Restart of the Concorde flights

By Läslo Evers

1. Recording of infrasound

The Concorde restarted its flights after a period of (infrasonic) silence. DIA recorded infrasound from a southwestern direction during the afternoon of 2001, November 5. The infrasound is related to the first public announced flight of the Concorde (infrasound from test flights was recorded during the previous weeks).
Figure 1 shows the records of the 15 microbarometers of the Deelen Infrasound Array (DIA). The time axis zero time is 15h32m GMT on 2001, November 05. After 140 seconds coherent energy can be identified. Coherent energy before 140 are so-called microbaroms. Microbaroms are infrasound caused by standing waves in the ocean, pushing on the atmosphere, close to storm centers.


Figure 1: Infrasound recordings of the 15 microbarometers of DIA. The time axis zero time is 15h32m GMT on 2001, November 05. Coherent energy related to the Concorde can be identified after 140 seconds.

2. Detection and localization

The coherent signals are detected and localized in the frequency domain. Time differences of the signals while traveling over the array are translated to phase differences in the frequency domain. Figure 2 shows coherency as a function of time and frequency. Maximum coherency of 360 is found after 182 seconds at a frequency of 0.78 Hz. The energy is localized by performing frequency-slowness analysis (see inlay). The event characteristics are resolved by the white vector in the inlay in figure 2. The energy appears from 249 degrees with respect to the North (thus from the southwest). The apparent sound velocity of the signal while traveling over the array is resolved as 397 m/s. The top frame of figure 2 shows the best beam. This is the sum of the 15 traces of figure 1 after being aligned following the resolved event characteristics.


Figure 2: Results of coherency analysis of the signals. Coherency is plotted as a function of time and frequency in the lower frame. The most coherent energy, around 180 seconds and 0.78 Hz, is analyzed in the inlay. The white slowness vector resolves 397 m/s as apparent sound speed and 249 degrees as azimuth. The top frame shows the best beam.

The best beam and 15 aligned traces are also plotted in figure 3. Clearly, the summation reduces the incoherent noise and therefore increases the signal-to-noise ratio. The Concorde signal appears as a series of coherent packages of energy. Several atmospheric refractions of the energy on its way to DIA explain this repetitive character.


Figure 3: The 15 traces of DIA in blue and the best beam in red. Summation reduces the noise on the signal and leaves a clear signature of the sonic boom caused by the Concorde.

The resolved azimuth of 249 degrees is plotted is figure 4. The Concorde is identified by the infrasound it emits when going form supersonic to subsonic speed (sonic boom) as preparation for landing.


Figure 4: Map showing the projected azimuth of 249 degrees from DIA. The infrasound is of sonic boom type caused by the Concorde lowering its speed for landing and going from supersonic to subsonic speeds.

November 2001
Läslo Evers