Testing Processes to Ensure Radar Altimeters Not Impacted by C-band

February 8, 2022 Anritsu Company

February 8, 2022

Even before the January 19th launch of 5G in the United States, the latest generation of wireless technology was getting a lot of press. Not all of it was positive, though, as the Federal Aviation Administration (FAA) voiced concerns that the C-band spectrum used to rollout 5G may cause interference with avionics equipment, specifically radar altimeters. While federal regulators and billion-dollar companies debated the issue, there was one thing without dispute – network testing is a necessity.

Fortunately, the two sides – mobile operators and the aviation community – have been trying to find a solution. It began in late 2021 when carriers agreed to lower the power on their 5G towers for six months. It continued in mid-January with an agreement not to turn on 5G antennas within two miles of airport runways.  

plane tower

Given the $80+ billion investment by mobile operators, the rollout of 5G was inevitable. Another certainty is that sound testing processes need to be in place to ensure interference does not occur.

Importance of Radar Altimeters

At the crux of the situation is that 5G near airports may interfere with the signals transmitted by radar altimeters. The Federal Communications Commission (FCC) conducted testing previously and concluded the 400 MHz guard band between the 3.8 GHz C-band and 4.2 GHz signal used by altimeters sufficiently addressed the issue. The FAA still has concerns, however.  

The reason for the trepidation is the importance of the altimeter. Data acquired from radio altimeters informs other safety equipment on aircraft, including navigation instruments, terrain awareness, and collision-avoidance systems. Altimeters do this by transmitting an RF signal from the aircraft down to the terrain and receive the reflected signal back. The time delay of the received signal is used to determine Above Ground Level (AGL) altitude.

Radar altimeters are either pulsed or frequency-modulated continuous wave (FMCW). Commercial aviation predominantly uses FMCW. The RF front-end in FMCW altimeters typically uses homodyne architecture, with the received signal mixed with the transmitted signal to measure the frequency difference. With linear FM chirps, this frequency difference is directly proportional to the propagation time delay, and thus altitude.

Total Test Approach  

Inherently wideband systems, radar altimeters have no requirements for front-end rejection currently. So, they may be susceptible to blocking. That is the reason for the interference concern – and why testing procedures need to be continuous.

Effective testing of base stations around airports will require an ongoing process. Proper deployment and installation will ensure that the 5G antennas do not cause interference with airline altimeters as they approach airports for landings. Continual and regular testing to ensure that base station transmissions continue to fall within the specifications and not impact an altimeter’s ability to provide accurate data to the safety systems of a plane must also be done.

All this testing requires a comprehensive toolbox for regulators, mobile operators, members of the aviation community, and/or any other professionals responsible to ensure the 5G transmissions on the C-band do not create interference. For deployment and installation, a real time spectrum analyzer (RTSA) and interference hunting solution are required. When it comes to ongoing maintenance and monitoring, interference hunting tools and remote spectrum monitoring systems are necessary. Fortunately, Anritsu provides a comprehensive portfolio to address all the requirements.

Real Time Spectrum Analysis

Anritsu’s Field Master™ Pro MS2090A (figure 1) RTSA delivers performance never previously available in a compact, handheld instrument. With continuous frequency coverage from 9 kHz to 54 GHz, The Field Master Pro, with a displayed average noise level (DANL) of -164 dBm and Third Order Intercept (TOI) of +20 dBm (typical), makes measurements such as spectrum clearing, harmonics, and distortion more accurate than previously possible.

Field Master Pro MS2090 Real Time Spectrum Analyzer.
Figure 1: Field Master Pro MS2090 Real Time Spectrum Analyzer.

A 110 MHz modulation bandwidth coupled with best-in-class phase noise performance maximizes measurement precision. Typical  amplitude accuracy of ±0.5 dB provides confidence when testing transmitter power and spurious – key measurements to prevent interference with altimeters.

Ongoing Spectrum Monitoring

A remote spectrum monitoring system (figure 2) will facilitate the identification of interference from 5G base stations. Patterns of unwanted signal activity can also be examined, providing an efficient way to characterize and locate the source of the interference problem.

Remote spectrum monitoring solutions can help ensure C-band transmissions do not interfere with radar altimeters.
Figure 2: Remote spectrum monitoring solutions can help ensure C-band transmissions do not interfere with radar altimeters.

Spectrum monitoring can serve to enforce compliance with the parameters agreed to by aviators, operators, and regulators. Also, remote spectrum monitoring allows manpower to be effectively multiplied over a wider area and when engineers are not scheduled to be on duty. Air traffic patterns and runway activity will be able to be maintained without interference from the 5G antennas efficiently with such a solution.

Anritsu remote spectrum monitors can be used with Vision off-the-shelf software application. Vision continuously monitors selected spectrum bands to locate any spurious and/or interfering signals. Historic records of the spectral activity over time are saved to a database that can be configured to sound alarms when out-of-band signals are detected. The spectrum records are searchable for detailed analysis over an extended period of time.

To learn more, you can visit a technologies page dedicated to interference, causes, and test solutions. 

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