AVSI is supporting efforts to preserve the safe operation of radio altimeters through research being conducted under AFE 76s2 – Out-of-band Interference with Radio Altimeters. Radio altimeters are safety-critical equipment that have been used for decades in all types of aircraft, providing accurate indication of height above ground level for applications such as cockpit display, terrain avoidance warning systems, and auto-landing systems. Most altimeters used in commercial transport aircraft are frequency-modulated continuous wave (FMCW) with a typical chirp bandwidth of ~100-180 MHz centered at 4300 MHz.
Recently proposed changes to the C-band spectrum between 3700 MHz – 4200 MHz will potentially alter the RF interference environment in which radio altimeters operate. One objective of AFE 76s2 was to determine if proposed mobile telephone use of spectrum between 3700 MHz – 3980 MHz would present harmful interference to radio altimeters. Laboratory measurements from AVSI testing indicated that commonly used altimeters are susceptible to OFDM waveforms centered in this part of the spectrum. Test results were provided to spectrum regulators to encourage that proposed C-band changes consider provisions that ensure that radio altimeters can continue to operate safely.