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Acoustic Vector Sensors (AVS) for Battlefield surveillance of Rockets, Artillery and Mortars (RAM), ground vehicles and UAVs

Information is critical to military success, and the data is gathered through sensors on a range of platforms, including aircraft, unmanned aerial vehicles, weapon systems, ground vehicles and even from soldiers in the field through smart wearable devices. The vision of sensors is to enable complete situational awareness under all-weather, day/night, and beyond-line-of-sight and through natural and man-made obstructions.


Electronic sensors have played a dominant role in a modern battlefield with a wide range of military applications: Sensors / seekers for smart weapon; Position Navigation and Timing sensors like MEMS Accelerometer & Gyro, FOG, RLG; Acoustic Sensors like Fiber Optic Acoustic Sensor, Hydrophone, Surface Acoustic Wave (SAW)  and CBRNE Sensors.


Sound is created by a vibrating object. Sound travels as a wave through a medium, for example, a liquid (such as water), a solid (such as the seafloor), or a gas (such as air). Therefore, sound does not exist in the vacuum of space.


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).


Acoustic and Seismic sensors can detect and track ground & air vehicles, gunshots, mortars, rockets targets; detect, classify, localize and track threat submarines, mines & IED, detection and very long-range infrasonic detection of events and targets of interest. They can detect, locate & assess activity associated with tunnels & UGFs


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; and 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.


Acoustic and seismic sensors integrated into a sensor network can help in detection, localization, characterization, recognition and tracking of stealthy   targets and their movements in urban environment.  This Fused Sensor Network can assist security forces by providing early warning of security breaches in high risk and vulnerable complexes like hotels, railway stations, markets, military facilities and sports stadium. This Network can also support Security forces in Counter Terrorism (CT) Operations by providing them real time intelligence for tactical planning and combat operations.


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 nonstationary noise. As a result, the detection range is weather-dependent and often anisotropic,
especially at long distances.


For seismic sensors, behavior 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).


Signal processing is the critical technology for extraction of data and intelligence from the signals generated by acoustic and seismic sensors. It consists of target detection algorithms, feature extraction, target classification algorithms and target tracking algorithms.


Acoustic vector Sensor

The traditional acoustic sensors were microphones which measured the sound pressure. A vector sensor is an underwater listening device used to detect sounds in water and convert acoustic energy into electrical energy. Vector sensors differ from hydrophones, which are also underwater listening devices, in that vector sensors measure both the particle motion and pressure changes associated with a sound wave, while hydrophones measure only the pressure changes. Vector sensors can determine the direction in which a sound wave is traveling using measurements made at a single point in space. Individual hydrophones are not sensitive to direction, making it necessary to construct hydrophone arrays made up of a number of hydrophones to determine the direction in which a sound wave is traveling.


The particle motion in a sound wave is described by displacement, velocity, and acceleration. These three quantities are vectors, because each has a direction as well as a magnitude. Vector sensors measure one of these vector quantities together with pressure. (Some vector sensors actually measure pressure gradient, which is proportional to acceleration, rather than particle motion itself.) For example, a vector sensor can be created by appropriately combining two geophones, which generate electric signals proportional to the particle velocity in a sound wave, and a standard hydrophone, which measures pressure.


Vector Sensors technology

While hydrophones are the most common underwater acoustic sensors, vector sensors have been used in U.S. Navy DIFAR (Directional Frequency Analysis and Recording) AN/SSQ-53 sonobuoys for many years. The DIFAR sensor consists of an omnidirectional hydrophone that measures pressure and two particle motion sensors mounted at right angles to each other that measure the horizontal components of particle motion associated with the sound wave. All three sensors are co-located in a single package that has nearly the same density as seawater so that the package moves with the sound wave in the same way as the surrounding water particles.


DIFAR sensors are most sensitive to signals arriving from a specified horizontal direction, while being less sensitive to signals arriving from other directions. The beam pattern, which describes the sensitivity of a directional receiving system (either an array of sensors or a single vector sensor) to incoming signals as a function of heading or direction, has the shape of a cardioid when a conventional beamforming approach (equal weighting for all components) is used with single vector sensor data.


In addition to determining the direction in which a sound wave is traveling, the DIFAR sensor also improves the signal-to-noise (SNR) ratio by rejecting noise arriving from directions other than the direction from which the signal of interest is arriving.


New, single-crystal piezoelectric material has recently made it possible to construct much smaller vector sensors without losing sensitivity. For example, three-dimensional (3-D) vector sensors that use separate single crystals to measure acceleration and a hydrophone to measure pressure can now fit in a housing that is 0.8 inches in diameter and 3.125 inches in length.


These new 3-D vector sensors provide more flexibility than the older 2-D sensors since they can make measurements in any orientation in space. Miniaturization allows for new applications as well as deployment approaches to reduce motion-induced self-noise, a source of noise contamination that has impeded wider use of acoustic vector sensor technology.


M/S Microflown Technologies Ltd. Netherlands

Acoustic vector sensors, developed by M/S Microflown Technologies Ltd. Netherlands, has come up with alternate ISR technology which can differentiate and determine attack sources by sensing and measuring high and low frequency sounds generated with weapons. They measure the particle velocity that is average velocity of the movement of air particles. The differential resistance between two micro machined metallic strips is measured in an air current to determine velocity.


“At any point in space, a sound field can be described completely by its two dimensions; the scalar value, and the vector value. The scalar value, sound pressure, is well known and well measured. But the other quantity in acoustics, the vector value Acoustic Particle Velocity, only recently became a directly measurable quantity with the invention of the Acoustic Vector Sensor (AVS) by Microflown,” write Microflown.


With the AVS, acoustic signatures of battlespace sources can be used to detect, localize and track hostile units. The advantages of the AVS compared to traditional sound pressure sensor arrays is that it offers more numerical manipulations, increased acoustic bandwidths, reduced system size since it relies on MeMs technology, low data transmission between nodes, and brevity of assembly. These traits allow the AVS to be more practically configured in a battle zone to determine the localization of mortars, artillery, gunshots from snipers, ground vehicles and aircraft.


According to an analysis done  by de Bree and Druyvesleyn, (An acoustic vector sensor based method to measure the bearing, elevation and range of a single dominant source as well as the ground impedance , Euronoise, 2009), multiple n-spaced AVS can find 4n2 uncorrelated sources in 3-D space; even more, current research has shown that 8n2 sources can be found if they are broad banded.


The AVS are accoutered with an MFSC-4 which is a 4 channel signal conditioner with powering, pre-amplification, and amplitude/phase correction capabilities. Signal processing is required in order to convert the real time acoustic data to a relevant format and relate the relevant data to a timestamp and location


The MEMS based AVS sensor, assembles a sound pressure transducer and three orthogonally placed Microflown sensors in compact (5x5x5mm) unit. The sensor can detect and localize gunshots, artillery, aircraft and vehicles within one shot, one second of time and is accurate within about 1 degree of source.


“Acoustic Vector Sensors (AVS) can detect, classify and locate all sorts of acoustic events in 3D space, for instance impulses, like Small Arms Fire (SAF), Rockets, Artillery and Mortars (RAM), but also tonal sound sources like ground vehicles and helicopters. AVS have a compact size (1cm), low weight (100 gram) and low power (<1 Watt), they can be deployed on all sorts of platforms, such as unattended ground sensors, vehicles, Unmanned Aerial Vehicles (UAV), dismounted soldiers and helicopters,” says Microflown.


The Microflown sensor is based upon MEMS technology, and uses the temperature difference in the cross section of two extremely sensitive platinum wires that are heated up to 200°C in order to determine Acoustic Particle Velocity. When air flows across the wires, the first wire cools down a little and due to heat transfer the air picks up some heat. Hence, the second wire is cooled down with the heated air and cools down less than the first wire. A temperature difference occurs in the wires, which alters their electrical resistance. This generates a voltage difference that is proportional to the Particle velocity and the effect is directional: when the direction of the airflow reverses, the temperature difference will reverse too



Wilmington police used ShotSpotter technology to pick up the sound of the gunfire and leading them to right block location where they found a car containing the body of a 33-year-old man who was the victim of a homicide. “This narrows the search down quite a bit and directs our officers pretty precisely and accurately as to where to go,” said Wilmington Police Captain Jim Varrone.


SST has developed and deployed “ShotSpotter”, the wide-area protection system designed for civilian and critical infrastructure applications. This system deploys multiple collaborative acoustic sensors perched atop roofs and light poles, throughout a coverage area up to 20 or more square miles. The sensors are paired with audio analysis software that identifies the unique signature of gunshots and other loud explosive sounds in real time.


Once the gunfire is detected, SST helps law enforcement respond safely and effectively to incidents by providing precise location of gunfire, both latitude/longitude and street address, number and exact time of shots fired, shooter position, speed and direction of travel (if moving). It can also provide Gunfire incident history and do pattern analysis.


DARPA had developed “Shooter Localization” technology under its Network Embedded Systems Technology (NEST) program. Ad-hoc wireless network of cheap acoustic sensors were used to accurately locate enemy shooters. Nodes detect shockwave and muzzle blast and send back their data to the base station and Base station then determines shooter location. Fast and accurate enemy shooter localization are key in reducing friendly casualties and neutralizing enemy combatants. The Performance parameters were Average accuracy of 1 meter and Latency of 2 seconds.


Wireless Sensor Networks (WSNs) based on acoustic signals are also useful for battlefield surveillance, border monitoring, and traffic control to classify moving targets, especially ground vehicles. Battlefield surveillance is accomplished by distributing sensor nodes using remotely-controlled vehicles,  UAVs or low-flying aircraft. Attached sensors transmit acoustic, seismic, and visual signatures of each of the adversary’s vehicle which allow for classification and threat capabilities. The signal data gathered by these electronic warriors will be relayed back to the Forward Operating Base (FOB) for evaluation and correlation.

On the battlefield, helicopters are well suited to avoid detection from radar and optronic
sensors as they are able to “hug the ground” and hide behind relief and vegetation, but their rotors radiate very distinctive very low and low frequency sounds that are not significantly attenuated by these obstacles.



UAV based reconnaissance is usually based upon optical sensors only. However, many events of interest such as gun fire or explosions can be easily heard, but are difficult to see with a live video stream sent from a UAV. Microflown says AVS technology can be integrated into existing UAV technology, adding a whole new dimension to UAV based ISR.


AVISA Sensors on military vehicles

The Dutch Ministry of Defence awarded Microflown AVISA with a 1 MEURO contract to adapt its game changing Acoustic Vector Sensor for installation on vehicle platforms. The acoustics project will have duration of two years. Acoustic Vector Sensors will be mounted on reconnaissance vehicles, armoured personnel carriers and tracked vehicles, providing them with an all-around, full spherical, 3D acoustic situational awareness


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