3 Instrument design and modules.
Figure 2 displays the system architecture where one can see the receiver part only called SARA and the complete tomographic instrument named JHAS which is obtain by adding a transducer and its driving electronics (power amplifier and impedance matching). A main microcontroller drives a number of other dedicated modules. The main controller and the modules communicate together by using the intermodule bus comprising an open collector loop, a 1MHz frequency signal from which are derived various modules frequencies and a synchronisation pulse.
Figure 2: General Architecture of SARA and JHAS.
Acoustic Receiver : This module is designed as a general acoustic receiver with either a single hydrophone or a 4-channel array, a very low noise large bandwidth preamplifier, an automatic gain control, a switched-capacitor lowpass filter, a 16-bit analog to digital converter and a powerful micro controller to perform some signal processing : bandpass filter, complex demodulation, crosscorrelation... The sampling frequency is fixed to 4 times the incoming carrier frequency. The samples are temporarily memorized in an EEPROM memory (2Mb) before storage on an hard disk.
Storage unit : This module is built around a PC/104 unit with a SCSI interface. The storage capacity is 1Gb but can be easily increased. A 10-line parallel bus is used to transfer the data beetween the acoustic receiver and the disk at a speed of 50kb/s.
Navigator : It is possible to sacrifice a part of the acoustic data set to find the mooring positions in the inverse problem [8], but it is better to correct the data from the mooring motions. To increase the travel times measurement accuracy, the range or the range variations beetween the instruments must be known to a few meters. The navigator is part of a long baseline positionning system which includes also 3 acoustic tranponders deployed on the sea floor about the base of the mooring. It periodically interrogates those transponders and records the travel times from each transponder. Triangulation gives the relative position of the mooring all along the experiment with a precision of a few meters [9].
