Radar altimeters are used on aircraft to measure the height above the surface. They have been in use for decades onboard large airliners and helicopters alike. There are many different systems using these height measurements and the operational impact of misleading or missing data varies greatly depending on the circumstances. Recently, there has been significant media coverage about interference caused by 5G mobile communication networks. In this article we look at the root cause of this.
As we shall see, this problem is not new at all…
A déjà vu
For readers in Europe, Africa, Middle East and Russia: Remember FM immunity? In 1979, the International Telecommunication Union (ITU) extended upper limit of the spectrum for FM broadcast stations from 100 MHz to 108 MHz in the aforementioned regions. This led to potential interference with existing ILS/VOR receivers. Based on this, ICAO Annex 10 vol. I was revised, and new equipment standards (immune receivers) were mandated [1].
Root cause
So, what is happening here appears to be a “natural cycle”. Once a certain aviation system has been developed and is in use on a large scale, technology will inevitably move on. This will enable other (usually non-aviation) systems to emerge. The frequency spectrum is then the confined medium, where conflicts will occur. So, nobody is to blame here, it is simply a natural process.
In the case of today’s radar altimeters, the technical standards were established in 1974 (!) [2] [4]. For comparison: ABBA were in the charts in those days… Many things have changed since then, and technology has significantly progressed.
There is just one problem: Your radar altimeter still “listens to ABBA”, so to speak.
Looking at the present conflict, the often-cited argument that “aviation has been there before” is not a very powerful one. It does not make sense to limit technological evolution due to historic events. HOWEVER: The rulemaking should consider the burden for operators and manufacturers to adapt and allow for a smooth transition.
The key is to find acceptable means for ALL stakeholders, to cope with the transition periods between the technology phases.
Why not simply use another frequency for mobile communication?
A fair question. The answer lies primarily in physics. In simple terms, very low frequencies propagate well in a populated environment, but only allow small data rates. High frequencies on the other hand, permit higher data rates but are limited in their propagation. The C-Band (4-8 GHz) is a very good compromise for good data rates with reasonable propagation. To put a price tag on this: In the US, 1 MHz of C-Band is worth about 65 million dollars! [8]. Now it should be clear, why there is mounting pressure to move other services closer to the radar altimeter band.
Radar altimeter basic principle
Most of the radar altimeters used in aviation employ the so-called Frequency-Modulated Continuous Wave (FMCW) principle [2]. A carrier signal is modulated to produce a “sweep” over a given frequency range. This signal is transmitted via a transmission antenna (red in Figure 3). After propagation, the reflected signal is received via another dedicated receiver antenna (green in Figure 3). Part of the transmit signal is coupled into the receiver path, where it is mixed with the received signal. This permits the determination of the frequency shift, which is representative for the propagation time and thus the distance travelled. The principle is depicted graphically in Figure 3.
The electronics involved are often analog and thus quite sensitive to interference. Only few, more modern designs are based on digital technology and are thus more tolerant to disturbances.
How is this possible? Well, here is the problem:
The Minimum Operational Performance Standard (MOPS) RTCA DO-155 in its legacy form does not mandate specific interference protection [4].
The good news is that the typical radar altimeter installation allows for a Line-Replaceable-Unit (LRU) upgrade, so the wiring and antennae can remain untouched in many cases, should an upgrade become necessary.
Wait a minute – what about HIRF tolerance?
Those involved in airworthiness certification might ask the following question: How come, the existing High Intensity Radiation Field (HIRF) guidelines do not provide sufficient protection? Aircraft can be exposed to a variety of HIRF environments, and the industry has developed standard “HIRF” environments years ago to account for radiation exposure, such as ATC radars, weather radars and other transmitters. Some of these emit much more power than a 5G antenna will. Here is the catch:
The HIRF environments used for current airworthiness certification were calculated based on known transmission sources at the time and assumed certain slant ranges [9]. 5G was simply not there.
The consequence of this is that the electric field strength levels in the current HIRF environments might not ensure sufficient protection against 5G interference [8].
What the industry did: RTCA special committee SC-239
After it became known that the 5G communication system will use part of the spectrum quite close to the radar altimeter band, the Radio Technical Commission for Aeronautics (RTCA) established a task force to investigate the potential interference risk [2]. The results of this investigation were published in a special report [2]. The applied method involved gathering data from mobile communication companies and radar altimeter manufacturers. The interference tolerance of typical radar altimeters was determined using bench testing. Then, a simulation was carried out, to determine the typical interference levels “seen” by an operational aircraft in a 5G-environment. The combination of these two data sources allowed to determine, where the interference margin is insufficient. The principle of this investigation is depicted in Figure 4 below:
The report determined that significant interference has to be expected, should the 5G communication systems be rolled-out without precautions [2]. The transport category airplanes seem to be affected predominantly by the intentional 5G emissions, while helicopters in an urban environment appear to be more susceptible to the spurious emissions [2].
Systems affected
The radar altimeter is usually the only system that permits a direct measurement of height above the surface on aircraft. Other systems, such as barometric or GNSS altitude are referred to another datum (MSL or Ellipsoid). As mentioned earlier, the impact of missing or incorrect data of the radar altimeter can be significant. The following list provides some examples (not exhaustive) of systems that might be affected [3]:
• Terrain Awareness Warning Systems (TAWS)
• Traffic Alert and Collision Avoidance Systems (TCAS II)
• Take-off guidance systems
• Flight Control systems
• Tail strike prevention systems
• Windshear detection systems
• Envelope Protection Systems
• Altitude safety call outs/alerts
• Automatic Thrust Control
• Thrust reversers
• Primary Flight Display of height above ground
• Alert/warning or alert/warning inhibit
• Stick pusher / stick shaker
• Engine and wing anti-ice systems
• Automatic Flight Guidance and Control Systems (AFGCS)
So, every installation will yield a different risk profile. The combination of radar altimeter product, airframe and operating environment will determine the hazard level.
Summary of technical problems
Figure 5 below provides a graphical summary of the technical problems associated with 5G radar altimeter interference.
The legacy MOPS was based on technology at the time and allowed for a relatively “wide” front-end filter without specific interference protection requirements [4]. The manufacturers of radar altimeters have therefore different solutions and varying levels of resilience of their products.
The radar altimeter is typically the only direct source of information about height above the surface and linked to many different systems.
Avionic retrofitting will take time, even if only the LRU’s have to be changed.
Why is there so much activity in the US?
One important aspect to note is that the US frequency allocation permits the 5G mobile communication stations to use frequencies closer to the radar altimeter band and higher transmission power than in Europe [2]. In December 2021, the FAA issued several documents regarding this. They are summarized below:
SAFO 21007, Risk of Potential Adverse Effects on Radio Altimeters when Operating in the Presence of 5G C-Band Interference [3]
Information to operators about the problem and introduction of four different NOTAM types that may be issued if interference is detected.
SAIB AIR-21-18R1 Risk of Potential Adverse Effects on Radio Altimeters [5]
A call for information about product sensitivity and interference tolerance to radar altimeter manufacturers. Additionally, an instruction to aircraft manufacturers to provide risk assessments for misleading / missing radar altimeter data.
Airworthiness directives 2021-23-12 and 2021-23-13
Mandatory revision of AFM/RFM enforcing operating limitations in a 5G environment, unless alternative means of compliance have been accepted by the FAA. The investigation is ongoing and specific aircraft may be cleared for operation as more data becomes available.
The way forward
Based on recent discussions amongst the ICAO Air Navigation Commission and regulatory authorities, the way forward will most likely consist of short-term and long-term mitigation measures as depicted in Figure 6.
The topic is evolving on a daily basis at this time and we shall see over the coming months how the global aviation community deals with this. There is good news on the horizon: The first "5G-tolerant" radar altimeter products are beginning to be advertised and the RTCA is updating the MOPS to include mandatory interference protection for future radar altimeters.
Interested in rotorcraft?
If you find this blog useful and are looking for something similar in the rotary wing domain, I can highly recommend the website of Jakob Dingemans:
Enjoy!
Revision/20220419
References
[1] ICAO, Annex 10, Aeronautical Telecommunications vol. I, 6th ed. 2006
[2] RTCA, Assessment of C-Band Mobile Telecommunications Interference Impact on Low Range Radar Altimeter Operations SC-239, Paper No. 274-20/PMC-2073, 2020
[3] FAA, SAFO 21007, Risk of Potential Adverse Effects on Radio Altimeters when Operating in the Presence of 5G C-Band Interference, December 2021
[4] RTCA, DO-155 Minimum Performance Standards Airborne Low-Range Radar Altimeters, 1974
[5] FAA, SAIB AIR-21-18R1 Risk of Potential Adverse Effects on Radio Altimeters, December 2021
[6] FAA, AD 2021-23-12 Transport and Commuter Category Airplanes: Amendment 39-21810; Docket No. FAA-2021-0953; Project Identifier AD-2021-01169-T, December 2021
[7] FAA, AD 2021-23-13 Various Helicopters: Amendment 39-21811; Docket No. FAA-2021-0954; Project Identifier AD-2021-01170-R, December 2021
[8] RTCA, online presentation: Interference Risk to Radar Altimeters from Planned 5G Telecommunication Systems, November 2020
[9] SAE, ARP 5583, Guide to Certification of Aircraft in a High Intensity Radiated Field (HIRF) Environment, 2003
[10] TUV Rheinland, Grundlagen der Luftfahrzeugtechnik in Theorie und Praxis, 1985