A new technology for broadband passive bearing estimation

Many, perhaps all worldwide marine preserves are experiencing illicit entry and resource exploitation, in some cases with human trafficking involved. It is extremely costly to provide sufficient surveillance over these often remote areas with conventional assets such as manned aircraft or manned ships, and several consortia of government, academic, non-profit, and for-profit groups are searching for alternatives. Teledyne Benthos has developed a new acoustic sensor that offers a unique ability for long term, persistent surveillance. In combination with Teledyne Webb´s Slocum glider, detection and tracking of passing ships, even over deep water, is a practical and relatively low cost alternative to traditional attempts at protection of remote marine resources. Most surface vessels utilize some form of propulsion system which radiates broadband acoustic energy. This energy typically extends from a few hundred Hertz to well in excess of 10 kHz, but the spectral location of the dominant portions of the energy are specific to the class, loading, speed, and manoeuvring of the vessel. Any portion of the spectrum is useable with our sensor, provided it is sufficiently stronger than the ambient noise, However, the lower the frequency we tune for, the greater the distance obtained due both to lower attenuation and higher signal to noise ratio. Teledyne Benthos has developed a (patented) method for determining the direction of arrival (DOA) of any energy radiated into our system´s frequency-specific receiver band. This method stems from our existing commercial wideband USBL technology. The method does not require conventional detection or classification algorithms, and a simple embedded tracker is sufficient to identify temporal continuity of measured bearings. Bearing accuracy varies with SNR and frequency, but is on the order of 5 degrees or less. The device is called the Passive Bearing Detector (PBD). The PBD is integrally combined with our acoustic modems to provide ti- ed track and spectral information to another passing modem, such as might be carried on a Slocum glider or other mobile asset. Where high spectral frequencies are appropriate (e.g., around 10 kHz), the PBD itself can be carried on the glider if the acoustic propagation conditions warrant. The PBD is essentially immune to frequency dilation/compression caused by speed, is robust against vertical multipath, and functions with any combination of narrowband and broadband energy. However, being an interferometric device, it works best when monitoring a single ship in isolation. It also has application as a collision-avoidance tool for surface or near surface unmanned platforms. In some cases the devices are deployed simply by lowering them over the side of a small boat and allowing them to sink with an attached weight. These versions will contain modems with acoustic releases, enabling easy recovery even after extended deployment. Several successful experiments and demonstrations have been conducted, some at roughly 11 kHz, others at roughly 1.4 kHz. These clearly show the value of a small, 2-person deployable, deep- or shallow-water deployable surveillance system that can provide coverage over many square kilometres, for extended periods of time.