Acoustic modem unit

Abstract

This paper will describe a device for transmitting data using a ships hull or frame as the acoustic path. This device is called a "HullCom" Acoustic Modem Unit (AMU). The initial purpose for the development was wireless data transmission on Volunteer Observing Ships (VOS). VOS offer a very attractive platform for meteorological and oceanographic measurements over much of the global oceans. Many VOS are fast container ships whose operators seek minimal time in port. There is not enough time to install cables and in addition, these ships are sold frequently in order to match the ship capability to contracted loads (thereby making cables unusable). In addition, installing cables on ships can be more expensive than the cost of the custom electronics described in this paper. The device consists of two sets of equipment, a local AMU (AMU-L) in an accessible place such as the main deck and a remote AMU (AMU-R) in a non-accessible place such as a sea surface temperature instrument mounted inside the hull near the waterline. Each AMU consists of an acoustic transducer, acoustic transmit and receive electronics, battery power and a controller. The acoustic portions of the AMU were designed and fabricated by Harris Acoustic Products and the controller was designed at the Woods Hole Oceanographic Institution. The acoustic portions of the AMU are based on early work by SeaBeam Instruments Inc on a "Hull Phone". The "Hull Phone" uses a voice modulated carrier frequency to provide communications in ships that do not have working standard communications due to damage or other circumstance. The translation from voice to a digital frequency shift key data signal is straight forward. Establishing an acoustic protocol that can tolerate the many reflections and noise interference was not straight forward. The final result is a 20 baud modem.