Passive acoustic radar

With the use of two (or more) USP sensors the location of an aircraft can be derived. The system can be seen as a passive radar for acoustic noise emitting objects. Several different detection techniques are available that are deployed for different situations. Some situations are summarized below. One of the outstanding features is that the acoustic radar operates in a passive manner and also without line of sight: is the direct sight of an aircraft is blocked by e.g. a mountain, it still can be detected and localized.
Localisation with USP sensors.

The probe distance is set 25.0 meter and the probes are placed 1.2 meter from the ground (grassland). Measurements are performed for helicopter movements (Eurocopter EC 120) during landing and take off. The 27Hz component, which is the blade-passage frequency of the main rotor, running at 410 rpm is used for detection.

Precision guided munition (PGM)
It is possible to acoustically target a source (e.g. armoured vehicle). This is e.g. shown in a brilliant anti-armor (BAT) munitions developed by Northop Grumman. This acoustic system is used to find rough direction. Other sensor are used for precision. If a Microflown sensor is used, the need for the fins is vanished because a single USP in the nosecone is sufficient to do the task. Advantaged are the smaller size, the better deplorability and the improved accuracy.
Acoustic sensors in Unmanned air vehicles (UAV)
If one want to apply directional acoustic sensors in an UAV it is possible to use sound pressure microphones. However array techniques (that is required with pressure transducers) require space (an array should be large compared to the wavelength), the array techniques require considerable computing, a small array (less than 1 meter) lacks directionality and the noise of the UAV itself is difficult to suppress.

Situation awareness

A directional sensor in the air can perform different tasks. It is possible to acoustically map a battlefield situation by locating and classifying all sources that are below a UAV. It is then possible to find e.g. gunshots, armoured vehicles, etc

	Acoustic miss distance indication system for target drones
	Air based gun shot localisation / Sniper detection
	UAV Acoustic Collision-Alert System
	UAV guidance-Ground based sensor for navigation unmanned air vehicles (UAV)
	Ground based sensor for take off and landing (Automatic take of landing ATOL) and Unmanned air vehicles (UAV)
	Perimeter defence / Border control
	Helicopter blind landing

The Microflown based passive acoustic radar system can be used for blind landing of helicopters. Two possibilities are mentioned here:

	Helicopter blind landing near by a ground base
	Helicopter blind landing in unknown terrain

Gun shot localisation / Sniper detection

In the past ten years, in-air acoustic sensor systems have been developed for the localization of small arms fire. A number of countries have systems at different stages of advancement that they have deployed on operations. These systems are being mounted on vehicles to provide crews with better situational awareness, thereby enhancing crew safety and force protection. The acoustic system is starting to be incorporated into vehicle equipment as a standard item, especially in new vehicles

Air-to-ground source localisation

A three dimensional sound probe based on three ½” PU-mini probes is mounted in a windscreen (see also chapter 12: “DC flow, Protection & Windshields”). This setup is mounted under the steps of a helicopter, see Fig. 21.19. The signals of the probe are recorded with a multi channel sound card and a laptop that is placed inside the helicopter. On the ground a impulse sound source was deployed on a known location,

Underwater acoustics

With the Microflown technology it is possible to measure the acoustic particle velocity in air. In cooperation with Thomson some R&D effort was taken to find out if the Microflown technology can also be used under water. It showed that this is possible in principle but that the actual realization for in air use is not suitable for under water use. New R&D is now started to create a Microflown for under water use. Traditional underwater vector sensors developed for DIFAR Sonobuoys, encompassing both military and civilian applications are over 25 years old with limitation in operational frequency band, sensitivity and poor horizontal angular resolution and they are not commercially available as sensors but as a part of operational navy systems (Sonobuoys).

Hydroflown for underwater harbour protection

With a single underwater sound pressure transducer (hydrophone) it is possible to extract seabed information. A simple propeller driven aircraft is used as source.

If underwater velocity sensors are used, much more information can be retrieved with more detail.