Computerised Research Echo Sounder Technology (CREST)
Our Computerised Research Echo Sounder Technology (CREST) is a highly adaptable approach to fisheries and other acoustic data acquisition. It is computer based, using the concept of a “software echo sounder”. It combines flexible digital signal generation and transmitters, multi-channel, wide dynamic range receivers, digitisation and advanced digital signal processing with data storage and management on a general purpose computer. Acoustic data can be combined with navigational, environmental and other data as required.
CREST is designed to preserve all the information present in the received signals including phase data. It allows all the popular approaches to fish stock assessment and fish target strength measurement to be implemented and in addition permits novel techniques such as transmission of swept-frequencies (chirps). Data can be output in a wide range of formats and can, for example, be used for bottom typing with products such as Quester Tangent’s QTC VIEW™.
CREST will accommodate a wide range of frequencies and at present, with suitable transducers, covers the frequency range 3.5 kHz to 200 kHz.
CREST systems consist of a transceiver incorporating one or more transmitting and/or receiving channels based upon the Motorola DSP56002 digital signal processor, communicating with a general purpose UNIX computer for control. The transceiver has surface mounted and underwater versions. The surface version can be installed at any convenient location and connected to either hull-mounted or towed transducers. The underwater version is housed in a watertight cylinder 250 mm in diameter and 1 m long rated to a depth of 1,000 m. This version also incorporates a leak sensor and has provision for a variety of telemetry sensors including depth, pitch and roll and three-axis acceleration.
The control computer is used to download software to the transceiver, manage echo sounder operation, data acquisition and storage. Communications are via Ethernet or a purpose-built high speed serial link. Any computer compatible with SVR4 UNIX or LINUX may be used.
CREST towed bodies for marine use are of a flat-nosed torpedo shaped “heavy weight” design. They feature robust low-cost construction and an “opening nose” mechanism to allow them to flood and drain quickly to minimise the time taken crossing the air-water interface during deployment and retrieval. We currently have two models: a large one of 580 mm diameter and 3 m long, designed to house the underwater transceiver, together with a Simrad Model ES38D or EDO Model 6978 transducer (or similar sized or smaller transducer). The other model is a smaller version, 315 mm diameter and 1.68 m long, intended for use with a surface transceiver and a Simrad Model ES38-12 (or similar sized or smaller) transducer.
Other towed bodies, suitable for use from small boats in lakes are also available.
Echo sounder software
The echo sounder software currently available for CREST implements a conventional high performance, fixed frequency, single channel or split beam vertical echo sounder. A range of different transmitted signals can be generated including chirps in a variety of combinations (e.g., alternate chirps and conventional fixed-frequency pulses. Data may be completely unprocessed or demodulated, filtered and corrected. The latter uses complex demodulation (preserving amplitude and phase), a 100 tap FIR low pass filter (Parks-McClellan optimal design) and 20LogR+2αR or 40LogR+2α time varied gain. The filter pass band is matched to the pulse length selected.
Data management and control software
The CREST control software runs on the control computer and downloads code to, communicates with and controls the transceiver. It receives echo sounder data from the receiver, saves these data into files and incorporates navigational and other information. It prints echograms to both a printer and the terminal screen, provides the operator with monitoring information and allows them to set parameters and control system operation.
The operator interacts with the system via an X-Window graphical user interface.
Data Analysis: ESP2
ES2P enables the user to check, list, plot and perform echo-integration on echo sounder data collected with CREST. ESP2 will also accept Simrad EK500 and EK60 data. The data to be listed, plotted and integrated can be selected by depth, ping range and amplitude. Selected and integrated data can be output for further processing using statistical packages such as SPlus and MATLAB. The check function ensures data integrity and provides a range of sophisticated bottom tracking algorithms. It is very portable and is available on a wide range of UNIX computers.