Fig. 13.

Fig. 13.

Course structure at representative location.

Simulation data indicates that an A380-800 positioned within the yellow area of the right parallel taxiway (Fig. 12) induces signal degradation equivalent to 50%–75% of the ICAO tolerance limit. More critically, an aircraft located on the Rapid Exit Taxiway (RET) generates interference that surpasses the threshold, constituting a violation of operational safety standards. To exemplify this severe distortion, Fig. 13 plots the distorted course structure resulting from an aircraft stationed at Point C.

If an aircraft is positioned at point C, it will cause an unacceptable impact on approaching aircraft within approximately 2500 m to 3500 m of the localizer.

Conclusion

1. For standard operational scenarios, the delineation of protection areas should be predicated on the aircraft classification and the localizer antenna aperture. This standardized methodology guarantees signal integrity and operational safety, effectively obviating the necessity for complex, site-specific technical evaluations.

2. While ICAO SARPs provide a baseline for delineating protection areas based on aircraft classification, the effective boundaries are governed by the specific positions and orientations of aircraft capable of inducing critical signal degradation. Although standard safety margins are inherent, optimizing this area requires site-specific safety assessments. These evaluations must account for the antenna specifications, the static obstacle environment, the design aircraft, and the specific runway-taxiway configuration. This targeted approach allows for the refinement of protection boundaries, thereby maximizing airport throughput without compromising navigation safety.

3. Given the inherent idealization of environmental parameters (such as terrain and obstacle topology) within computational models, discrepancies between simulation results and empirical data are inevitable. Consequently, regarding engineering applications in airport construction, the final design proposal mandates a rigorous validation framework: primarily relying on computational modeling, but definitively calibrated and verified through on-site flight inspection.

4. For airports characterized by a restricted fleet mix—typically small-to-medium hubs—the operational protection area can be tailored to the specific scattering characteristics of the operating aircraft, rather than a generic worst-case model. Moreover, the implementation of dynamic protection area management or predictive interference warning systems—driven by real-time aircraft tracking (position and orientation)—offers a viable pathway to significantly enhance operational efficiency.

Author contributions

All authors contributed equally in the preparation, drafting, editing, and reviewing of the manuscript.

Funding

This work was supported in part by the Safety Capability Project of the Civil Aviation Administration of China (Project No: AADSA201S013S) and the Safety Capability Project of the Civil Aviation Administration of China (Project No: 2024NO168).

Data availability

All data used in the current study is available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.