Currently, the only available means of reporting clear air turbulence (CAT) is the pilot report (PIREP), whereby a pilot experiencing turbulence reports their location and associated data. In this report, a system is proposed that would allow the detection of CAT through infrasonic emissions.

This system is comprised of an array of three infrasonic microphones, where each microphone is spaced 30.48 m apart in an equilateral triangle. The infrasonic microphone is Model 377M06, designed by Langley Research Center and manufactured by PCB Piezotronics, Inc. under contract to Langley. It is prepolarized (electret), thus obviating the need for an external, polarizing voltage.

Each microphone is installed in a sub-surface, box-shaped windscreen made of closed-cell polyurethane foam with a removable, but tightly fitting lid. The windscreen material is waterproof, thus permitting all-weather operation. The microphone is mounted on an aluminum reflector plate such that the microphone cartridge is positioned at the center of the windscreen. The frequency response of the system is nominally 0.03 to 100 Hz, limited by the microphone response at the low end, and by the windscreen transmissibility at the high end. The infrasonic detection is periodically calibrated by a pistonphone, built in-house, generating 110 dB sound pressure level at 14 Hz.

The NASA Langley Clear Air Turbulence System (CATS) data acquisition, file handling, and post-processing data analysis are accomplished by software programs developed in-house. For identification of an infrasonic source from received signals, four criteria are Concomitancy, Coherence, Directionality, and Characteristic Signature.

With Concomitancy, when the source is present, the signals must appear. When the source is absent, the signals must disappear. In the case of atmospheric turbulence, the source can only be identified through a “marker” (PIREP), in which case only the first statement applies. If a PIREP subsequently disappears, the turbulence can still be present and yielding emissions. With Coherence, the received signals must “look alike” on all three microphone channels, i.e., reveal high coherence. Coherence is a measure as to whether the signals received across the array originate from the same source. With Directionality, the angle of arrival determined from an analysis of received signals must conform to the actual direction from a source to the array. The method used here to determine the angle of arrival is based on “slowness mapping.” With regard to Characteristic Signature, the most commonly used signatures of acoustic sources are based on spectral information, a practice that is maintained here. The spectrum of atmospheric turbulence emissions measured in this investigation differs substantially from those reported in the past. A single array provides the direction to the source, but a second array will permit the determination of the source location through triangulation. A national network of 35 ground-based infrasonic arrays is expected to cover the U.S. territory for early warning of clear air turbulence.

This work was done by Qamar Shams and Allan Zuckerwar of Langley Research Center. LAR-17317-1/836-1.