Acoustics in the land battlespace

"Before electrical and wireless communications became common on the tactical level, the sound of battle was often the quickest and most efficient method by which a commander could judge the course of a battle" - Charles D Ross1
Modern warfare in the land environment is not predictable. In a multitude of theatres, forces employing modern weapons and systems currently face an adversary who does not. The most extreme example is represented by forces in Iraq and Afghanistan (and recently in South Lebanon), but other examples exist (to a lesser degree) in Pakistan, and Chechnya.

In Iraq technologically advanced forces face an ever-expanding threat from organised and semi-organised groups and militias using relatively simple tools in order to pursue political (as well as some specific military) goals through chaos and destruction. Attacks by these groups target local populations as well as coalition forces to weaken nascent institutions, demoralise civilians, create social and cultural unrest, and weaken the will to continue with the security operation. These opposition groups use commercially available components, civil infrastructure such as mobile phone networks, and social infrastructure such as schools and other buildings rather than military-specific systems. The result is that activities by these personnel are almost impossible to predict, planning and preparation of pending attacks is difficult to detect, and ambush and other operations are conducted with little warning.

In Afghanistan, small groups use the challenging geography to mask activities and aim to make fleeting contact, cause casualties, and then disengage. In some cases engagements can be as short as a single shot at long range.

Paradoxically, the recourse for technologically advanced forces is to employ ever-more advanced technologies in order to gain some measure of warning, and to respond with the minimal in civilian causalities. The technological edge is still considered by planners and operational personnel in these forces to be a key discriminator in achieving operational success.

Network Centric Warfare (NCW) is one such response; with its increased coordination and information sharing across and within deployed forces. The 2005 Defence NCW Roadmap2 highlighted the importance of "linking Command and Control, Sensor and Engagement systems via a network, to facilitate enhanced situational awareness, collaboration and offensive potential". Within this framework there is a current (and intended future) pre-eminence of microwave and optical/IR systems, with acoustic sensors being somewhat overlooked.

Acoustics can however provide valuable information on events, on the location of those events, on activities taking place at the locations, and on the platform and weapon types and numbers at the locations. Acoustic-based information can be fused with other sensor outputs to develop a more complete situational understanding, and is therefore consistent with the NCW approach. It needs to be given higher recognition and priority.

One key advantage of acoustic systems over other sensors is that they are invariably cheaper and, in many cases, lighter thus enabling provision down to small groups such as infantry sections, or even individual soldiers. The result of such an implementation is that networked data can be provided within/between sections and the overall NCW goals advanced.

Acoustics has two main applications for land forces. The first is the provision of self protection through sniper detection and location. The second is a contribution to situational awareness and tactical planning through detection, localisation and classification of small arms, heavy weapons and vehicles.

The first application (self protection) has been applied principally through vehicle-based acoustic systems although Canberra-based Pacific Noise and Vibration (PNV) has developed what is considered to be the world's first man-wearable system with funding through the Defence Capability and Technology Demonstrator (CTD) program. Other vehicle based systems are also on offer: the US Boomerang (by BBN Technologies (initial development by DARPA) and PDCue -(AAI Corporation); Canada's Ferret (MacDonald-Detweiler); Israel's Sniper Acoustic Detection System (SADS - by Rafael); and France's Vehicle PILAR (by 01dB).

Acoustic sniper detection systems work by utilising the noise of the supersonic bullet (the 'crack') and/or the noise of the muzzle blast (the 'thump') to determine the bearing, in azimuth and elevation, and range of the weapon being discharged. Examination of the waveform can also provide classification information regarding weapon type. When coupled with an orientation sensor and Geographic Positioning System (GPS) the shooter location can be resolved to real-world coordinates and therefore be easily overlaid onto a topographic map. In this form the sniper location can be readily exchanged with other systems. It is this feature that enables the second application, the utilisation for situational awareness, and tactical appreciation and planning.

The PNV Acoustic Area Protection System (AAPS) utilises a core suite of signal processing algorithms, the Acoustic Threat Localisation System (ATLS), to provide a capability suitable for implementation on an individual soldier, on a vehicle, as an unattended ground sensor, or carried by an Unmanned Aerial Vehicle (UAV). Shooter locations can be determined in real time from each sensor node individually, or the output of multiple modes can be fused to provide increased accuracy. The result is that an almost instantaneous determination of shooter location is available to the soldier to enable accurate and immediate return fire.

The networked form is the basis for information sharing and situational awareness. The AAPS in the networked form can be implemented within the infantry section, and functions as an embryonic Soldier Combat System. The possibility to exchange information between different AAPS configurations adds to the inherent flexibility of the system. This is NCW from the bottom up, rather than the usual top-down.

The AAPS has been proven during tests to be able to accurately detect and locate (with minimal false alarms and missed detections) small arms, mortar base plates and Rocket Propelled Grenades (RPG) at launch. The Vehicle variant has proven an ability to perform at vehicle speeds up to 95 kph, using an array no larger than 10x10x10cm, with bearing accuracy better than +/- 6 degrees. Detection of a Steyr rifle has been achieved out to 1.5km, and the shooter is within a 250m box at that range. (At 300 metres, the size of the box reduces to 60m). Extension of the detection and localisation capability to other acoustic sources types such as artillery and vehicles can easily be undertaken.

The acoustic detection and localisation capability can be integrated with an optical capability to provide positive identification of an intended target, and integrated with a gun mount to enable the weapon to train on the origin of the gunfire. For a theatre such as Afghanistan such a capability provides a level of response and force protection that currently does not exist.

As the acoustic signature of a vehicle can be used to detect, classify and track that platform, it is intuitively obvious that the minimization of the radiated acoustic signature is needed as a counter. Signature reduction will increase the acoustic stealth of the vehicle, reduce detection ranges, and increase survivability and mission effectiveness. Moreover, the acoustic signature can be used to classify between vehicle (and weapon) types and therefore contains valuable tactical information.

In an environment such as Iraq it is only a matter of time before vehicle acoustic signatures are used as a passive trigger for Improvised Explosive Devices (IED). These devices could be readily constructed from commercially available components. They could be employed, completely unattended, to selectively target one particular type of vehicle whilst letting other types pass unhindered. The counter to this development therefore warrants attention now; before such devices appear in the area of operations.

Actions to overcome acoustic IED triggers involve measuring, monitoring, managing and reducing vehicle signatures. Parallel and supporting activities such as creating false signatures, signature masking and/or decoy vehicles also need to be considered.

Acoustics can be seen to have a significant place in the land battlespace in providing enhanced level of protection to both mounted and dismounted troops; through an ability to rapidly, reliability and accurately detect weapon discharge and other noise sources. The acoustic sensor can be portable, lightweight and unobtrusive, and can network with other sensors and sensor types to provide a valuable addition to situational awareness, force reaction and tactical planning. Furthermore, the application of acoustics in an offensive manner through triggering of IEDs needs to be considered and countermeasures considered before this becomes a serious threat.

Graeme Dunk is the Chief Operating Officer of Pacific Noise and Vibration Pty Ltd.

By Graeme Dunk, Canberra