Advanced submarines use Acoustic Intercept Systems (AIS) to improve situational awareness by
analyzing data from directional wideband sensors to provide DCL (Detection, Classification and
Localization) of nearby acoustic emitters. AIS systems perform this function using a three-step
process; The first step detects signals in the incoming multichannel data set using spectrum
analysis and beamforming to reduce noise and enhance signals of interest. The second step
estimates signal parameters including carrier frequency, pulse duration, pulse envelope, pulse
intensity, modulation type, and modulation characteristics along with inter-pulse characteristics
including pulse repetition interval and bearing rate. The third step attempts to determine the
emitter type and assess the threat level from the signal parameters. A major benefit of AIS is that
because it detects one-way transmissions, it provides a significant counter detection range
advantage over search sonars, which are relying on two-way transmissions. This counter
detection range advantage allows a submerged platform to have better situational awareness than
other platforms searching for it with active sonars.
Despite its utility, AIS capability is not widely available for current small and medium sized
UUVs because it requires complex sensors and high bandwidth processing. The processing
bandwidth needed for AIS is driven by the frequency band to be covered and the multiple
channels required to provide directionality. Emitters to be detected can be intentional or
unintentional. Examples of intentional emitters include fathometers, navigation sonars, fish
finders, fishing net monitors, search sonars, harbor defense sonars, imaging sonars and
emergency beacons, while examples of unintentional emitters include transients from other
underwater platforms and emissions from diver breathing apparatus. The AIS band to be covered
depends on the class of emitters to be detected with a low end which can go down to as low as 1
kHz and a high end which ranges from less than 100 kHz to 2 MHz. The AIS processing load is
a function of the frequency band to covered, the number of input channels, and the sophistication
of the processing. Because of the very large frequency band to be covered, submarine-based
systems require significant processing resources which would be impractical to duplicate in a
small to medium sized UUV.
This presentation discusses the issues that need to be resolved in developing a practical AIS for
UUVs with limited battery and computational resources. These issues include developing a
sensor geometry which provides for best coverage area and localization ability from a small
number of channels without affecting UUV hydrodynamics, desirable platform quieting to
minimize self-noise from the UUV’s BLDC motor, and required signal processing for spectral
analysis, beamforming, detection, parameter estimation and threat assessment.
The issues are explored by discussing the design and implementation of a ‘blade sensor’ AIS
capability for NSWC PCD’s DREWUV. This system uses a planar array of wideband channels to
provide nearly 4 pi steradian coverage while minimizing additional drag. Critical aspects of the
sensor design are discussed along with a suggested processing methodology based on a single
board DSP with power consumption of less than 50 Watts.