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RF Filters Can Keep Air Travel Safe in the 5G World

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by Adam Moya, Business Development & Strategic Partnerships, Benchmark

Fifth Generation (5G) wireless networks are filling frequency spectrum beyond 2.5 GHz. Earlier this year, North American 5G carriers started using newly licensed bandwidth at C-band frequencies from 3700 to 3980 MHz, raising concerns of interference with aircraft radar altimeters long operating from 4200 to 4400 MHz. Radar altimeters have coexisted for decades with low-level satellite communications (SATCOM) signals at C-band frequencies (3700 to 4200 MHz).

However, in the U.S. the latest addition of 5G spectrum is near in frequency to the bandwidth reserved globally for radar altimeters. When those C-band 5G signals are coming from cell towers and transmitters near airports, they pose the threat of interference for aircraft radar altimeters and endangering safe landings. The coexistence of telecom and altimeter signals is vital. However, it will require a fresh look at aircraft radar altimeters, especially updates that help them reduce 5G C-band signals as interference.

Aircraft radar altimeters have long operated within the globally allocated frequency band from 4200 to 4400 MHz, with only low-level fixed service (FS) and fixed satellite service (FSS) surrounding that band. The potential danger of spectrally close 5G C-band signals has spurred the U. S. Federal Aviation Administration (FAA) into action, producing guidance such as Airworthiness Directives (ADs) on specific aircraft and altimeters.

Recent FAA ADs regarding 5G C-band interference of radar altimeters have targeted older Boeing planes, such as 747 aircraft with untested, legacy radar altimeters. The ADs explain that airlines using those older Boeing 747s must revise their operating procedures, such as takeoff and instrument landing system (ILS) approaches, when 5G C-band interference is present.

The FAA, aviation industry, and interested researchers are feverishly using spectrum analyzers and other RF/microwave test equipment to detect and analyze 5G C-band signals to learn more. The behavior of C-band 5G signals may be predicted when they are propagating over the ground and terrain, but such signals are not well understood at the altitudes and flight angles of aircraft relative to 5G C-band transmitters (cell towers).

The computer-intensive nature of 5G wireless networks makes them difficult to model. Typical wireless network antennas (Figure 1) use electronic beam steering, and transmission power levels vary from cell sites near airports. The radar altimeters add to the complexity of the modeling by operating over changing terrain at different signal levels and changing altitudes.

What is known is that many critical-to-flight electronic aircraft systems rely on altitude data from radar altimeters, and they must be fully inter-operational within these adjacent 5G C-band signals. As many of these radar altimeters are fully established and certified as part of the aircraft operations, deploying an update is complex. Upgraded RF filtering is among the hardware upgrades that can ensure reliable and accurate radar altimeter operation in the presence of high-power 5G C-band EM emissions in a simplified manner.

Filtering to Improve Altimeters

Additional RF filtering within a radar altimeter system is part of setting its level of interference tolerance—how close it can operate to an interference source without suffering disruption of service. The FAA, FCC, and various organizations with interests in both aviation and telecommunications, such as the National Telecommunications Information Administration (NTIA) and Radio Technical Commission for Aeronautics (RTCA), respectively, are devoted to developing a “flight plan” that will ensure the safe and reliable coexistence of 5G cell sites and radar altimeters for both fixed-wing and rotary-wing (helicopters) aircraft as soon as possible.

Additional filtering is a practical part of the spectral management strategies needed to minimize the threat of interference to altimeters from these 5G C-band signals. More selective RF filters within commercial and civil radar altimeters can improve altimeter interference tolerance.

Both interference and distance from the ground impact an altimeter’s sensitivity and dynamic range. Radar altimeters typically have several microstrip or stripline patch antennas mounted to the bottom of an aircraft, with separate antennas for transmit and receive functions. By transmitting signals to the ground and measuring the reflected returning signals, the time between signals, t, can be used to calculate the range, R, to the ground and surrounding terrain (wherever the signals bounce) according to the relationship t = 2R/c, where c is the speed of light.

Radar altimeters are used on all types of civil, commercial, and military aircraft, including transport and cargo airplanes, helicopters, and unmanned aerial vehicles (UAVs). Both civilian and military aircraft are susceptible to interference. Civilian aircraft primarily use Frequency Modulated Continuous Wave (FMCW) radar, while military aircraft use pulse width modulation to support Low Probability Detect/ Low Probability Intercept (LPD/LPI). In both cases, inline filtering is attractive because it is independent of waveform and aftermarket capable.

Homodyne and heterodyne architectures are used in radar transceivers, with RF filtering following the antennas for radar receive and transmit functions: between the antenna and a low-noise amplifier (LNA) for receiving and between the antenna and a power amplifier for transmitting. A minimum 3 dB filter passband of 200 MHz from 4200 to 4400 MHz with low passband insertion loss enables a radar altimeter receiver to detect and process return signals reflected by the terrain while maintaining high sensitivity to return signals that have suffered high terrain or atmospheric attenuation.

At the same time, high stopband rejection suppresses EM energy outside of the radar altimeter passband, whether from 5G towers or other sources. Such filtering is typically accomplished with an RF/microwave bandpass filter with stopband rejection sufficient to suppress interfering signals to levels below the noise floor of the altimeter’s receiver.

Practical Solutions for Retrofits and Upgrades

Adding filtering to legacy aircraft and radar altimeter systems is not a simple plug-in solution because of the highly integrated nature of an aircraft’s aviation systems. However, for older, legacy aircraft and aviation systems, where coaxial interconnections between an altimeter’s antennas and receivers are accessible, a coaxial filter may be inserted into the cable path between the receive antenna and the altimeter receiver antenna port as a solution. A coaxial bandpass filter with a 200 MHz passband at a center frequency of 4300 MHz and at least -40 dBc stopband rejection will reduce interference from 5G C-band sources. The filter must meet system-level requirements such as low VSWR and sufficient power-handling capability.

An example of such an approach for older, legacy aircraft and their radar altimeter systems is a B series combline bandpass filter from Benchmark Lark Technology (Figure 2). The series covers center frequencies from 1 to 20 GHz with passbands as narrow as 1% of the center frequency, so a filter with 200 MHz passband at 4300 MHz center frequency offers better than -55 dB rejection in both stopbands. The filter is added to the altimeter system’s coaxial interconnections utilizing SMA connectors.

What’s Next in Radar Altimeters?

The FAA continues to pursue reliable solutions for compatibility between 5G C-band networks and radar altimeters at 4200 to 4400 MHz. The strategy will involve many new altimeter standards and new radar altimeter designs. Those new designs must also meet growing demands for shrinking size, weight, and power (SWaP) requirements. The latest aircraft radar altimeters employ a variety of modulation methods, including FMCW, pulsed CW, and frequency-hopping techniques, requiring low-VSWR bandpass filters that can provide the required out-of-band signal rejection with reflection- and distortion-free passband performance.

As aviation system integrators attempt to pack more functions into smaller spaces, selecting an RF filter for a new radar altimeter design will weigh several factors, including cost, performance, and SWaP requirements. To coexist with 5G C-band signals, bandpass filters for radar altimeter receivers must minimize insertion and return losses to capture low-level return signals. However, to achieve high performance in the transmit portion of radar altimeters, filters should exhibit excellent signal fidelity in the smallest size possible while also handling significant transmit power levels.

An FMCW radar altimeter may transmit at signal power levels as high as 2 W. For much shorter signal durations, a pulsed CW altimeter may transmit at power levels as high as 10 W, although the pulse lasts for 20 μs or less. The RF filters must preserve the FMCW or pulsed signal characteristics with the lowest possible passband signal loss while providing maximum rejection of unwanted interference within its stopbands.

An ideal radar altimeter filter passband would suffer low signal loss from 4200 to 4400 MHz while providing high rejection (-40 dBc or better) of signals below 4200 MHz. One challenge for any form of RF filtering meant to suppress signal energy from 5G C-band signals is their proximity in frequency to the radar altimeter’s passband. Any interfering signals must be attenuated without degrading the signals within the slightly higher frequency passband.

Threading this needle requires a filter with a sharp cutoff frequency. High signal rejection is desired immediately below the cutoff frequency (4200 MHz), in the filter’s stopband, with no loss just above the cutoff frequency, in the passband (4200 to 4400 MHz). Moreover, this sharp filter response is required without degrading other filter performance traits, such as passband amplitude ripple (which tends to be high for an RF filter with a sharp cutoff). Low passband filter loss, such as 0.5 dB, helps maintain good altimeter receiver sensitivity. Amplitude ripple within ±0.1 dB supports good radar altimeter amplitude response, while phase ripple within ±1 deg. helps to minimize phase errors when measuring radar return signals.

With many differences in radar altimeter transceiver architectures, no single drop-in filter solution can achieve the highest possible 5G C-band interference suppression for all radar altimeters. Depending upon the SWaP requirements of an aircraft and its radar altimeter system, altimeter upgrades for reliable suppression of 5G C-band interference may call for different mechanical filter construction.

Filters based on surface-mount technology support the miniaturization of new radar altimeter designs. Benchmark Lark Technology’s SD series bandpass filters occupy much less space on a PCB but provide minimum -42 dBc rejection of 5G C-band signals while passing desired altimeter signals from 4200 to 4400 MHz with low insertion loss (Figure 3). They meet MIL-STD-202 requirements and handle wide temperatures from -40° C to +85° C. Despite their small size, 0.7 x 0.36 x 0.2”, these SMT filters can handle typical continuous wave and pulsed power levels from radar altimeter receive antennas (less than 1 W) without performance variations due to thermal effects.

Growing global demand for 5G services has resulted in 5G’s occupation of a portion of C-band frequency spectrum once considered isolated. Once deployment began, various government agencies reexamined the risk of interference to essential radar altimeters operating in adjacent, slightly higher frequencies. Well-controlled, 5G wireless transmissions should pose no problems for aircraft radar altimeters. However, when those transmissions become distorted in any way, they can block or interfere with the vital, invaluable operation of aircraft radar altimeters. At that point, custom RF filters can be a valuable tool to upgrade the performance of many commercial and civil radar altimeters.

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