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Decomposition of Infrasonic Signals Using a Wavelet Transform

INTRODUCTION Since 1971 the infrasound was recorded at the recording stations, operated by the Swedish Institute of Space Physics, located between 59.8'N (Uppsa1a) and 6.78'N (Kiruna). The most dominating man-made infrasound with frequencies around 2 Hz was found to be generated by supersonic flights, in particular by regular flights of Concorde. These signals may be recorded at distances of at least 4000 kilometers due to a focusing mechanism (Liszka, 1978). The recording equipment used at all stations until 1994 was operated at a fixed frequency of 2Hz (for description see Lundqvist, 1986). The accuracy with which the angle of arrival was determined by that equipment was rather poor: on average ±1O degrees. For that reason the fine structure of the angle of arrival of infrasonic signals could not be performed. However, it could be suspected that at least a part of the structure observed on the azimuth vs time curves of the Concorde signal was real and not only due to measuring errors. Not until 1994, when a high resolution, broad band recording equipment (Waldemark, 1994) was put into operation at two infrasonic stations (Jamton and Uppsala), it became possible to study the fine structure of the angle of arrival of the Concorde signal. Results published in 1995 (Liszka and Waldemark, 1995) have shown a mode-structure of the signals.