Theory of Operation 1-5The Series 5000 departs from previous multibeam systems in that the swathforming process is implemented digitally using digital signal processing (DSP),rather than analog delay lines, phase shifters, or multipliers and adders. Theprimary advantage of this technique is a reduction in the size and weight of thetowfish. This affords a concomitant reduction in the size of the required surveyvessel and ancillary equipment, such as a winch and crane. Other advantagesinclude more flexibility in the swath processing, allowing software control ofoperating parameters. Previously this would have required hardware changes.The Series 5000 architecture is composed of the towfish, the tow cable, thecomputer, and the TPU. The towfish contains the transducer arrays and electronicsubsystems for transmission, reception and data acquisition, and telemetry. Thetowfish also includes standard and optional sensors for the monitoring of towfishposition and motion dynamics, pressure, depth and altitude (acousticallymeasured), temperature, and other pertinent information.The tow cable is simply a coaxial cable that provides power transfer and duplexcommunications between the towfish and the TPU.The TPU processes the data from the towfish, producing multiple simultaneouslyfocused swaths. The TPU outputs this data via a proprietary parallel port. Sonarprocessor control of the TPU is done via the local area network (LAN).1.4.2 TowfishThe towfish is composed of the towfish housing, which has external brackets forcable attachment, the transducer arrays (one for each side), the nose cone, the tailsection, and a pressure housing containing attitude and environmental sensors andelectronics. In a typical operation the towfish is towed behind the survey vessel bymeans of the tow cable. The depth at which the towfish runs below the surface ofthe water is a function of both the length of the deployed cable and the tow speed.The two transducer arrays are each composed of 12 piezoelectric ceramicsub-arrays that operate as both transmit and receive elements. Each set of12 sub-arrays is arranged into a continuous line array spanning an overall apertureof 1.2 meters.The towfish electronics is divided into five individual boards: a Transmitter board,two Receiver boards, a Sensor Interface board, and a Multiplexer board. A blockdiagram of the towfish is shown in Figure 1-3.NOTE The transducers are side specific and are not interchangeable.
