Hostile Fire Locator for Helicopters

Statistics show, that the biggest threat to rotary wing aircraft is unguided ammunition from infantry weapons, assault rifles, sniper rifles, machineguns, rocket propelled grenades (RPG) and anti-aircraft artillery (AAA). However, the currently available self defense systems (flares, chaff) assume the opponent to use advanced weapons. If the rotary wing aircraft is flying at lower altitude, unguided ammunition (low tech weapons) can damage the aircraft in a very serious way. A system that is able to detect this kind of threat is called a Hostile Fire Indication (HFI) system. It is only in recent years that HFI systems are being developed mainly because of the difficulty in detecting this threat.

Though the likelihood of hitting the helicopter is low, the number of attempts is most likely very high. However, as most attacks remain unnoticed to the crew, insurgents are not facing any countermeasures. There is no penalty on an attack right now. Breakthrough Acoustic Vector Sensors will change this situation as they provide robust detection and accurate 3D localisation.

Acoustic Vector Sensors are, acoustically speaking, broad banded. They hear all relevant sound sources, including both supersonic and subsonic Small Arms Fire (SAF), as well as RPGs.

Acoustic Vector Sensors are directional, they can ignore the background noise caused by the helicopter platform itself.

Acoustic Vector Sensors point in the direction from where the assault was carried out.

Existing advanced self-defense systems protect the helicopter and the crew against IR surface to air missiles but offer limited to no protection against RPG and Small Arms Fire and do not actually locate the opponent for counter-fire. Acoustic Vector Sensors both detect the threat and locate the place it was launched from, providing two new capabilities. The crew may decide to escape from the threat now knowing for sure in what direction to fly (until now, this is often a matter of luck), or decide to take offensive actions against the enemy.

A CHANGE IN DOCTRINE
When the attack is detected, and as the crew knows where the attack came from, they may decide to strike back. It will make (some) insurgents think twice before considering an attack, putting a heavy penalty on their actions. Obviously, as the propagation speed of sound is slow, Acoustic Vector Sensors cannot trigger active self-defense measures on the helicopter and will not offer protection against a well aimed missile. But they will reduce the number of attempts and hence reduce the loss of lives.

CURRENT DEVELOPMENTS
Microflown AVISA develops a new type of acoustic sensor that is capable to detect and localize all sorts of acoustic signatures: the acoustic vector sensor (AVS). This sensor is in daily use on a RAM (Rocket, artillery and mortar) localization system that is operational at the ASK, ‘t Harde.2 Apart from this it shows that SAF localization on both ground as ground vehicles is both accurate as reliable. The sensor showed also useful on airborne platforms. A prototype UAV is able to locate RAM and SAF on the ground.

Key feature of the AVS is that it can locate acoustic signatures without the need of a spaced array. A single AVS has a similar functionality as a spaced array. Therefore the a huge traditional array system can be miniaturized to a single spot sensor. This allows installing the entire system in a pod. It shows that the AVS has a better operational behavior than a spaced array of microphones. The exact technical explanation for this statement goes beyond the scope of this document but the main reason for this is that the AVS determines the direction of the sound wave in one point in space.

Together with the Dutch Armed Forces on June the 7th 2013 an extensive Small Arms Fire test was done at the ASK ‘t Harde. The main purpose of the test was to investigate if small arms fire (SAF) can be localized with the acoustic vector sensor (AVS) from a helicopter platform. In order to realize this test, fire was openend on their own Cougar-Transporthelicopter. This hostile fire indicator (HFI) is requested world-wide and is tested for the first time with Acoustic Vector Sensors.

During the tests the Acoustic Vector Sensor have proven to function succesfull as a Acoustic Hostile Fire Indicator as well as Acoustic Hostile Fire Locator. The sensor is considered as a true ”game changer” in the battlefield.