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Acoustic/Seismic Ground Sensors for Detection, Localization and Classification on the Battlefield. Including Small Aircraft


Vivere est militare!
A. Sedunov & N. Sedunov & A. Sutin & H. Salloum. Stevens Institute of Technology. 2007 said:
Passive acoustic localization of small aircraft

Low Flying Aircraft (LFA) may be used to smuggle illicit drugs or illegal immigrants across borders. Stevens Institute of Technology has built Acoustic Seismic Aircraft Detection (ASAD) system that demonstrated detection, tracking and classification of LFA by their radiated noise using passive acoustic sensors. It consists of several nodes deployed in a wide area, where each node acquires signals from an attached cluster of five microphones. This paper presents the basics of the methodology of target localization using this specific setup, based on the direction of arrival (DOA) finding sensors followed by triangulation of the direction of arrival estimates from two or more nodes. DOA is found using the time difference of arrival (TDOA) estimates from several pairs of microphones. Software for prediction of performance was developed and was used to provide accuracy of localization estimation in field tests. The field tests were conducted with various types of LFA - Cessna, Helicopter, ultralight and the acoustic tracking results were compared with GPS ground truth.

1. Introduction
Small planes, helicopters and ultralight aircraft are frequently used to cross borders, due to their potential for smuggling large quantities of illicit drugs as well as bringing illegal immigrants. These aircraft are typically being flown at low altitudes and are hard to detect with radar, thus exploiting gaps in coverage by the interdiction detection network [1].

The typical approach is to use special radars to detect Low-Flying Aircraft (LFA) having low radar cross section. The application of passive acoustics can provide several advantages over radars: acoustic methods are simpler and the equipment that is needed is low-cost and easy to deploy. Acoustics may allow detection of Targets of Interest (TOI) at any altitude, including ground targets, as well as slow-moving, and stationary targets. Additionally, such systems do not produce any radiated signals and cannot be detected by EM wave detection equipment. In the literature, many examples of acoustic systems distributed in a wide area can include coherent processing of widely separated microphone sensors [3], while others rely on processing compact clusters and DOA finding [3,5,6]. A combination of acoustic detection and other types of surveillance may prove complementary. A system based on multiple sensors is presented in [7,8].

Stevens Institute of Technology has developed the Acoustic Seismic Aircraft Detection (ASAD) and conducted a test in a mountainous terrain to establish the applicability of the technology and estimate the performance. A high level summary of the results has been published in [2].

This article is focused on the localization aspect of the system's functionality: the approach used to resolve the direction of arrival (DOA) by using as few as 5 sensors arranged in compact volumetric arrays and then to localize the source based on such measurements from multiple widely separated such microphone clusters. A similar approach is recommended in [4], however it is based on a more computation-intensive MUSIC algorithm and only a two-dimensional problem is considered. The basic approach to predicting the performance of such a system in the context of mountainous terrain is discussed, creating a simple model that can predict the usefulness of a given sensor placement and thus can inform deployment decisions of the user of the acoustic system.

Continue reading: Passive acoustic localization of small aircraft by A. Sedunov, A. Sutin, H. Salloum, N. Sedunov. Published in Journal of Acoustical Society of America, 2013
A. Lemer & F. Ywanne. Thales Underwater Systems. 2013 said:
Acoustic-Seismic Ground Sensors for Detection, Localization and Classification on the Battlefield


1.2 Advantages and drawbacks of Acoustic and Seismic sensors
In Air Acoustics and Seismic sensors exhibit several interesting advantages for battlefield applications which among which Non-Light-Of-Sight (NLOS) detection, Fully passive (stealthy, low power) and panoramic (360°) coverage, Non Cooperative Target Recognition (NCTR) capabilities : acoustic and seismic signatures of some targets contain highly revealing features (e.g. helicopters) and low cost (potentially expendable).
These advantages are somewhat counterbalanced by the following drawbacks :
  • Their performance are sensitive to the environment. For acoustic sensors, performance are weather sensitive, mainly because long range acoustic propagation, rather complex, depends on wind and temperature evolution with altitude. Wind also creates an additional low frequency non stationary noise. As a result, detection range is weather dependent and often anisotropic, especially at long distance. For seismic sensors, behaviour strongly depends on ground composition (attenuation, wave velocity, interaction with acoustic waves).
  • The sound celerity in air is rather slow (340 m/s), which induces significant detection delay at long range. Seismic waves velocities are very variable (depending on ground, but also on range through the depth of propagation paths).
Acoustic/Seismic sensors can be used as stand alone systems, or can be coupled with other sensor technologies. Indeed, they can offer :
  • Alert and target cueing for LOS passive sensors,
  • A reduction of active sensors vulnerability,
  • Complementarities for coverage, localisation and NCTR purposes.
Depending on the context, in-air acoustic and seismic sensors can therefore be considered as good complements (or even sometime alternatives) to more traditional battlefield sensors such as cameras or radars.

Continue reading: Acoustic/Seismic Ground Sensors for Detection, Localization and Classification on the Battlefield [PDF, English] by Alain Lemer and Frederique Ywanne, Thales Underwater Systems S.A.S, 2007.
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