The various companies purchased the rights to US-wide spectrum from the Federal Communications Commission in December of 2020 for $69 billion, allowing them to gain access to up to 280 MHz of combined bandwidth. This bandwidth provides faster upload and download speeds than the current 4G LTE services at a frequency which demonstrates better range and obstacle blockage performance.
The sale of this spectrum brought about major issues within the FAA, the commercial airline industry, and the DoD (Department of Defense), who all addressed concerns over the potential danger of interference with the radar altimeter system on aircraft. For this reason, the FAA proclaimed that pilots are prohibited from utilizing auto-landing and other flight systems used at low altitudes where the new 5G signals may interfere with radar altimeter systems. This ruling sparked mixed reactions from air travel industries and securities markets.
Unfortunately for travelers, this resulted in major international airlines canceling flights to the US. Furthermore, several domestic routes were canceled at 50 airports which were highlighted as areas of concern by the FAA. The consequences of such actions were wide-ranging, causing global air travel difficulties, making aircraft manufacturer stocks plummet, and driving avionics manufacturers to begin testing and certifying their systems against the new threats.
Thousands of aircraft around the globe began facing scrutiny, from major commercial airliners to personal aircraft and helicopters. Although past and current generations of radar altimeters were never designed with the capacity to anticipate or reject strong adjacent signals, radar altimeter developers and aircraft manufacturers are now forced to check and test their designs to assess how they will perform when flying near a 5G tower during takeoff or landing. By the end of January, the FAA announced approvals for about 78% of the US commercial fleet to carry out low-visibility landings at airports where wireless companies deployed C-Band.
Radio frequency (RF) systems are regularly used on aircraft platforms for a myriad of missions, such critical components for autopilot and instrument landing systems (ILS). Previously, the RF band of 3700-4400 MHz was designated for use by Fixed Service and Fixed-Satellite Service links. Operators that utilized this bandwidth had very low emissions or uplink stations that were positioned far from airport locations.
While radar altimeter use was internationally recognized and protected in the range of 4200-4400 MHz, in March of 2020, the FCC made a decision to reallocate the band, dedicating the 280 MHz of spectrum from 3700-3980 MHz to support the telecommunication industry’s need for additional bands. Radar altimeter systems generally operate between 4200-4400 MHz. You may ask yourself: “What was the problem if they do not use the same frequency band as telecommunications companies?” Turns out that altimeter systems are particularly sensitive to interference.
In the event of interference, it is possible for a radar altimeter to provide inaccurate readings of the distance to the ground. Moreover, these systems are often coupled with automated landing and takeoff systems which can make flight controls quickly reduce or apply engine thrust, reduce flaps, lower landing gear, or deploy other automatic safety actions that can cause aircraft to stall and jeopardize passenger safety.