Post on 13-Apr-2018
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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