ATSEP Use-Cases: Interference Errors Due To Airborne Collision Avoidance Systems 

Airborne Collision Avoidance System (ACAS) error refers to a failure or malfunction in the system designed to prevent aircraft collisions in the airspace.

Airborne Collision Avoidance System (ACAS) is a system that uses radar to detect other aircraft in the vicinity and provide pilots with resolution advisories to avoid collisions. However, the radar signals emitted by ACAS can interfere with other radar systems, such as those used by Air Traffic Control (ATC). This interference can cause problems such as false alarms, loss of radar returns, or even total radar failure. To mitigate this issue, ACAS is designed to use a specific frequency band that is separate from the frequency bands used by ATC radar and to automatically reduce its transmission power when it is not needed. However, in some cases, interference may still occur.

As ACAS is used by Pilots and its transmitted signals also have an impact on ATC Radar hence we will shed light on both ways.

ACAS Dependency on ADS-B

Airborne Collision Avoidance System (ACAS) is a system that uses radar to detect other aircraft in the vicinity and provide pilots with resolution advisories to avoid collisions. It is based on the Automatic Dependent Surveillance-Broadcast (ADS-B) system, which is a cooperative surveillance technology in which aircraft periodically transmit their position and velocity to other aircraft and ground-based systems.

Last Line of Defense for Pilots

ACAS is designed to be a last line of defense against collisions, and it is intended to supplement, rather than replace, other collision avoidance systems, such as visual lookout and Traffic Collision Avoidance systems (TCAS). 

ATC Radar detecting false targets

One of the most significant sources of interference is the radar's transmitter. The ACAS system emits a radar signal in the 1030 MHz frequency band, which is also used by ATC radar systems. This can cause the ATC radar to detect false targets or to lose track of real targets. This can occur due to the transmission of the ACAS RA signals, which can cause false echoes on the radar display. Additionally, the ACAS system's high power output can overload the ATC radar's receiver, causing it to fail. 

ACAS System detecting false targets

Another source of interference is the ACAS system's receiver. The ACAS system is designed to detect radar signals in the 1090 MHz frequency band, which is also used by ATC radar systems. This can cause the ACAS system to detect false targets or misinterpret the signals from the ATC radar. ACAS (Airborne Collision Avoidance System) is an aviation system designed to provide alerts to the flight crew of an aircraft in the event of a potential collision with another aircraft. When two aircraft are equipped with ACAS, they can interfere with each other if they are close enough and within each other's range of surveillance.

This interference can occur because the ACAS on each aircraft is designed to continuously scan the surrounding airspace for other aircraft and issue alerts to the flight crew if it determines a potential collision is imminent. The ACAS systems on aircraft typically operate on either 1090 MHz or 1030 MHz frequencies. If two aircraft are equipped with ACAS systems that operate on different frequencies, they should not interfere with each other. However, if two aircraft are equipped with ACAS systems that operate on the same frequency, they can interfere with each other, as both systems may issue alerts simultaneously. This can lead to confusion for the flight crew, who may not be able to determine which aircraft is the potential collision threat. To minimize this type of interference, aircraft are equipped with filters that reduce the number of false alerts generated by other aircraft.  If two aircraft are nearby, both ACAS systems may issue alerts simultaneously, confusing the flight crew.

Built-in Mitigation

To mitigate this issue, ACAS is designed to use a specific frequency band that is separate from the frequency bands used by ATC radar. It also uses a technique called pulsed radar. This means that the radar signal is emitted in short bursts, rather than continuously. This reduces the amount of interference caused by the ACAS system's transmitter, and it also increases the system's sensitivity, allowing it to detect targets at greater ranges.

Power Reduction Leading to Reduction in Interference

ACAS also has a feature called "power reduction." This feature automatically reduces the system's transmission power when it is not needed. This further reduces the amount of interference caused by the ACAS system's transmitter.

However, in some cases, interference may still occur. For example, if two equipped with ACAS are flying close to each other, the radar signals emitted by one aircraft's ACAS system can interfere with the other aircraft's ACAS system, causing both systems to fail. To mitigate this issue, ACAS is designed to automatically reduce its transmission power when it is close to another aircraft.

ACAS interfering with Air Traffic Control Radar

ACAS operates in the 1030 MHz and 1090 MHz frequency bands, which are also used by ATC radar systems. The transmissions from ACAS can create false echoes on the radar display, making it difficult for the controller to accurately track aircraft. The false echoes can be caused by multiple factors, including the strength of the ACAS transmission, the relative location of the aircraft and radar, and the sensitivity of the radar system.

Mode S Signals Interference with ATC Radar

Additionally, some ACAS systems use Mode S transponder signals, which can also interfere with ATC radar. Mode S signals can create false targets on the radar display, or cause actual targets to disappear. In some cases, the false targets can be mistaken for real aircraft, leading to confusion and potential safety hazards.

ATC Radar a Critical System to Ensure Safety

Air Traffic Control (ATC) radar systems are critical for the safe and efficient operation of aircraft, and any interference with these systems can have serious consequences. One potential source of interference is the Airborne Collision Avoidance System (ACAS), which uses radar to detect other aircraft and provide pilots with resolution advisories to avoid collisions.

How Radar Interference due to ACAS is depicted

Airborne Collision Avoidance System (ACAS) radar interference can manifest in various ways. Some common patterns of interference include

False targets

In some cases, the ACAS may detect false targets that are not present in the airspace. These false targets can be caused by reflections or delays in the signal from buildings or other structures.

Increased noise

 The ACAS may detect an increase in noise or background clutter, making it more difficult to detect actual targets.

Loss of targets

In some cases, the ACAS may lose the ability to detect actual targets due to interference.

Reduced range

The range of the ACAS may be reduced due to the interference, making it more difficult to detect targets at a distance.

Interference on specific azimuth or elevation

The interference may occur in specific azimuth or elevation, which can be caused by buildings or other structures that are reflecting or delaying the signal.

Interference at a specific time of the day

The interference may occur at specific times of the day, which can be caused by factors such as changes in atmospheric conditions or the movement of buildings or other structures.

Interference on specific altitude

The interference may occur at specific altitudes, this can be caused by the aircraft equipped with ACAS operating at the same altitude.

Interference on specific speed

The interference may occur at a specific speed; this can be caused by the aircraft equipped with ACAS operating at the same speed.

It's important to note that the pattern of interference can vary depending on factors such as the location of the ACAS-equipped aircraft, the type of ACAS being used, and the type and intensity of the interference. ATSEP should conduct regular monitoring of interference patterns to identify the cause and take necessary actions to rectify the issue.

Possible errors caused by ACAS interference with ground-based air traffic radar

 Interference can lead to a range of errors that can have serious consequences for the safe operation of aircraft.

False Alarm

One possible error caused by ACAS interference is false alarms. When the ACAS system's radar signals are detected by the ATC radar system, it may interpret them as real targets, leading to false alarms and unnecessary evasive maneuvers. This can create confusion and stress for the pilots and air traffic controllers, and it can also lead to other aircraft being misdirected or delayed.

Loss of Radar Return

The ACAS system's radar signals can overload the ATC radar system's receiver, causing it to fail. This can result in the ATC radar system losing track of real targets, leading to a loss of situational awareness and an increased risk of collisions.

Misinterpretation of Signals

Another possible error is a misinterpretation of signals. The ACAS system's radar signals can cause the ATC radar system to misinterpret the signals from other aircraft, leading to incorrect information being provided to the pilots and air traffic controllers. This can lead to miscommunication, misdirection, and other errors that can compromise the safety of aircraft.

Total Radar Failure

In the worst-case scenario, the ACAS system's radar signals can overload the ATC radar system's receiver and cause it to fail. This can result in a loss of radar coverage and an inability to detect other aircraft, leading to a high risk of collisions and other serious incidents.

Other Problems

In addition to these errors, ACAS interference can also lead to other problems such as increased workload for air traffic controllers and pilots, increased fuel consumption, and delays. These issues can have a significant impact on the safety and efficiency of air traffic operations.

Air Traffic Safety Electronics Personal rectifying ACAS

Interference to ground base Air Traffic Radar

To rectify ACAS interference with ground-based ATC radar systems, several measures can be taken. One approach is to use filtering and signal processing techniques to separate the signals from the ACAS system from those of other aircraft and ground-based radar systems. This can be done by using filters that reject the frequencies used by the ACAS system and by using signal processing algorithms that can distinguish between the signals from the ACAS system and those from other sources.

Antenna Design and Location of Radar

Another approach is to use antenna design and location to reduce the impact of the ACAS system's radar signals on the ATC radar system. This can be done by using antennas that are specifically designed to reject the frequencies used by the ACAS system, or by locating the ATC radar system's antennas in areas where they are less likely to be affected by the ACAS system's radar signals.

Adaptive Radar

Another possible solution is to use a technique called adaptive radar, which adjusts its parameters according to the current environment. This can include reducing the power of the radar signal when it is not needed or changing the radar's frequency or modulation scheme to avoid interference.

Regular Testing and Maintenance

In addition, to ensure that the systems are working properly, regular testing and maintenance should be conducted on both the ACAS system and the ATC radar system. This includes testing for proper frequency and power output, as well as checking for any signs of damage or wear.

Several measures can be taken to rectify ACAS interference with ground-based ATC radar systems. These include using filtering and signal processing techniques, antenna design and location, and adaptive radar, as well as conforming to specific standards and regulations and conducting regular testing and maintenance. By taking these steps, it is possible to ensure the safe and efficient operation of aircraft while minimizing the potential for interference with other radar systems.

Preventive Measures against ACAS Interference with ATC Radar

Adherence to Standards Laid Down By ICAO or FAA

It is important to note that while these issues can be caused by ACAS interference, it is designed to minimize the potential for interference with other radar systems. ACAS systems are required to conform to specific standards and regulations to minimize the potential for interference with other radar systems. These standards are developed by organizations such as the International Civil Aviation Organization (ICAO) and the Federal Aviation Administration (FAA).

In conclusion, ACAS interference with ground-based ATC radar systems can lead to a range of errors that can have serious consequences for the safe operation of aircraft. These include false alarms, loss of radar returns, misinterpretation of signals, and total radar failure. It is important to take steps to minimize the potential for interference, such as conforming to specific standards and regulations, and conducting regular testing and maintenance, to ensure the safe and efficient operation of aircraft.

References and Further Reading

• Transmission on 1030/1090 MHz (2019), Jerome Bodart
• Airborne collision avoidance system (2022), Eurocontrol