Bpsk

Data

M-sequence signal

Sensors

Sound velocity meter

Transducer

4 array

TX sound pressure

180 dB re uPa at 1m

RX sensitivity

-210 dB re 1V/uPa at 1m

Table 1. Specifications of the USBL

solutions with ROVs, UROVs and HROVs. In recent years, data traffic on networks has drastically increased with the evolution of broadband networks. In order to meet the demand, developers are trying to develop a 40 Gbps optical communication system using a dense wavelength division multiplexing technique for land and submergible cable applications.

For wireless remote control and status monitoring of AUVs, an acoustic communication system or an acoustic modem is used. This is also effective for monitoring an UROV or an HROV. For close-range communication, electromagnetic communication would be useful because radio communication performance would be less affected by multi-pass interference. Optical communication systems having a capacity of 622 Mbps and 2.488 Gbps are generally used for underwater vehicles. Prizm Advanced Communication Electronics Inc. provides a communication board with an HD-SDI interface. Canare in Japan manufactures fiber-optic products including an 8-channel coarse wavelength division multiplexing HD-SDI transceiver module. Neither of these manufacturers produces an all-in-one optical transceiver, which would consist of video interfaces, serial data interfaces, and parallel interfaces on one printed circuit board. Yoshida et. al. (Yoshida 2007b) have developed two types optical communication boards: one is an optical-electrical communication system for the ABISMO system and the other is a high speed device for an UROV vehicle, with the prototype being installed in the PICASSO system. a. An Optical-electrical Communication System

The ABISMO system consists of a launcher and a vehicle. The support ship and launcher are mutually connected by optical fiber cable for data transmission. The launcher and the vehicle are mutually connected by a metallic cable. Three-point-communication (the ship -the launcher - the vehicle) is therefore needed in the ABSIMO system. The block diagram of the optical communication system model, JT3 for the ship-launcher communication and the radio frequency digital communication device, JT3-RC for the station-probe communication, are depicted in Figure 6. Its optical communication bit rate is the same as the SONET (STM-4) standard but the protocol is an original one. Every input signal is sampled, time shared, Manchester encoded, and then transmitted at a bit rate of 622 Mbps. The JT3-RC is a full duplex transceiver with 8 RS-232C channels. In the JT3-RC circuit board, its synchronization is achieved by a sequential synchronization using Manchester encoding with a 16 bit preamble. The time-division multiplex data rate is 12.96 Mbps. Maximum transmission range is designed to be 200 meters by using 2.5-2 V standard coaxial cable. A pre-emphasis circuit reduces deformation of the transmission wave caused by loss through the cable. This system was practically tested in the Marianas Trench in June 2008 at a depth of 10300 m.

Fig. 6. The block diagram of the optical communication part of the JT3 (upper) and the blockdiagram of the JT3-RC. The synchronizer in JT3-RC regenerates the sampling clock.

b. A Low Cost 2.5 Gbps Optical Communication System with HD-SDI Interface

The system consists of a pair of transceiver units for the vehicle and the ship side. The transceiver unit consists of two printed circuit boards: a protocol converter board and a power supply board (each board size is 120 x 80 mm). Major devices for the converter are a 2488 Mbps optical transceiver module produced by Sumitomo Electric Industries, Ltd. and a TLK3101 transceiver chip by Texas Instruments Incorporated which is composed of 2.5 Gbps to 3.125 Gbps Serializer / Deserializer. The transceiver has the interfaces: one HD-SDI data interface for an HDTV camera, three NTSC interfaces, four RS-232C interfaces, two RS-485 interfaces, and 8-channel parallel I/O interfaces.

c. Acoustic Modem Using Time-Reversal Waves in Shallow Water

An advanced acoustic communication method utilizing time-reversal waves has been developed (Kuperman 1998, Shimura 2004). In most acoustic communications the ship-vehicle configuration is vertical because there are many multi-path signals in the horizontal configuration. It would be better to use a time-reversal technique for communication under multi-path fading in the shallow water zone. Shimura did a simulation for communication between a ship and a vehicle in the shallow water zone using high frequencies (Shimura 2006). He reported that the method of time-reversal process with an adaptive filter provides good communication results. When the vehicle, however; moves, the advantage of the method is depressed. We will try to modify the method and choose the best parameters, aiming at better ship-vehicle communication up to 500 m in distance.

d. Communication by Electromagnetic Field.

In seawater the attenuation coefficient, a in the HF band and below is obtained by equation 3 which is derived from Maxwell equations.

where p,o is the permeability, go is the conductivity of the seawater, and f is frequency in Hertz. Substitution of p,o = 4n x 10-7 and g = 4 S/m into equation 3, one obtains, a = 3.45 x 10"2T7 (dB/m). (4)

The equation means that an RF wave in seawater is rapidly damped, for example 128 dB/m at 10 MHz. A number of tries at RF communication in seawater have been made. Siegel attempted propagation measurements in seawater at 100 kHz and 14 MHz (Siegel & King 1973) by preparing a special underwater antenna. They concluded that the experimental data are in good agreement with theoretically obtained data from asymptotic formulas. A new approach to electromagnetic wave propagation through seawater has been proposed (Al-Shamma'a 2004). In their theory, there are conduction currents in the near field and displacement currents in the far field. This causes rapid signal attenuation in the vicinity of the antenna but in the far field the attenuation is comparable with the dielectric loss. JAMSTEC has also carried out propagation measurements in seawater from a quay. The propagation characteristics in the ELF roughly agreed with the theoretical characteristics. The curve according to the HF measurement data as shown in figure 7 is similar to the one that Al-Shamma'a obtained. This means that someone should make a careful investigation at HF.

"Static B by the Loop

■Background noise MAX."

--•--Electrode 1 kHz -o- Electrode 10 kHz —+— Loop ant 1 kHz —X-Loop ant 10 kHz f=14MHz, Vertical Simulation Experimental

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