First submarine phone booth

In November 2001, France Télécom had already announced the first submarine telephone booth. In co-operation with Amphicom, a French company specialised on underwater communication, they had developed the first phone connection to a diver working underwater. The system comprises a buoy fitted with a GSM phone relay that handles two-way communications with an underwater terminal. The terminal is connected to the buoy by a wire and is equipped with a dial pad, a special mouthpiece, a light, and a buzzer.

This is how it works: The buzzer and a flashing light alert the diver of an incoming call. The sound wave from the surface goes through the system to the mouthpiece. The diver just has to bite down on the mouthpiece and push a button to accept the call. Sound vibrations propagate to his ear via his skull, which acts as a resonance chamber. He can then clearly hear the incoming call and also talk back. With the dialling pad, the diver can also call anybody on a fixed or wireless phone.

Application areas

The `submarine phone booth' has been tested by archaeologists at the Alexandrine Research Centre in Egypt, who have been carrying out underwater excavations at the presumed site of the Alexandria Lighthouse. By providing direct, instantaneous communications between the divers and excavation managers, the system eliminated the need for frequent returns to the surface for reporting new findings. Other possible application areas include oil platforms, shipyards, scientific research, salvaging ships, civil security, and military use. Divers can quickly signal any sign of discomfort or danger, or report to the surface.

However, the new underwater communications system is not yet commercially available. This is due to the fact that several research issues have not yet been solved. To make underwater GSM really popular among professional and leisure divers, the wire link between the buoy and the submerged terminal has to be eliminated. Divers would then be totally independent and could wirelessly communicate with each other under water.

Underwater transmission of sound

Today, when scuba divers want to talk to each other underwater, they use hand signals or write on a slate. There is, however, already a wireless technology for underwater communications commercially available.

Devices for two-way, wireless underwater communication have been available for many years. They are based on ultrasound waves. However, ultrasound has some severe practical limitations. It is very sensitive to obstacles. This can be anything: rocks, weeds, and even dirt particles muddying the water.

Exploring current fields

For this reason, the research team at France Télécom's partner Amphicom near Toulouse explores a different way of transmission: current fields. Current field transmission uses the conductivity property of water. The main advantage is that current fields are less sensitive to water than ultrasound. Water can be considered as a wide range conductor, with a low load resistance. Bipolar electrodes put in this conductor, and supplied with DC or AC voltage, will deliver a DC or AC current. This current will generate an ionic field in the whole conductor. The current wave goes through the water, along the surface and the bottom. The wave can bypass obstacles and is not sensitive to changes of water temperature or the degree of water pollution.

In summer 2002, Amphicom successfully tested a prototype of a wireless underwater communication device using current fields. Despite the advantages of current field transmitters, Amphicom's marketing manager Kenneth Mackern could not yet specify a date for the market launch of communication systems based on current fields. Many specific questions regarding range, power consumption and battery power have still to be answered.

Phonation under water

Furthermore, there is another problem: phonation. How to speak underwater and get the sound into the transmitter? Technically, this problem is already solved. Half-face masks with a voice transmitter integrated in the mouthpiece are available on the market. However, they are not very popular among scuba divers. "Scuba divers don't like masks," explains Kenneth Mackern. They prefer to have just a mouthpiece with a regulator connected to the air tank. Integrating the communications device into the mouthpiece is the easy part, but doing it in a way that not only a gurgling sound but understandable speech is transmitted has proven to be quite difficult.

Thus, communicative scuba divers probably will have to wait some more years until they can describe colourful sea plants and exotic fishes to their loved ones at home via GSM.

Further information on the topic of underwater communication is available in the online version of this article at: http://www.eurescom.de/message/