CERTIUM Blog

A tale of three interferences

It all started when the European ANSP upgraded their voice communication system with modern IP-based equipment. What seemed like a straightforward enhancement soon evolved into a complex technical puzzle. After switching to IP-based communication the digital radios began revealing previously undetected issues within the communication infrastructure. This led to a series of operational challenges that would require sophisticated analysis to unravel.

#1: Abnormal amount of short SQL events

The initial symptoms appeared in the form of what was referred to as "ghost calls" - unusually short squelch events lasting less than 300 milliseconds. In radio communications, squelch activation occurs when a receiver detects a signal strong enough to potentially carry voice traffic, opening the channel for reception. These ghost calls were occurring at an alarming rate - approximately 50 times per minute per channel, with bursts reaching up to 450 events per minute on some frequencies.

What made this situation particularly challenging was the responsiveness of the digital radios. In analog mode they did not signal these brief interruptions, while in IP-mode the radios' sensitivity and fast response time were now revealing these previously invisible issues. The radios were doing fine, however other solution elements had a hard time to cope with the large amount of squelch events. For the air traffic controllers, these manifested as persistent "clack" sounds in their headsets, creating a distracting work environment.

The situation quickly escalated beyond technical concerns into a significant business dispute. The ANSP, having recently invested in the new communication system, attributed these issues solely to the new equipment, as these problems previously hadn't been visible. This led to a payment dispute, with the ANSP refusing to pay for the new system components claiming they were defective. In response, engineering teams were dispatched across the country with specialized measurement equipment to investigate the root cause.

The investigation revealed a more complex reality. Rather than a fault in the new system components, the analysis showed that the interference sources had likely existed even before the upgrade. The digital radios, with their superior sensitivity in IP-mode, were simply making these pre-existing issues visible for the first time. Only specific operational frequencies were affected, suggesting a systematic rather than a random issue.

Through careful analysis, the source was eventually traced to a defective system installed near the ATC receiving antennas, generating electromagnetic interference (EMI) strong enough to trigger these phantom activations. This discovery highlighted an important lesson: sometimes new technology doesn't create problems - it merely reveals existing ones that were previously below the radar.

#2: The intelligent intruder

The second interference pattern presented an even more intriguing puzzle. Unlike the random ghost calls, these interruptions followed distinct patterns - some occurring every 70-85 seconds, others precisely every 5 minutes. Each event generated two audio packets, each lasting 20 milliseconds, from the radio to the Voice Communication System (VCS).

This regular pattern suggested a systematic issue rather than random electromagnetic interference. Further investigation using the R&S®AVQA monitoring system indeed revealed distant systems generating sweep signals across the entire ATC band. In one particularly interesting case, two sweep signals, separated by 5MHz, would trigger two short squelch events within 10 seconds of each other. The interference sources could not be shut down but are now being closely monitored to ensure their impact on air-traffic-communications stays within acceptable bounds.

#3: Elevated noise

The third interference pattern manifested as significant elevations of the background noise floor - a critical parameter for radio communications. Monitoring revealed periodic power peaks, that could cause severe receiver desensitization.

The cascade effect

What made these interference sources particularly problematic was their interaction with the ANSP's VCS System. In particular with cross-coupling, which is a common practice in ATC operations, allowing controllers to transmit simultaneously on multiple frequencies. However, this meant that a single interference event on one frequency would be automatically relayed to multiple channels, creating a cascade of phantom transmissions across the entire system.

This multiplication effect had severe operational consequences. The voice recording system, crucial for safety and legal requirements, was not sized for the sheer volume of these short transmissions being replicated across multiple frequencies. The system, designed to capture regular voice communications for post-incident analysis, needed to be enhanced to cope with the load of these phantom transmissions.

Resolution and lessons learned

The resolution of these issues highlighted several important lessons for ANSPs worldwide. First, the transition to modern IP-based radio systems, while offering superior capabilities, may reveal previously undetected interference sources due to their enhanced sensitivity.

Second, the interaction between different systems - in this case, the VCS with cross-coupling operations and recording systems - can amplify seemingly minor issues into significant operational challenges.

Most importantly, this case demonstrated that communication problems in ATC environments often have multiple, independent root causes that can interact in complex ways. The solution required not just identifying individual interference sources but understanding their cumulative impact on the entire communication ecosystem.

This real-world experience serves as a valuable reminder that maintaining clear and reliable communications in air traffic control requires constant vigilance and sophisticated monitoring capabilities. As aviation communication systems continue to evolve, the ability to quickly identify, analyze, and resolve such complex interference patterns becomes increasingly crucial for maintaining safe and efficient operations. This is exactly what R&S AVQA is all about.

Network issues: An unexpected discovery

During the investigation of the interference problems, the analysis tools revealed another significant finding - one that, while not the primary focus, highlighted additional concerns about the network infrastructure. The monitoring system detected issues that could potentially impact the quality of voice communications across the entire ATC network.

The key metric for voice quality in ATC communications is the Mean Opinion Score (MOS), as defined by the ED136 standard - a crucial regulatory requirement for air traffic control systems. This standard mandates that voice communications maintain a MOS score above 4.0 consistently. However, the analysis revealed concerning deviations from this requirement.

The most striking discovery was the detection of extreme jitter values - reaching up to 398 milliseconds - in the network traffic. To put this in perspective, such high jitter values are extraordinarily unusual for an ANSP's backbone network and can cause significant disruptions in voice communication. These delays create noticeable gaps in the packet stream, potentially affecting the clarity and reliability of pilot-controller communications.

What made this finding particularly noteworthy was its system-wide nature. Unlike the interference issues that affected specific frequencies, these network problems impacted all radios across multiple sites. The analysis pointed to packet loss and jitter as the primary culprits, symptoms that typically indicate underlying network infrastructure issues.

While this discovery was secondary to the main investigation, it demonstrated the value of comprehensive monitoring in ATC systems. What started as an investigation into radio interference led to the identification of network infrastructure issues that might have otherwise gone undetected until they caused more serious operational problems. It served as a reminder that in complex ATC systems, problems in one area can often lead to the discovery of potential issues in others, emphasizing the importance of regular, system-wide monitoring and analysis.

Advanced monitoring: The key to complex problem resolution

The resolution of these complex issues was greatly simplified through R&S AVQA’s advanced voice quality assurance technology that provides comprehensive monitoring of ATC voice communications. This monitoring system operates as a passive solution, capturing and analyzing voice communications in real-time without interfering with operational systems. By implementing fixed time-slice analysis of call quality - examining data in 5-second segments rather than relying on averaged metrics - the system could pinpoint exact moments of interference and quality degradation.

This granular approach proved crucial in this case study. While traditional monitoring might have missed the brief 40-millisecond interference events or masked them in averaged data, the system's ability to analyze each transmission segment revealed the pattern of ghost calls.

Similarly, the detection of the 398-millisecond jitter spikes in network performance would have been difficult to isolate using conventional tools that typically focus on averaged measurements.

The system's capability to simultaneously monitor both voice quality and network performance metrics proved invaluable in understanding the interplay between different issues - from radio interference to network problems. This comprehensive view allowed engineers to distinguish between symptoms and root causes, ultimately leading to the identification of three distinct radio interference sources and the network issue.