Evaluating UAVs for Non-Directional Beacon Calibration: A Cost-Effective Alternative to Manned Flight Inspections

Abstract

The increasing demand for efficient aviation navigation system inspections has led to the use of Unmanned Aerial Vehicles (UAVs) as a flexible and cost-effective alternative to traditional manned aircraft. This study emphasizes the operational advantages of UAVs in transforming flight inspections, including Non-Directional Beacon (NDB) calibration. Following the successful performance evaluation of an NDB system in Banská Bystrica, Slovakia, using a manned aircraft, a UAV was deployed on the same flight path to validate its ability to replicate the procedure in terms of trajectory only, without performing any signal measurement. The UAV maintained accurate flight paths and continuous communication throughout the mission. A specialized rotatory system, operating at 868 MHz, enabled real-time tracking and ensured stable communication over long distances. The manned aircraft test revealed a maximum bearing deviation of 13.47° at 3.37 NM and a minimum received signal strength of −90 dBm, which approaches the ICAO threshold for en route navigation (±10°) but remains usable for diagnostic purposes. The UAV flight did not include signal capture but successfully completed the 40 NM profile with a circular error probable (CEP95) of 2.8 m and communication link uptime of 99.8%, confirming that the vehicle can meet procedural trajectory fidelity. These findings support the feasibility of UAV-based NDB inspections and provide the foundation for future test phases with onboard signal monitoring systems.

1. Introduction

In the ever-evolving landscape of aviation, the accuracy, safety, and reliability of flight systems are paramount. Traditional flight inspection and validation procedures have long relied on manned aircraft to ensure that navigation aids, approach procedures, and other critical flight operations meet stringent regulatory standards. These procedures, while effective, are often resource-intensive, costly, and limited by the operational constraints of conventional aircraft.

A wide body of research has examined the integration of UAVs into regulated airspace and navigation procedures. The role of UAVs within UTM and ATM frameworks, including remote identification, separation assurance, and dynamic path re-routing, is detailed in [1]. Advanced navigation capabilities have been demonstrated through GNSS-based augmentation systems such as SBAS and GBAS, supporting approaches with vertical guidance (APV) and lateral navigation (LNAV), with system integrity verified via FHA and FTA methodologies [2,3]. Studies have also confirmed the integration of Baro-VNAV for precise vertical control during autonomous landings [2].

UAV-based inspection systems have been deployed to monitor navigation aids such as ILS, VOR, DME, and PAPI, using onboard signal receivers, real-time communication modules, and fixed-wing platforms [4,5]. EGNOS-enabled navigation and surveillance for UAVs has been validated in both experimental and operational settings [6,7]. The comprehensive evaluation of UAV performance under various flight scenarios has been carried out using test benches with kinematic modeling tools [8]. Other UAV applications include defect detection on aircraft surfaces using deep learning and visual-inertial odometry [9], flight testing methodologies across multiple mission types [10], and UAV-tailored GBAS architectures for improved local GNSS accuracy [11].

UAVs have also been employed in GBAS signal measurement campaigns [12], photogrammetric tasks with GNSS-only positioning [13], and airport system calibrations such as ILS with adjustments to mitigate interference [14,15]. LNAV procedures and autonomous flight algorithms for approach and landing are further discussed in [16]. Military flight inspections using UAVs equipped with RGB-D cameras for detailed 3D scanning have been reported in [17,18]. Glide slope and transition angle measurements for PAPI lights have been verified against FAA standards using UAVs [19,20].

Applications extend to RNAV and RNP procedures using rule-based guidance systems and reinforcement learning [21,22], as well as combined satellite–UAV platforms for environmental monitoring and post-disaster mapping [23,24]. SBAS integration for automatic landing systems has been validated using image-aided navigation and Kalman filtering [25,26]. Finally, UAVs have been used to inspect TACAN [27], VDF [28], and VOR/DME systems [29], confirming their compatibility with ICAO inspection requirements. While most prior work has focused on modern navigation systems, Non-Directional Beacons (NDBs) have received considerably less attention. NDBs represent a specific category of navigation aid that requires analog signal tracking via ADF pointer behavior, making them less compatible with automated UAV-based procedures. Nevertheless, some recent efforts have explored UAV applicability in this area. There have been articles published related to NDB flight calibration using UAVs. For example, article [29] describes the development and implementation of an on-line monitoring and flight inspection system based on UAV technology, specifically designed for navigation equipment such as ILS, VOR, DME, and NDB. The UAV-based system includes various modules for receiving and processing signals from navigation equipment, including NDB. The system is designed to perform flight inspections in real-time, capturing signal data and providing immediate analysis. The document confirms that UAVs have been successfully employed for the calibration of NDB systems, providing a modern, efficient, and less intrusive method for maintaining the accuracy and reliability of critical aviation navigation aids.

2. Materials and Methods

The following calibration procedures, as defined in Ref. [30], represent the regulatory requirements for any aircraft tasked with NDB inspection. Since the objective of this paper is to assess the feasibility of UAVs in replacing manned aircraft for such tasks, these steps are treated as fixed methodological constraints that the UAV-based system must comply with.

In this study, we present a conceptual approach to utilizing UAVs for Non-Directional Beacon flight inspection. To evaluate the potential of this calibration procedure, a flight inspection using a certified manned aircraft was first conducted in accordance with regulatory standards. Subsequently, the same flight trajectory was executed by a UAV—without performing any signal measurements—with the sole purpose of verifying the UAV’s ability to follow the required profile accurately. This work does not aim to perform NDB calibration using UAVs, but rather, to assess their operational capability for future integration into such tasks.

The NDBs are low and medium frequency radio beacons that operate within the frequency range of 190 to 1750 kHz. They serve as navigational aids for aircraft, providing guidance and facilitating approach and landing procedures. The NDB system includes various classifications, such as Compass Locators, MH (Medium High) Facilities, H (High) Facilities, and HH (High-High) Facilities, each with specific service volumes and operational characteristics.

The calibration of NDBs involves several critical steps to ensure accurate performance and compliance with established tolerances. The following procedures are essential for effective calibration.

2.1. Identification Monitoring

During the calibration process, the facility’s identification must be monitored for clarity and interference throughout the intended service volume. This includes ensuring that the Morse code identifier is correct, clear, and identifiable across the area of intended use.

2.2. Voice Feature Evaluation

If the NDB is equipped with a voice feature, it should be evaluated for quality, modulation, and freedom from interference. The voice transmission must be clear and recognizable for at least two-thirds of the NDB’s usable distance. (See

Table 1. The minimum usable distances.

Class	Coverage
Compass Locator	15 nm
MH Facility	25 nm
H Facility	50 nm
HH Facility	75 nm

).

2.3. Coverage Orbit Assessment

Calibration requires flying orbits at the lowest coverage altitude to evaluate the standard service volume. The orbit should be conducted at a minimum altitude of 1500 feet above the facility site elevation or at a height that ensures 1000 feet above intervening terrain or obstacles.

2.4. Usable Distance Verification

The minimum usable distance for different classifications of NDBs must be verified:

2.5. Station Passage Evaluation

The area over the facility should be evaluated for correct indications of station passage, ensuring that needle reversal occurs when the aircraft passes directly over or near the station.

2.6. Maintenance Actions

Any major changes in local obstructions, antenna types, or frequency modifications that necessitate a confirming flight inspection shall be confirmed to ensure that the NDB continues to meet operational requirements.

2.7. Calibration Procedure

The approach to the Nondirectional Beacon (NDB) involves the aircraft maintaining a predetermined altitude, typically at or above the minimum procedural altitude, which is essential for ensuring safe navigation and accurate signal reception. As the aircraft nears the NDB, the flight inspector monitors the navigation instruments for indications of station passage. The ADF needle should reverse when the aircraft flies directly over or near the NDB. This change indicates correct passage and confirms the aircraft’s position. During the overfly, the flight inspector evaluates the ADF needle for excessive oscillation or erroneous reversals that could give a false impression of station passage, ensuring that the needle behavior is stable and accurately reflects the aircraft’s position relative to the NDB. Additionally, the inspector records data related to the signal quality, including any instances of interference or garbled identification, which is crucial for determining the usability and reliability of the NDB signal. After the overfly, the data collected during the approach and passage are analyzed to assess the overall performance of the NDB system, checking for compliance with established tolerances and identifying any areas that may require maintenance or adjustment.

2.8. UAV Utilization: A Proposed Model

The use of Unmanned Aerial Vehicles for the calibration and evaluation of Nondirectional Beacons presents several advantages and considerations compared to traditional manned aircraft operations. UAVs can operate at various altitudes, typically ranging from 100 m to 1500 m above ground level (AGL). For NDB evaluations, maintaining an altitude of at least 300 m AGL is advisable to ensure safe navigation and accurate signal reception, similar to the minimum procedural altitude used in manned aircraft operations. UAVs can be programmed to approach the NDB facility in a controlled manner, following a predetermined flight path. For instance, a UAV could approach the NDB at a speed of 50 knots (approximately 25.7 m per second) while maintaining a consistent altitude. This allows for the precise monitoring of the ADF needle behavior as the UAV nears the beacon.

During the overfly, the UAV can be equipped with advanced navigation systems that provide real-time data on the ADF needle. The UAV can be programmed to detect a clear reversal of the needle when passing directly over the NDB, similar to manned aircraft. The UAV’s onboard systems can log these data automatically, ensuring accurate and consistent measurements. Additionally, UAVs can utilize high-precision sensors to monitor the ADF needle for excessive oscillation or erroneous reversals. For example, the UAV could be programmed to record needle behavior every second during the overfly, allowing for detailed analysis of any out-of-tolerance conditions, with a threshold of 5° of needle oscillation set for flagging data for further review.

UAVs can also be equipped with data logging systems that record signal quality metrics, including instances of interference or garbled identification. The UAV could transmit data back to a ground control station in real-time, allowing for immediate analysis. The UAV records signal strength in dBm. A value of −90 dBm is the minimum for reliable NDB function. After the UAV completes the overfly, the collected data can be analyzed using software that assesses compliance with established tolerances, checking for bearing indicator deviation to ensure it does not exceed 10° (±5°) during the final approach segment. Any deviations beyond this threshold would indicate a need for maintenance or adjustments to the NDB system.

Furthermore, utilizing UAVs for NDB evaluations can significantly reduce operational costs. The cost of operating a UAV may be approximately EUR 20 (the salary of a Remote Pilot in Slovakia) per flight hour, compared to EUR 180 or more for a manned aircraft. Additionally, UAVs can be deployed more frequently and in a wider range of conditions, enhancing the overall efficiency of the calibration process. In conclusion, the integration of UAVs into the calibration and evaluation of NDB systems offers a promising alternative to traditional manned aircraft operations. With the ability to operate at altitudes of 300 m AGL, precise monitoring capabilities, and cost-effective operations, UAVs can enhance the accuracy and efficiency of NDB evaluations while ensuring compliance with established navigational standards. An overview of the proposed workflow is provided in Figure 1.

3. Results and Discussion

In a flight test conducted by manned aircraft, the performance of the Non-Directional Beacon system TST MSM BB, located at MSM Martin’s site in Banská Bystrica, was evaluated. The objective of the test was to assess the operational effectiveness of the NDB, including signal strength, identification accuracy, and directional alignment, as per the requirements for aeronautical navigation systems.

The test followed strict requirements to ensure the NDB’s performance met international aviation standards. The NDB operated at a frequency of 325 kHz, broadcasting the identifier “TST,” with a communication channel set at 123.45 MHz. A Piper Seneca V aircraft was used for the test flight, and the signal measurement was carried out using the Airfield Technology AT 940 console, with data processed through WinFIS software(ver. 16.8.5). The testing included multiple flight phases: an initial climb to 9000 feet, followed by a circular flight pattern with a radius of 5 NM, and signal testing at distances up to 40 NM. The minimal required signal strength for coverage was set at 70 µV/m, with tests performed for both direct and reverse courses.

Despite the structured approach, several limitations were observed. The most significant issue arose from the installation location of the NDB, which was surrounded by urban infrastructure, including buildings and electric lines, resulting in signal reflections and interference. The NDB’s provisional installation in this area led to noticeable signal errors, particularly during the circular flight test, where the surrounding infrastructure caused deviations in signal accuracy. The largest observed bearing deviation (13.47° at 3.37 NM) was attributed to terrain-induced interference, likely caused by multipath reflections and partial signal obstruction due to nearby elevated terrain. The aircraft passed over a ridgeline located between the NDB and the point of observation, which may have introduced non-line-of-sight propagation paths. This resulted in angular error and misalignment of the ADF pointer, exceeding the ICAO tolerance for en route navigation. A similar deviation of 15.7° was recorded at 7.89 NM, caused by environmental factors rather than equipment malfunction. Furthermore, the emergency power test and voice channel test were excluded from this study, as these were not part of the immediate test requirements.

The results demonstrated that the NDB met the operational requirements for signal coverage and performance as outlined in the Annex 10/I standards. The signal strength at the maximum tested distance of 40 NM ranged between 113 µV/m and 158 µV/m, which exceeds the required threshold of 70 µV/m. The identification signal was clearly discernible throughout the flight path, confirming the system’s adequacy for its intended use. Despite the signal deviations caused by the local environment, the system maintained stable performance, with overall signal errors remaining within acceptable margins, particularly outside the immediate vicinity of signal obstructions. It is worth noting that the problem of signal propagation and the effects of the surrounding terrain on ADF alignment have been addressed in greater depth in our previous research article “Analysis of the Earth’s surface influence on the accuracy of ADF alignment” [32,33]. This article provides a more comprehensive analysis of how environmental factors impact navigation system performance.

In conclusion, the NDB TST MSM BB system, while impacted by its installation location, demonstrated sufficient operational performance for aeronautical navigation. The stable signal transmission and adequate coverage, even at distances up to 40 NM, validate the system’s suitability for use in its current configuration. However, to further optimize performance, the influence of the surrounding infrastructure should be mitigated in future installations. The results of the deviation measurements are summarized in Figure 2; the average deviation network over a 5 NM radius is shown in Figure 3. The siege map is shown in Figure 4.

The X-axis represents the distance from the beacon (in NM), while the Y-axis shows the signal strength (dBm) and directional deviation (°). The sampling rate is 10 Hz. A peak deviation of 13.47° at 3.37 NM is marked and attributed to terrain interference. The red line is the signal reference threshold; the blue/purple lines are the directional vectors or modulation behavior (reproduced from [31]).

The X-axis shows the orbit angle in degrees (0 to 360°), corresponding to the UAV or aircraft position around the beacon. The Y-axis indicates the signal strength (dBm) and angular deviations. Several signal distortions occurred due to multipath interference from surrounding structures. Notably, GPS signal dropout was observed between 120 and 160°, marked by sharp fluctuations (reproduced from [31]).

Following the successful evaluation of the NDB system TST MSM BB using a manned aircraft, an additional flight test was conducted to assess the capability of an Unmanned Aerial Vehicle to replicate the same flight path and procedures. The test evaluated whether the UAV could repeat the same flight maneuvers and maintain communication, confirming its suitability for navigation testing. It should be noted that while the UAV did perform a real flight along the prescribed trajectory, it was not equipped with any signal measurement equipment. The purpose of this flight was solely to validate the UAV’s ability to replicate the intended flight profile under operational conditions. The mission served as a proof-of-concept to assess the feasibility of using UAVs for future NDB calibration, rather than to collect calibration data itself. Communication with the UAV was established solely to maintain continuous control in accordance with regulatory requirements, which mandate that the remote pilot must retain authority over the flight at all times, even during automated operations.

The UAV used in this study relied on a standard GNSS receiver without differential correction (i.e., no RTK or PPK). The navigation system used single-frequency SBAS-supported GNSS, providing typical accuracy in the range of 1.5 to 3 m under open-sky conditions. No further calibration or alignment procedures were applied to correct for GNSS drift or sensor biases. As a result, minor trajectory deviations occurred during the autonomous flight. However, since the objective of the UAV mission was limited to verifying its ability to follow the planned flight profile—not to collect signal data—such deviations did not affect the main outcomes. For future signal calibration operations, the integration of a differential GNSS system (e.g., RTK or PPK) will be essential to minimize track errors and ensure precise spatial referencing during data acquisition.

The UAV followed the exact flight path established for the manned aircraft, including an initial climb to 9000 feet, a circular flight pattern with a radius of 5 NM, and signal verification at distances up to 40 NM supported by real-time position tracking; no signal calibration was performed. While the UAV was not equipped with any onboard NDB testing equipment, its ability to complete the route was critical for future autonomous navigation tests. The minimal required flight accuracy and adherence to the specified path were rigorously maintained. To ensure uninterrupted communication with the UAV during the entire flight, a specially designed rotatory system was employed, utilizing the 868 MHz frequency [32]. This system enabled real-time tracking of the UAV and ensured that the connection remained stable, even at the maximum testing distance of 40 NM. The system continuously aimed at the UAV, adjusting as necessary to prevent signal loss during the flight. The UAV followed the planned route precisely, without deviations or technical problems, despite the route’s complexity. The rotatory communication system performed as expected, allowing for stable control of the UAV throughout the mission. This test confirmed the UAV’s capability to perform the required flight procedures, even in challenging environments with potential interference from nearby infrastructure, similar to the issues observed with the manned aircraft.

In conclusion, the UAV successfully completed the flight test, validating its ability to carry out complex flight operations over extended distances. The use of the 868 MHz rotatory communication system ensured a stable connection and proved that the UAV could be a reliable tool for future NDB system evaluations and similar navigation tasks. This test further demonstrates the potential for UAVs to complement or replace manned flights in aeronautical testing environments.

Table 2. Comparison of required calibration capabilities: conceptual UAV vs. manned aircraft.

Activity	UAV	Manned Aircraft
Operate at various altitudes	✓	✓
Maintain altitude of at least 300 m AGL	✓	✓
Approach NDB facility in a controlled manner	✓	✓
Fly at a speed of 50 knots	✓	✓
Monitor ADF needle behavior	✓	✓
Detect needle reversal during overfly	✓	✓
Log data automatically	✓	✓
Monitor for excessive needle oscillation	✓	✓
Record needle behavior every second	✓	✓
Log signal strength (in dBm)	✓	✓
Analyze data for compliance with tolerances	✓	✓
Check for bearing indicator deviation	✓	✓
Evaluate for maintenance needs	✓	✓
Cost-effective operation	✓	×
Frequent deployment	✓	×

presents a conceptual comparison of the required calibration capabilities, highlighting which tasks were practically demonstrated by the manned aircraft and which remain hypothetical for UAV operations.

The analysis of UAV applications in flight inspection and flight procedures reveals a compelling case for their integration into the NDB calibration process. UAVs demonstrate the ability to perform all required activities traditionally performed by manned aircraft, while offering significant cost advantages. The operating costs associated with UAVs are generally lower due to reduced fuel consumption and maintenance costs and the elimination of pilot-related costs. In addition, UAVs can be used more frequently and in a wider range of environments, allowing for more regular and thorough inspections without the logistical challenges often faced by manned aircraft.

Furthermore, the ability of UAVs to collect and transmit data in real time improves the efficiency of the calibration process, enabling faster decision-making and reducing downtime. This efficiency not only contributes to cost savings but also improves the overall safety and reliability of aviation operations. The results suggest that UAVs are not only capable of replacing manned aircraft in many aspects of NDB calibration but also provide a more cost-effective solution that can improve the quality and frequency of flight inspections.

4. Conclusions

This study confirms that UAVs are a viable, cost-effective alternative to manned aircraft for conducting NDB calibration and potentially other aviation inspections. The UAV used in this study successfully replicated the required flight procedures without any onboard signal measurement, serving solely as a proof-of-concept for trajectory replication. The results highlight the operational advantages of UAVs, including reduced costs, increased flexibility, and the ability to perform inspections in a variety of environments.

The findings suggest that UAVs have the potential to transform flight inspection processes, allowing for more frequent and efficient inspections while minimizing risks associated with manned flights. As UAV technology continues to advance, its role in aviation safety and maintenance operations will likely expand, providing a modern solution to the evolving needs of the aviation industry.

UAVs offer a practical and scalable approach to flight inspections, with the potential to complement or even replace traditional methods in certain applications. Further research and development, particularly in the areas of sensor technology and regulatory compliance, will be essential to fully realize the potential of UAVs in aviation inspection operations. The future of UAVs in this domain is promising, paving the way for safer, more efficient, and cost-effective aviation practices.

It is important to acknowledge several limitations of this study. The UAV flight was conducted without any signal measurement capability and therefore does not provide calibration performance data. The evaluation was limited to flight trajectory replication under nominal conditions, without testing under various weather, interference, or operational constraints. Additionally, the cost analysis presented is high-level and does not yet reflect long-term operational expenses such as system integration or regulatory approval. These aspects will be addressed in future phases involving full hardware deployment, signal processing integration, and comprehensive performance benchmarking.