Acoustic Mortar Localization System – Results from OIF

Stephen Tenney, Brian Mays, David Hillis, Duong Tran-Luu, Jeffrey Houser andChristian Reiff

US Army Research LaboratoryAdelphi, MD 20783

ABSTRACT

The US Army Research Laboratory developedand deployed an acoustic mortar and rocket detectionand locating device, in a little over two months. Afterreceiving a direct request from Iraq, ARL mergedcomponents from a number of programs to enable therapid fielding of this capability. As of mid 2004, fourUnattended Transient Acoustic MASINT Systems(UTAMS) have been operated detecting mortar fire,rocket launches and IED’s while displaying the localizedresults. Due to the typically high angle of flight ofmortar fire, the acoustic signal of the launch can arrive atthe array before the mortar impact. This can provide ashort time for advance warning. Results were sosuccessful that the Multi-National Corps – Iraq (MNC-I)generated an Operational Needs Statement (ONS), inaddition CJTF76 has requested UTAMS units forAfghanistan. A portable Acoustic Mortar DetectionSystem (AMDS) has also been developed in support ofSOCOM. This version is lightweight, low-power, air-deployable and automated for mortar launch detection.

1. INTRODUCTION

ARL has studied the problem of mortardetection for some time in investigating impulsive signaldetection and long range acoustics. Explosions createwide band signals; those generated by artillery cancontain low enough frequencies that will propagate verylong ranges. The mortar signature is somewhatspecialized because of its low amplitude and the mortartube itself. Using low cost hearing aid microphonesconfigured in a tetrahedral array (Fig. 1),

Fig. 1 – Tetrahedral Microphone Array

battlefield activity like mortar fire, rocket launches andIED detonations can be detected. By exploiting existingCOTS based hardware and ARL developed algorithmsand signal processing, we were able to make minormodifications to existing designs and deliver operationalequipment directly to the field.

2. COMPONENTS

Acoustic array configurations had been studiedand were in existence. It became our task to match aconfiguration to an existing 24-bit A/D that we couldpurchase off the shelf. The application of the 24-bit A/Dto this problem is significant in that the large availabledynamic range can handle very low amplitude (longrange) signals as well as near, very loud sounds.Combining this with COTS extended temperature rangesingle board computer, operating under a real-timeoperating system, and a viable platform could bedelivered. This hardware was selected and the packagingdesigned for extended temperature range application.The package consisted of an environmental enclosurethat was selected to keep its internal temperature belowthe upper limit of the extended temperature rangeelectronics at outside temperatures that would likely beencountered in Iraq. By deploying multiple arrays (up tofour) at each site, and wireless connectivity between thearrays and a laptop computer, lines of bearing from thearrays to the source can be created. At the locationwhere these lines-of-bearing (LOBs) cross is a potentialtarget source.

3. SIGNATURE

The mortar launch signature is predominantly inthe 100 Hz region, while the impact signature containsmore energy and wider frequency content. Automaticsignal identification is being implemented to allow thesignal processing to indicate which signal is a point oforigin (POO) and which is a point of impact (POI) (Fig1). Detection ranges for each of the various types ofmortars is typically accomplished at the tactical range ofthe weapon. UTAMS has demonstrated that it detectsboth POOs and POIs. It is more difficult to detect thePOO since it comes from a further distance and containsless energy.

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4. ALGORITHM

The detection algorithm is based on sufficientcoherent energy (energy weighted by coherence at eachfrequency) across the full array. An adaptive filteradjusts for the acoustic background and enhances theability to find impulsive events in the time waveform.Once an event is detected that passes the coherent energylevel requirement an angle of arrival is developed fromthe timings across all of the microphones. Statistics fromthe content of the detected signal suggest whether thedetection is a valid acoustic wave.

5. DISPLAY

Using a standard mapping package LOBs aredisplayed over a map or overhead imagery of the areaaround the array location. The tracking algorithmevaluates the LOBs and where they cross to determinewhether the detections are likely to be acoustic signalsand if so plots an icon over the crossing. This is shownin Fig. 2 below.

Launch

Impact

Fig. 2 – Display of a Launch and Impact

The tracking algorithm will remove ghosting andcrossings that could not have been created by acousticsignals. A log of the LOBs is kept on the laptopcomputer where it can be reviewed at a later time andreplayed through the display to recreate the display andlocalized results. Once a detection is reported, the usercan click on the LOB and view the time waveformgenerated. This allows the user to evaluate visually if thesignal was in fact a mortar, rocket an explosion orsomething else of no interest.

6. PERFORMANCE

UTAMS has demonstrated useful accuracies,though it is not intended to be a counter battery device.While high winds will degrade system performance andtemperature inversions during the heating of the day willshorten detection range, the wide spectral content of theexplosive launch and impacts tend to ease the detection

problem. The relatively short range of mortars and smallrockets also tends to be favorable to the UTAMSdetection range.

7. RESULTS

Through the assembly of off the shelfcomponents and commercial hardware ARL hassuccessfully delivered acoustic mortar and rocketdetection capability to the soldier. Up to four sites havebeen operated in Iraq detecting and locating insurgentmortar and rocket fires. Success can be judged by theformal request for more UTAMS units in Iraq andadditional units for Afghanistan. We have quantified theperformance capabilities and can provide the classifiedperformance capabilities upon request. A classifiedbriefing was presented at the recent MSS BattlefieldAcoustics and Seismics Conference and is referencedbelow. The availability of AMDS as a low power lightweight mortar and rocket detection device offersadditional capability for SOCOM.

ACKNOWLEDGEMENT

This capability could not have been deliveredwithout the aid of many individuals in ARL’s AcousticSignal Processing and Sensor Integration Branches. Inparticular, Lenora Isaiah-Weathers was the driving forcein assuring the assembly of each of these sets of UTAMSunits.

Also, without MAJ Greg Holifield’s originalrequest for the device and his ability to successfullypresent the concept to other soldiers in active combat andsell them on it, we would not have been successful. Hisoriginal vision encouraged all of us to bring thetechnology forward. His replacements, MAJ MarcusVarnadore and MSG Mark Brzezinski, were important incovering the second wave of training and maintenance.And finally the soldiers in the field who have operatedUTAMS and provided feedback to us are invaluable intheir contribution. May they stay safe and all returnhome.

REFERENCE

Tenney, S. M., 2004: Unattended Acoustic MasintSystem (UTAMS) (SECRET), Military SensingSymposium, Battelfield Acoustic And Seismic.