| A well-controlled source of repetitive infrasound emissions was previously identified and has
been related to development and acceptance tests of the European Space Agencies
ARIANE-5 main engine. The propulsion testing facility of the German Aerospace Agency
(DLR) near Heilbronn, Southern Germany, is a distance of about 320 km away from the
International Monitoring System (IMS) station IS26 in east-southeasterly direction. In the
past, signals associated with these propulsion tests could normally be detected at IS26 during
winter months, but not during summer months, reflecting the changes in atmospheric
conditions between winter and summer.
Over the last year, DLR has prepared to conduct a series of seven propulsion tests which
started in November 2011; with interim times between tests of 3-4 weeks it will last until late
March or early April 2012. With mobile infrasound recording equipment available at BGR
we planned to record the infrasonic wavefield along the path to IS26 at regular distances
starting as close as 20 km from the source. Our aim is to study sound propagation
from direct paths mainly involving the tropospheric layer through the “zone of
silence” to distances close to IS26, where paths through stratospheric layers are
followed.
Preliminary results show that during the relevant winter season direct path propagation
can be observed to some 40 km from the propulsion test source, even at seismographic
stations where the acoustic wave couples into the ground. The tests are also observed at IS26,
and waveform duration and f-k-analysis confirm the signals to be associated with the GT-type
propulsion tests. |