RADALT Issues

5G high-speed telecommunication networks are being slowly rolled out across the world. It is a fast, high-density information streaming medium that operates in the 3.4 to 4.0GHz frequency range. It is short-range and is susceptible to attenuation by buildings and trees, which necessitates a vast network of antennae. The associated infrastructure is both resource-intensive and very expensive, so 5G implementation is going to be slow.

The potential problem is that most Radio Altimeters (RADALT) work in the 4.2 to 4.4GHz band. The US Radio Technical Committee for Aeronautics has conducted an assessment of 5G in the frequency band 3.7 to 3.98GHz and found the potential for harmful interference to aviation RADALT systems. The RADALT is a mandated critical aircraft safety system used to determine an aircraft's height above terrain or water. RADALT equipment can be directly integrated into several aircraft systems and functions for various flight profiles.

These can include terrain awareness, aircraft collision avoidance, wind shear detection, flight controls, auto-land, auto-hover and transition up/down systems in rotary aircraft. Interference could lead to malfunctions in aircraft systems and a potential loss of situational awareness during critical phases of flight. Both 5G enabled portable electronic devices and ground-based antennae have the ability to compromise RADALT equipment.

Worldwide testing and evaluation have been limited so far with various nations looking into potential 5G flight safety issues. Canada, Japan, and the USA are introducing guard bands of between 0.2 and 0.7GHz on either side of the 4.2 to 4.4GHz RADALT frequency band. Canada is introducing both exclusion and protected zones around major airports and heliports that have auto-land systems (CAT II and III ILS). Ground-based antennae will be tilted downwards and transmission power levels reduced.

Uncrewed drone operations will not be permitted in the 3.45 to 3.65 GHz band. Individual aircraft types will require testing and certification with regard to exposure to 5G signals, which will be a time consuming and expensive process. Transport Canada will be producing maps showing 5G zones, allowing crews to plan to avoid them in the low-level environment. ICAO is liaising with aircraft manufacturers to decide the best way ahead to mitigate this issue. In the interim, operators are reminded to remain vigilant regarding their aircraft behavioural characteristics and report any undesirable or objectionable aircraft behaviours observed during manual or automated RADALT systems operations.

In simple terms, the problem with 5g (specifically in the United States) is the power it uses, somewhere typically in the region of 2.5 times that of other countries' 5G networks. The antennas point upwards, which means powerful signals going right into the RADALT of aircraft, whereas other countries point them down. This might be why the USA has seemingly bigger issues with interference than elsewhere. Also, the interference potential from 5G in C band (3.7 to 4.2GHz) is greater in the US because the 5G allocated band goes up to 3.98 GHz, whereas in other places, including Europe, it stops at 3.8GHz or even 3.7GHz.