#ATOMIC CLOCK PC#
CAT operation is always monitored by another small micro PC, called PIC, and the SH7145F micro PC is forced to be reset by PIC when the communication of the PIC with the SH7145F stops for a predetermined period with unknown troubles. CAT system is slept between two consecutive transmissions to reduce power consumption for the case of larger transmission intervals.
#ATOMIC CLOCK SERIAL#
The status of system operation is monitored via Bluetooth serial interface for the land-based CAT. As a result, receiving window can be selected widely to receive transmission signals safely. As an alternate method, correlation calculation is performed swiftly by carrying the SD data with raw data to the external PC. However, correlation calculation by the internal PC is not recommended because it exhausts much computational time especially for data acquired with a wide receiving window.
The correlation calculation can be performed by the SH7145F microcomputer, equipped inside the CAT controller. As a result, a matched filter is constructed through the correlation with M sequence. The received signals are also cross-correlated by one-period replica of M sequence, used in the transmission to increase remarkably the signal-to-noise ratio (SNR). After the 10-bit A/D conversion, the received data are sampled at twice the carrier frequency and stored as two-channel raw data in the 2 GB micro SD memory card. By taking a product of the received signals with cosine and sine functions, they are branched into two channels in-phase and quadrature components and low-pass filtered to pick up only signals related to the M sequence through complex demodulation. Acoustic signals coming from the counter station are received by the same transducer (called mono-static sonar), band-pass filtered, and amplified.
#ATOMIC CLOCK SOFTWARE#
A carrier phase modulated by pseudo random signals ( M sequence and Gold sequence), constructed by the software program, is transmitted at a predetermined time interval from a broadband transducer after power amplification. For the moored-type CAT system, GPS clocks are replaced by chip-scale atomic clock (CSAC). All clocks are provided by 1 Hz and 10 MHz clock signals from GPS. The timing of sound transmission and reception and A/D conversion is controlled coherently by SH7145F micro PC. In a field operation of CAT, this provides a critical advantage to protect the internal electric circuits from the attack by salty wind and makes a long-term stable operation of CAT system possible.īlock diagram of the land-based CAT system is shown in Fig. Thus radio waves from global positioning system (GPS) can propagate directly to the GPS receiver stored inside the container without opening the upper lid of the container. CAT controller is stored in a plastic container.
Solar panels that supplement the rechargeable batteries are equipped as an option of the CAT system when a long-term operation is required. A tomography domain is surrounded by multi land-based CAT systems, and only sound traverses the tomography domain surrounded by the acoustic stations. The system is composed of a CAT controller and rechargeable batteries on land, and subsurface transducer and cable in water ( Fig. A land-based coastal acoustic tomography (CAT) system provides a new observation tool to enable oceanographic observation from the shore ( Zheng et al., 1997 Zheng et al., 1998).
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