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MULTI PURPOSE ACOUSTIC VECTOR SENSORS FOR BATTLEFIELD ACOUSTICS A passive sensor to detect multi events that can be used on multiple platforms Dr. Ir. Hans-Elias de Bree, Dr. Ir. Jelmer Wind, Prof. Dr. Ir. Erik Druyvesteyn, Microflown Technologies, The Netherlands Major Henk te Kulve, Chief Target Acquisition and Sensor systems, Ministerie van Defensie, The Netherlands Abstract Acoustic signatures of both battlefield and underwater sources can be passively exploited towards detecting, localizing and tracking hostile units. Acoustic vector sensors (AVS’s) have come to play an increasingly significant role in this technology with application focus on border control, harbor protection, gunshot localization, and situational awareness. Any sound field can be described by the scalar value sound pressure and the 3D vector value acoustic particle velocity. An acoustic vector sensor (AVS) is a 4 channel sensor capturing the sound pressure and the three orthogonal components of the acoustic particle velocity. A sound pressure microphone is omni- directional: its sensitivity is not dependant on the direction of arrival (DOA) of a sound source. A Microflown particle velocity has a figure of eight sensitivity: its sensitivity is dependant on the cosine of the DOA. An AVS is a small acoustic sensor that is capable of determining the direction of arrival (DOA) of a sound source instantly from the relative amplitudes of the three orthogonal components and for the entire acoustic bandwidth. Local processing on the sensor node itself is therefore possible. The output of such a node is the classification of the acoustic event, providing at least the azimuth (or bearing) and elevation. With some events the range can be determined as well. Traditional systems use the time of arrival at microphones spaced apart in an array to determine the DOA. This technique has some drawbacks: large system size, limited bandwidth and accuracy loss due to wind and temperature changes. Acoustic measurements in air have always been based upon (arrays of) sound pressure microphones. After the invention of the Microflown sensor in 1994, capable of measuring directly the acoustic particle velocity in air, AVS’s have become available. In addition, Microflown based AVS’s for underwater use are now being developed. The acoustic vector sensor can be placed on all kind of platforms and it can detect multiple simultaneous acoustic signatures: rockets, artillery and mortars (RAM), gunshots, UAVs, rotary wing and fixed wing (both fast jets and propeller driven aircraft) and ground vehicles. The acoustic vector sensor represents a completely new generation of acoustic sensors that measures a completely new physical quantity, it is not simply a set of microphones with smart processing. Microflown Technologies is conducting several co-funded R&D projects over the next three years including, a dual AVS buoy, mortar localization, sense and avoid for UAVs, RPG detection, and acoustic radar. In Gdansk, Poland, 12 AVS systems are being deployed to enrich an urban video surveillance system. The Dutch Ministry of Defence (MoD) has acquired a Microflown shooting range safety system with 10 AVS nodes. The Microflown Sensor The Microflown sensor, invented in 1994 is the world’s only true acoustic particle velocity sensor [1]. As can be seen in the picture below, the sensor consists of two wires which are heated to 200°C above the ambient temperature during its operation. As air flows across the sensor, the upstream wire cools down and gives off some heat to the passing air. Hence, the downstream wire cools down less due to the now heated air. This difference in temperature is measured electrically, making it possible to measure the acoustic particle velocity directly. The heating of the wires requires about 70mW.

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MULTI PURPOSE ACOUSTIC VECTOR SENSORS FOR

BATTLEFIELD ACOUSTICS A passive sensor to detect multi events that can be used on multiple platforms

Dr. Ir. Hans-Elias de Bree, Dr. Ir. Jelmer Wind, Prof. Dr. Ir. Erik Druyvesteyn,

Microflown Technologies, The Netherlands

Major Henk te Kulve, Chief Target Acquisition and Sensor systems,

Ministerie van Defensie, The Netherlands

Abstract

Acoustic signatures of both battlefield and

underwater sources can be passively exploited

towards detecting, localizing and tracking

hostile units. Acoustic vector sensors (AVS’s)

have come to play an increasingly significant

role in this technology with application focus

on border control, harbor protection, gunshot

localization, and situational awareness.

Any sound field can be described by the scalar

value sound pressure and the 3D vector value

acoustic particle velocity. An acoustic vector

sensor (AVS) is a 4 channel sensor capturing

the sound pressure and the three orthogonal

components of the acoustic particle velocity.

A sound pressure microphone is omni-

directional: its sensitivity is not dependant on

the direction of arrival (DOA) of a sound

source. A Microflown particle velocity has a

figure of eight sensitivity: its sensitivity is

dependant on the cosine of the DOA.

An AVS is a small acoustic sensor that is

capable of determining the direction of arrival

(DOA) of a sound source instantly from the

relative amplitudes of the three orthogonal

components and for the entire acoustic

bandwidth. Local processing on the sensor

node itself is therefore possible. The output of

such a node is the classification of the acoustic

event, providing at least the azimuth (or

bearing) and elevation. With some events the

range can be determined as well.

Traditional systems use the time of arrival at

microphones spaced apart in an array to

determine the DOA. This technique has some

drawbacks: large system size, limited

bandwidth and accuracy loss due to wind and

temperature changes.

Acoustic measurements in air have always

been based upon (arrays of) sound pressure

microphones. After the invention of the

Microflown sensor in 1994, capable of

measuring directly the acoustic particle

velocity in air, AVS’s have become available.

In addition, Microflown based AVS’s for

underwater use are now being developed.

The acoustic vector sensor can be placed on all

kind of platforms and it can detect multiple

simultaneous acoustic signatures: rockets,

artillery and mortars (RAM), gunshots, UAVs,

rotary wing and fixed wing (both fast jets and

propeller driven aircraft) and ground vehicles.

The acoustic vector sensor represents a

completely new generation of acoustic

sensors that measures a completely new

physical quantity, it is not simply a set of

microphones with smart processing.

Microflown Technologies is conducting

several co-funded R&D projects over the next

three years including, a dual AVS buoy, mortar

localization, sense and avoid for UAVs, RPG

detection, and acoustic radar. In Gdansk,

Poland, 12 AVS systems are being deployed to

enrich an urban video surveillance system. The

Dutch Ministry of Defence (MoD) has

acquired a Microflown shooting range safety

system with 10 AVS nodes.

The Microflown Sensor

The Microflown sensor, invented in 1994 is the

world’s only true acoustic particle velocity

sensor [1]. As can be seen in the picture below,

the sensor consists of two wires which are

heated to 200°C above the ambient

temperature during its operation. As air flows

across the sensor, the upstream wire cools

down and gives off some heat to the passing

air. Hence, the downstream wire cools down

less due to the now heated air. This difference

in temperature is measured electrically, making

it possible to measure the acoustic particle

velocity directly. The heating of the wires

requires about 70mW.

Figure 1: the Microflown sensor.

From 1994 to 2004, a large body of scientific

research has been done worldwide by many

universities and industry, exploring a wide

variety of Microflown measurement techniques

leading to hundreds of scientific papers. From

around 2004, the sensor has become widely

accepted, primarily in the automotive industry.

The technology is currently being used to

improve the interior sound quality of the

products of almost all major car manufacturers.

Figure 2: An acoustic vector sensor consisting of

a sound pressure microphone and three

optionally placed Microflown sensors.

The acoustic vector sensor can be used in

many applications. These will be mentioned in

the following paragraphs. The Dutch MoD has

prioritized the areas of interest. Acoustic

sensors command attention because they are

passive and cannot be jammed.

1) Replacement of weapon location radar. In

this project there is a specific need for a mix of

sensors to detect RAM and UAV’s. The

acoustic vector sensor can be used to detect,

localize and classify threats. Apart from that it

can confirm RADAR tracks in order to reduce

the false alarms inherent with the current

RADAR system.

2) Gunshot detection is a requirement for the

convoy and maneuver units.

3) There is a need for unattended ground

sensors (UGS) that have the capability to

detect, classify and locate ground vehicles.

4) An additional air defense system to detect

and track rotary wing (RW) aircraft for local

use on a light armored vehicle. The system

must provide warnings in situations where the

air defense radar has gaps for RW.

5) A shooting range safety and plotting system

for incoming mortars and artillery shells. The

system provides the location of the artillery

impacts for safety and training purposes. The

Dutch MoD has acquired such a system based

on ten acoustic vector sensor nodes.

These requirements translate on a technical

level into three focus areas:

A) classification and localization of

acoustic signatures

B) mounting on the specific platforms

C) the use of different sensors on different

platforms in a networked battle field

management system to increase the

effectiveness of the collected intelligence

In the following sections the state of the art of

the acoustic signatures is summarized, the

mounting on various platforms is shown; and

the current state of system integrations (or

applications) is presented.

Part I: Acoustic signatures

Battlefield acoustics can be divided into

several groups of signatures. The first division

of signatures is impulsive versus non impulsive

(or time varying) sources.

The group of impulsive noise sources is further

divided into low frequency blasts (mortars,

explosions, etc.), mid frequency blasts (hand

gun, small caliber muzzle blasts) and high

frequency impulses caused by, e.g. supersonic

bullets.

Non impulsive sources are divided into low

frequency tonal sources like rotary wing, or

propeller driven aircraft, mid frequency tonal

sources like UAV’s and broad banded noise

sources like jets and missiles.

In a series of field tests undertaken over the

last few years it has been proven that the AVS

is able to detect and localize all of the above

acoustic signatures with high accuracy.

There are traditional systems that detect and

localize acoustic events. However these

systems are dedicated to a single type of

signature. Systems that detect low frequency

blasts are large and must therefore be ground

based. Other smaller systems are designed to

detect and localize high frequency impulses

(gunshot localization). Those systems are not

able to detect low frequency blasts, or non

impulsive signatures.

There is a system available that is able to

detect rotary wing aircraft. The system is large

and dedicated to this specific task.

The previous paragraphs are summarized in the

table below.

Mortars

AVS based localization of mortars was studied

in a Dutch Ministry of Defence sponsored

project in 2009 [2]. The outcome is that it is

possible to use Microflown AVS to localize

mortars. This was tested at up to 6km distance.

In a successive experiment at the German

Baumholder it was shown that the accuracy of

a mortar launch is below 2 degrees (equivalent

to 30m/km), measured in non line of sight

conditions, in hilly terrain relatively close to a

forest line, see Figure 3 and Figure 4.

Figure 3: Panzer Howitzer localized to 16m

accuracy at 680m.

With one AVS it is possible to find the

direction of a mortar launch and the impact. It

is not possible to find its location (because the

range must be known for this).

With two or more AVS the location can be

found with more accuracy than expected from

the angular accuracy alone. This is because

apart from the DOA information of each AVS

the event detection timestamp can be used to

further improve the location prediction.

The current status of mortar localization is that

the angular accuracy has been proven to be

below 2 degrees in an operational test. The

detection range is higher than 6km and with

multiple AVS deployed, the localization can be

better than 15m/km.

Figure 4: Mortar shots and Howitzer shots

located with a single ground based AVS in

Baumholder (D).

The Dutch MoD has acquired a system

consisting of ten Unnatended Ground Sensors

(UGS) for a shooting range safety and plotting

system for incoming mortars and artillery

shells. This system will be realized in 2011.

Small Arms

Gunshot localization is done by detecting two

acoustic events: the shock wave created by the

supersonic bullet and the muzzle blast created

by the weapon.

First trials were conducted by measuring small

arms fire from various weapons including

9mm handguns, 5.56mm and 7.62mm rifles,

and .50 calibre machine guns. In successive

trials real time software was tested.

Competitive systems use the time of arrival at

microphones spaced apart in an array to

determine the DOA. This method is dependent

on temperature and wind speed.

In an extensive test of Microflown AVS by a

large system integrator, it was proved that the

angular accuracy is better than 2 degrees [3].

This test has been repeated at the Dutch MoD

in 2010.

Figure 5: Gunshot localization trial with the

Dutch MoD using a Diemaco C7 rifle.

A theoretical framework to find the shooter

location using the DOA of the shock wave and

the muzzle blast has been developed and was

applied to a field test at the Dutch MoD in

2010.

To calculate range, the DOA of the shock

wave has to be known in 3D (so both direction

and elevation). Traditional systems appear to

have problems with elevation measurements.

Apart from not knowing the shooters elevation,

this also introduces a problem in calculating

the distance to the shooter.

Missiles

A first proof of concept showing that it is

possible to detect and localize missiles was

established at the Dutch MoD. by measuring

small civilian rockets. During a later large-

scale field test, HOT missiles were launched at

1800m. The noise of such an event is easy to

detect at this distance.

Figure 6: Helicopter Bo 105 / HOT anti-tank

missile LFX at Baumholder, Germany.

Stinger missiles were detected and tracked at

the Greek NAMFI base.

Figure 7: Stinger LFX at NAMFI Greece.

The current state of the art is that the launch of

a missile can be detected and localized in a

range of more than 2km. The impact can be

detected and localized at range of greater than

3km.

Rotary Wing Aircraft

A first field test of tracking rotary wing aircraft

by triangulation was undertaken in 2007 [4],

[5]. It showed that it is possible to track a

helicopter in 3D with only two AVS ground

sensors by triangulation of the emitted noise of

the helicopter. This method is being developed

further in a European FP7 project.

Two AVS are required for the localization of

helicopters in 3D space. If the AVS are

positioned optimally, the initial accuracy (no

advanced processing; only the localization of

each AVS is used) is in the order of 30m/km in

bearing; the elevation accuracy is increasing

for increased elevation angles. With advanced

processing the signals of both AVS sensors are

used in one algorithm. This enhances the

accuracy.

Propeller driven aircraft

Propeller driven aircraft are assumed to fly in a

straight line. This assumption makes it possible

to determine an aircraft location with just a

single acoustic vector sensor.

In 2009, the method was refined by using

properties of the spectrum of the emitted noise.

It is possible calculate the Doppler shift as a

function of time. With this method it is

possible to determine the closest point of the

aircraft and the speed of the aircraft [6].

The Doppler method was refined in 2010 [7].

By using both the Doppler and the direction of

arrival (DOA) spectrum as a function of time,

it is possible to determine the speed, heading,

altitude, and true RPM of a propeller driven

aircraft using a single acoustic vector sensor

(AVS).

A method was developed that eliminates the

need to know the acoustic properties of the

ground [8].

With the time-frequency-DOA representation

it shows that it is possible to find the acoustic

signature of a plane even when the

measurement is disturbed by background noise

from gunshots and a nearby diesel engine [7].

Mathematic filters have been developed that

can filter in the frequency domain (e.g.

propeller noise), time domain (e.g. gunshots)

and in the DOA domain (an arbitrary sound

source in a certain direction) [7].

Unmanned Aerial Vehicle (UAV)

The signal processing of the detection and

classification of UAV’s is carried out in a

similar manner as for rotary wing aircrafts. A

few tests have been undertaken with a civilian

UAV, and another study is made in Crete in

October 2010, where UAV’s are used as

targets for LFX stinger training. End 2010 a

field test was done to measure the Raven mini-

UAV under in realistic battlefield conditions

(at the Dutch MoD).

Figure 8: Raven mini-UAV under is measured in

realistic battlefield conditions at the Dutch MoD.

Jet aircraft

The main difference (acoustically speaking)

between jet aircraft and propeller driven

aircraft is the lack of clear tonal components

for jets that might make the Doppler algorithm

more difficult to use.

Another point of attention is the high speed

that jet aircraft have. One needs to take into

account the low propagation speed of sound

and the relative low range of an AVS

(compared to RADAR). The AVS must

therefore be placed far from the location to be

protected. This makes the jet aircraft detection

and finding its direction more suitable for

border protection applications.

Apart from those issues there is a great

similarity between jet aircraft and propeller

driven aircraft.

At the NAMFI base tests in Greece, an F16

fighter jet was detected and localized using

AVS.

Part II: Platforms

The AVS is small, light weight and draws just

a small current. The algorithms to determine

the DOA do not require much processing.

These specifications allow the sensor to be

used on practically any platform.

Each platform has its own specifics. Some

results of the ongoing investigations are

presented in the following paragraphs.

Ground based

The first ground based sensors were used in

2007/2008. Since then the applications have

become clear and development has

accelerated. First models were made on a

standard tripod, but after the method for

cancelling the influence of the unknown

ground impedance was developed, [8] the AVS

were placed directly on the ground.

Placing the AVS on the ground has multiple

advantages: easy camouflaging, simpler

ruggedization, wind speed is reduced on the

ground, and mathematic models become

simple.

The ground based system consists of, apart

from the AVS, a mini computer, a GPS, an

electronic 3D compass, a 3D electronic gravity

sensor, a 4 channel A/D convertor and a

battery.

Figure 9: AVS ground sensor

Ground vehicle platform

AVS can also be mounted on ground vehicles.

Differences with the ground sensor are: wind

noise rejection capabilities, smaller size,

powering and geo-referencing from the host

vehicle, different design.

The initial aim is to integrate the AVS on the

camera of a reconnaissance vehicle. A

reconnaissance vehicle should not be easily

detected and therefore should use passive

sensors where at all possible. With the AVS

mounted on the camera housing it is possible

to detect, localize and range various battlefield

acoustics (e.g. mortar launches and impacts,

RPG launches and impacts, gun shots, snipers,

helicopters and jets, etc.). This increases the

reconnaissance capabilities and offers

protection, in the shape of a warning system, to

the vehicle itself. It is also possible to detect

approaching persons close by the vehicle and

outside the field of view of the on-board

camera.

Figure 10: Prototype clip-on of the AVS to be

mounted on the Fennek reconnaissance vehicle.

A first prototype of the AVS to be mounted on

the Fennek reconnaissance vehicle has been

developed as shown above. The total thickness

is less than 10mm and the sensor package can

be easily and quickly mounted and removed

from the camera of the Fennek.

Another application on a Stinger Weapon

Platform Fennek is described later in the

armored reconnaissance protection section.

Soldier worn

Soldier worn AVS are being pursued in

collaboration with a large system integrator.

Helmet, shoulder and rifle mounted options are

being explored. The AVS small size and light

weight are suitable to the application on these

platforms, see picture below. The sensor is

mounted on the Picatinny railing above the

soldier’s left hand.

Figure 11: An AVS mounted on a Diemaco C7

rifle.

The objective is to provide an arrow on the

periphery of the optics when a gunshot is

detected, such that the rifle is then moved in

the direction of the arrow, with the arrow

subsequently moving to the centre of the sight

when the correct bearing and elevation is

reached. This allows quick sighting without

losing situational awareness by looking away

from the sight to obtain the target information.

Unmanned Aerial Vehicle (UAV)

In 2009 the idea to put an AVS on a UAV was

proposed [9]. The idea to sense and avoid other

aircraft was adopted on a broader forum in

2010 (MIDCAS) [10].

First successful tests were carried out in

cooperation with Delft University [11].

Figure 12: An AVS mounted on a rotary wing

UAV.

Microflown has developed prototypes of AVS

that operate on UAV’s that can be tested in-

house.

Figure 13: An AVS mounted on a fixed wing

UAV.

The UAV application that is being developed

at the moment is gunshot and mortar detection

and localization from an UAV. First test flights

have been undertaken [11] and a propeller

noise canceling algorithm has been developed.

A special wind cap has been developed for

mounting on a UAV.

In parallel, additional UAV mounted AVS

tests are being carried out by a third party in

India using Microflown’s AVS sensors [19].

Rotary Wing Aircraft

First tests under a helicopter were undertaken

in 2009 in a field test in Poland. These tests

showed that the AVS sensor can be used on a

helicopter under flight conditions.

Figure 14: An AVS mounted on a rotary

aircraft.

In early 2011 a test is scheduled in the US. In

the test scenario, shots will be fired at an

operational helicopter and it will be tested if an

AVS mounted under the helicopter can localize

the shots.

Ships

In order to track propeller driven aircraft, AVS

were bought and tested on ships in India by

ADE in 2008.

Figure 15: An AVS tested on a ship in India.

The acoustic vector sensor is also proposed to

be used for mine sweeping purposes by major

system integrators. The direction of the blast is

detected in air and the range is determined by

combining video data with the acoustic data.

Buoys

Microflown Technologies is working on a

European project with the objective of

developing a buoy capable of measuring with

two AVS simultaneously, both in air and

underwater. Once a threat is detected and

classified, a UAV will be launched from a

buoy that can survey the threat and broadcast

the images to the command centre.

Underwater platforms

An underwater acoustic vector sensor is being

developed in co-operation with Suasis, in

Turkey, in a three year Eurostars program

(Hydroflown). The prototype has been tested

[12], and the first product to be developed is a

sea-bottom based sensor for, e.g. harbor

protection.

Figure 16: An underwater AVS prototype tested

in Turkey.

Part III: System Integration

In the previous paragraphs the sensor

capability to detect and find the DOA of most

battlefield acoustic signatures is presented and

the use of the AVS on several platforms is

shown. The subjects of the following

paragraphs are the use of the AVS on specific

platforms for specific situations.

Remarks on source localization

Localization of acoustic events requires the

determination of the direction to the source

(the DOA, direction of arrival of the sound

wave) and the determination of the distance to

the source.

With only a single acoustic vector sensor, the

DOA can be determined directly. To find out

the range, extra information is required.

In the case of supersonic gunshots the extra

information lies in the fact that both the bullet

and the muzzle blast provide a measurable

noise source.

Range information can also be derived from a

Doppler signal in case of moving tonal sources

like propeller driven aircraft.

If an acoustic event is spotted visually, the

range can be determined by determining the

delay. It is also possible to use VHF for this.

If multiple acoustic vector sensors are

applied, the localization can be done with

straightforward triangulation. If multiple AVS

are synchronized, the accuracy of the

localization increases further.

Remarks on signal processing

Signal processing is required in order to

convert the real time acoustic data to a relevant

format and relate the relevant data to a time-

stamp and location.

The sensor itself has a very high dynamic

range (-10dB – 130dB). High-tech hardware is

used to protect the sensor from wind effect,

overloading and other distortions.

Current research and development is based on

the following signal processing philosophy.

First, the signal is examined for relevant

signatures (triggering). This can be in the time

domain (e.g. gunshots), the time-frequency

domain (e.g. Doppler of a passing propeller

driven aircraft) or even in its time-frequency-

DOA representation (e.g. tracking an aircraft

using measurement data which is interrupted

by gunfire). When such a signal is detected, it

is determined if the signal is within preset

limits (classification). If the signal is classified,

models are applied to generate an appropriate

output.

When linked, these outputs are combined to

improve the classification precision and the

localization accuracy.

Figure 17 shows an example from a time-

frequency-level-DOA representation of the

output of a single AVS of an airplane flyover

at a shooting range in the presence of an idling

diesel engine at 200 meters distance. The time

is shown on the x-axis, the frequency is on the

y-axis, the level is represented with the

brightness of the color and the color represents

the DOA as indicated in the legend (right).

The gunshots are seen as vertical lines in the

graph. Such signals can be detected, classified

and then removed in the time domain and the

graph cleans up as shown in Figure 18.

The diesel engine generates two harmonics at

55Hz and 110Hz. Because the DOA is not

changing, the color remains the same. It is

relatively simple to classify and clean up the

signals from such sources. This is shown in

Figure 19

The airplane remains as a Doppler frequency

shifted and DOA varying signal. This signal is

relatively easy to detect and classify.

Out of the change in DOA over the ground and

the Doppler shift it is also possible to calculate

the closest approach distance, the elevation, the

heading and the speed of the aircraft.

Gun shots

Idling car

Idling car

n

i

A

s

ir

s

ca

ra pftg

Figure 17: A battlefield acoustic multi event

represented in a time-frequency-DOA

representation.

Figure 18: same as Figure 17 but with gunshots

removed with signal processing.

Figure 19: same as Figure 18 but with the static

diesel engine removed with signal processing.

Traditional vs. AVS systems

Traditional systems localize sound by using the

time of arrival at microphones spaced apart in

an array. Traditional systems are optimized for

a certain bandwidth and the array size is

inversely proportional with frequency (20

meters for mortars down to 50cm for

gunshots), the accuracy is affected by

temperature and wind, and to calculate the

DOA quite some processing is required.

The Microflown Acoustic Vector Sensor can

be used from below 1Hz up to 20kHz, and for

some applications higher still. The ability to

measure the DOA is not affected by wind and

temperature (or system size) and requires

almost no processing. It is possible to find the

DOA in 3D, i.e. both bearing and elevation can

be determined.

(Re)confirmation in relation to

weapon location RADAR

A weapon locating RADAR is able to detect

and localize mortars. From the localized

trajectory it is possible to predict the point of

impact and estimate the launch location. For

some trajectories it is possible to predict the

direction of the projectile but difficult to

estimate the launch location. With an AVS it is

possible to enhance the prediction.

Sound propagates at a speed of roughly 1

kilometer every 3 seconds. A single AVS is

therefore not the best candidate to act as a

mortar warning system. It can however be used

to dramatically decrease the false alarms, or be

used at moments when a weapon locating

RADAR is not operational.

Long range weapon locating RADAR is

inaccurate at shorter ranges. An acoustic vector

sensor will provide extra information for these

short ranges.

Armored recce protection

Two applications are addressed here:

1) Extra reconnaissance tool in combination

with the optical camera for the reconnaissance

armored vehicle as described before

2) An additional air defense system to detect

and track rotary wing (RW) aircrafts for local

use on a light armored vehicle. The system

must provide warnings in situations where the

air defense radar has gaps for rotary wing

aircraft.

A passive system is crucial because, e.g. a

Stinger Weapon Platform Fennek or HMMWV

(Humvee) must remain passive (to avoid

warning red forces). The reconnaissance

platforms should not give away its presence.

The only situational awareness available to

occupants of the Fennek is by looking out of

the narrow windows or by the narrow field of

view optical camera. Currently, the force

protection of the Fennek is done using soldiers

outside the vehicle.

Situational awareness in the air is done by

remote RADAR systems. To detect, classify,

identify and track (i.e. a flying object) takes

some time (between 5-15 seconds). Rotary

wing aircraft that fly below the horizon and

pop up nearby cause a significant threat due to

this. These scenarios cannot be countered by

the Stinger Weapon Platform (SWP) Fennek

and/or the AGBADS (Army Ground Based Air

Defense System, i.e. multiple RADAR systems

on the ground).

An AVS on the SWP Fennek that is able to

detect and determine the DOA of low flying

rotary wing aircraft (especially behind trees,

buildings or hills) will be a solution for this

specific threat.

Based on local sensing, our aim is to identify a

helicopter even if it is sheltered (e.g. below a

tree line). The optical camera can be aimed at

the detected direction. Once the helicopter

pops up and causes a serious threat, the Stinger

Fennek can take the appropriate defensive

action.

Situational awareness

Armored vehicles are completely closed and

operators cannot hear a threat approaching.

This is also the case even in open vehicles with

firepower ability because the soldiers are using

hearing protection. An AVS on such a vehicle

provides the possibility to get general close

range passive situational awareness. This

situational awareness ability can be provided

simultaneously with other AVS tasks such as

helicopter, gunshot and/or RAM detection.

Border control

Unattended ground sensors (UGS) equipped

with acoustic vector sensors can be used for

border control. Especially in long borders of

difficult terrain (such as mountains, or dense

forest, rocky shorelines), other systems like

radar systems are less practical. The AVS-

UGS system must be set up in a manner which

uses hardly any energy. Once the UGS system

will detect a specific sound signature, the

sensor will wake up from its dormant state,

locate the source and then transmit a short

warning to a command and control. As an

example, a grid of one AVS-UGS per 10km is

foreseen if propeller driven aircraft or jets are

the threat to be detected.

AVS and RADAR systems

Radars and Acoustic Vector Sensors (AVS)

work on completely different principles and

have therefore completely different operational

features. For detection, RADAR system

requires an object that reflects electromagnetic

waves, an AVS requires an object that emits

sound.

RADAR transmits; hence it is an active system

and can therefore be detected. An AVS is

passive and can therefore not be detected.

RADAR systems requires line of sight and it is

therefore difficult to detect low flying aircraft.

An AVS can ‘hear’ around obstacles.

The ‘cone of silence’ of a RADAR image has a

limited elevation (RADAR cannot ‘see’

upright). An AVS has a 3D spherical detection

ability; it can detect in all directions at the

same time.

At closer range RADAR has an electronic

blind sector in the order of 1-2km, which is

caused by the switching time. Acoustic source

localization has optimal accuracy in this

complementary range.

Clutter caused by rain, clouds, fog, air layers

with different temperatures, ground objects

(trees, buildings) or sea waves do not cause

problems with AVS.

A search (rotating) RADAR system can only

detect in the line of sight, so once per rotation.

Once the sound reaches the AVS, it can detect

in all directions at the same time.

Once the sound emitted by a threat reaches an

AVS, the localization, detection and

classification is done in the order of hundreds

of milliseconds.

For an army ground based air defense system it

takes some time to detect, classify, identify and

track a flying object. Threats flying low and

therefore mostly in the blind spots of the

RADAR system (e.g. rotary wing that pops up

or UAV’s) are almost impossible to detect. The

crews of weapon systems cannot hear the

threats because of their own platform noise.

These threats can be detected and localized by

an AVS.

From a technical point of view, the

specifications of an AVS are complementary to

those of a RADAR system. It can therefore be

used as a sensor to enrich the RADAR

detection capability. In some cases it can even

replace a RADAR system or be used where

RADAR systems are not feasible to

implement.

AVS cooperative with video

An armored vehicle with firing capability that

detects a suspected threat with an AVS (e.g. a

rotary wing aircraft (RW) below the tree line)

can direct its optical camera to the suspected

threat. Once the RW pops up for a few seconds

it can become a threat so fast that it is

impossible for an army ground based air

defense system to classify and identify in time

to warn and permit counter fire. With the AVS

detection and localization, and with the optical

camera subsequently cued toward the threat, it

is possible to identify the threat quickly and if

necessary engage it.

Another development is ongoing in Poland in a

crowd control application (for UEFA EURO

2012 soccer). The AVS is capable of detecting,

classifying and localizing acoustic signatures.

This ability is used to steer pan-tilt-zoom

optical cameras towards locations of interest.

If just optical systems are used (CCTV images)

the amount of data is enormous. Finding

possible threats is possible with video

processing but these techniques are not entirely

sufficient. With the mix of sensors (video and

audio, known also as VAUDEO), the

pre-processing of the data becomes more

powerful at the sensor level such that the

amount of data required to be transferred to the

command center is dramatically reduced.

AVS on shooting range

A shooting range safety and plotting system

for incoming mortars and artillery shells is

being acquired by the Dutch MoD. The system

provides the location of the impacts for safety

and training purposes. At the safety control

centre impacts of mortars and artillery shells,

location of shooters and the trespassing of

closed airspace is plotted.

Air control

In a Dutch co-funded project a ‘pop up airport’

should be realized with several UGS-AVS

scattered around a possible landing area.

Several aircraft can be located at the same

time. This information will be used to guide

small aircraft in by radio.

Outlook

Within the framework of a NIAG study

concerning the protection of civil aircraft

against attacks with MANPADS, a dense mesh

of sea based and land based autonomous and

wireless AVS nodes (6000) will be proposed.

Such a solution meets the requirements of a

complex scenario monitoring an airport close

to sea, mountains and an urban area. The

solution is part of a demonstration plan of

NIAG 146. In parallel, the sea floating AVS

will be developed in a large Aselsan led project

(Reconsurve), the land based solution will be

developed with Dutch national funding. It is

envisaged that such an AVS network can be

used for many other less demanding

applications such as CRAM, UAV threats,

wide area compound protection, etc.

Conclusion and discussion

Already in 2000-2003 it was scientifically

proven that Microflown sensors are suitable for

battlefield acoustics [17], [16] this vision was

adopted by both Microflown Technologies and

ADE in India [14], [15], and by undisclosed

system integrators [13] in 2008, and the US

Naval Postgraduate School [18] in 2009 [3].

As of the end of 2010, for several acoustic

vector sensor based applications, Microflown

Technologies has an R&D cooperation with

several large system integrators.

It has been proven that the Microflown

Acoustic Vector Sensor (AVS) has the ability

to detect and determine with high accuracy the

direction of arrival of all relevant battlefield

acoustic events.

A Microflown Acoustic Vector Sensor is very

small, making it suitable for deployment on all

types of platforms.

A single Acoustic Vector Sensor can determine

the bearing of any gunshot. In the case of

supersonic gunshots, the bearing and range can

be determined. In combination with other

sensors (optronics, underwater sensors, ground

sensors, etc.) the range can be determined from

other sources.

If multiple Acoustic Vector Sensors are used

the accuracy increases and the computation of

localization becomes more straightforward.

References

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[3] Evaluation of the Microflown Particle

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