Challenges to Use RF Signals in Underwater

Acoustic waves have been the primary means of underwater communications and detection. They also have disadvantages: because they are low frequency waves, they are unsuitable for use in higher-speed communications and cause significant propagation delay. For these reasons, other means of communication, including radio waves, have been studied. However, certain properties of seawater make underwater radio wave use difficult.

Seawater has electrical properties very different from air.

Its relative permittivity of approximately 81 (compared to approx. 1 for air) and electrical conductivity of approximately 4 S/m* (compared to approx. 0 S/m for air) greatly attenuate radio waves.

Lower frequency radio waves travel through seawater more easily. Some scientists have also been researching visible light communications underwater because certain wavelengths of blue light (which gives the ocean its blue appearance) is known to be more resistant to attenuation.

Although the use of visible light enables high-speed communications, it is strongly directional and therefore requires careful emitter-receiver alignment.

In addition, visible light may scatter in turbid water, preventing it from reaching the receiver. Radio wave-based underwater wireless technologies may therefore complement acoustic wave and visible light communications when conditions render them ineffective. In another research project, we developed a portable vector network analyzer capable of measuring electrical properties in the range between 10 kHz and 10 MHz and an electromagnetic response measurement system that can be connected to an underwater cable for use in shallow seas. The system is equipped with interfaces to which a variety of sensors (e.g., cameras, CTD meters, tilt meters, and underwater altimeters) can be connected to record environmental conditions.

We also constructed a frame (an underwater channel sounder (UCS)) onto which these measurement devices and sensors can be mounted that can be suspended in water for various experiments.

UCS has a transmitting antenna and three receiving antennas. The transmitting antenna was mounted on a movable stage and is able to move 900 mm in either direction from the central position in parallel with the array of receiving antennas. The receiving antenna array is also mounted on a movable stage and can be moved within the frame, enabling the distance between it and the transmitting antenna to be adjusted.

The control unit controls antenna switching, communications with the network analyzer, various sensors, and other functions. A fiber optic cable connects this control unit with the terminal controller on the ship, enabling the ship to transmit control commands to and receive data from the UCS. The control unit, the network analyzer, and the battery are mounted in a row.

These components are enclosed in pressure- resistant containers capable of withstanding water pressure up to about 100 m in depth. Using this measurement system, we were able to evaluate the propagation properties of 10 MHz radio waves while varying the inter -antenna distance from 60 cm to 140 cm.

We are also developing an underwater wireless measurement system with under-seabed sensing capabilities. This technology uses radio waves to detect buried objects invisible to acoustic waves and other sensory technology. The system is designed to detect objects buried under the seafloor by measuring radio wave propagation properties (amplitudes and phases) between antenna arrays directed at the seafloor. We have been using the system to measure various types of buried objects, such as metals, dielectrics and their composites. We are also examining methods of estimating the sizes and shapes of buried objects using the collected data.