VELOCITY TOMOGRAPHY SYSTEM
The block diagram of the velocity tomography system is shown in Fig. 1. The system is composed of four portions of transmitting, receiving, timing and system control.
The transmission signals are formed, amplified and sent to the transmitter through a cable. The one period of the Gold sequence that modulates a carrier of frequency 5.5kHz with BPSK (binary phase-shift-keying) is used as a transmission signal. The Gold sequence with various codes is prepared for different stations. The coding of signals is done by the CDMA (code-division multiple-access)
Transmitted signals are detected by a hydrophone. Received signals are processed through amplifier, coherent demodulator, 16-bit A/D converter and digital signal processor (DSP), and stored in the hard disk of a card-type computer. The correlation of the received signals and the corresponding Gold sequence is promptly calculated by the DSP.
All timing clocks of transmission, demodulation, and A/D conversion are synchronized by the pulse signals from GPS with the aid of timing circuit. When the GPS is unavailable, it is replaced by a high-accuracy time base module.
The system controller is a card-type computer, which manages all operations of the system including the low power consumption and the communication with an external computer by IrDA (infrared data association).
For long-term velocity measurement, the following special design is considered
1) Using GPS (or time base module) for high timing accuracy,
2) Using the high frequency sound and the coherent demodulation to improve time resolution,
3) Using the spread spectrum sequences as transmission signals to identify the multi-station signals, and
4) Low power consumption design for long-term use.
Figure 2 show the photo pictures for various parts of the system. The left picture is the surface buoy and pressure housing. The upper right picture is the GPS antenna and electronic package. This unit is put into the upper part of pressure housing. The lower right picture is the transmitter and hydrophone.
The major parameters of the velocity tomography system are presented in Table 1. The maximum station number of sixteen can be processed for this system. When using a high power transmitter, the sound source level reaches 200 dB. Timing accuracy is less than I μs by for the GPS and less than 8 μs for the time base module.
