Defense Intelligence Reference Document Detection And High Resolution Tracking Of Vehicles At Hypersonic

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Infrasound stations, or observatories, could also be used to monitor the acoustic signature of
meteors or supersonic aircraft based on their shock waves or the disturbances caused by
their turbulent wake. Some meteors, for example, produce subsonic sound from 1 to 5 Hz
for several minutes during their flight through the atmosphere. This data could be developed
to identify the speed, altitude, and type of object moving through the atmosphere.
Seismic
When a sonic boom is generated by a meteor or an aircraft, the sound waves pass through
the atmosphere at the speed of sound in air at about 343 m/s. Infrasound stations pick up
the pressure variations caused by the shock wave many minutes after the event occurred,
depending upon the distance between the supersonic object and the station. When a Mach
cone generated by a shockwave strikes the ground as shown in Figure 18, the pressure pulse
is propagated into the ground and travels as a high-speed N-wave at a velocity far exceeding
the speed of sound in air.
Ground-based vibrations caused by sonic booms have been detected by seismic sensors
originally designed to detect earthquakes as documented by Cates and Sturtevant. 27 Caltech
and the U.S.G.S. operate 200 seismographic stations throughout Southern California as part
of the TERRAscope seismic network. The network employs detectors that can sense ground
vibration between 1 and 20 Hz.
The network was actually used to detect the sonic boom and pressure vibration induced by
the flight of supersonic aircraft and reentry vehicles. On 9 December 1993, the network
successfully detected the flight of an SR-71 flying over Edwards Air Force Base at a
maximum speed of Mach 3.2. On 30 January 1992, the network was also used to detect the
reentry of the space shuttle Discovery, STS-42, over the Pacific Ocean and landing at
Edwards Air Force Base.
Another Discovery landing on the west coast was monitored by 66 seismic stations in
Washington and Oregon. The shuttle was monitored by this seismic network for 500 km of
its trajectory through the atmosphere. The seismic data from these stations showed the
hyperbolic shape of the Mach cone as it impacted the surface of the earth. The angle of the
Mach cone showed that the shuttle was traveling at Mach 14, corresponding to an altitude of
55 km.
One interesting investigation that involved the use of these seismic arrays involved
mysterious sonic booms heard over Southern California in 1991 and 1992. By using data
from the TERRAscope array, the source of these disturbances was traced to two F-4 Phantom
aircraft flying over Edwards Air Force Base at speeds near Mach 1.
Since the TERRAscope array is designed to detect seismic activity, including earthquakes and
volcanoes, the detection of meteors and supersonic aircraft generate unwanted data.
Caltech has been adding atmospheric pressure transducers to its seismic network to detect
and remove airborne signals from the seismic data. Conversely, by combining the infrasound
network data operated by the Los Alamos National Laboratory with data from seismic arrays,
data from explosions, volcanic eruptions, and other terrestrial sources can be eliminated
from infrasound data to better detect the presence of meteors, reentry vehicles, and
supersonic aircraft.
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