Electromagnetic Under-the-Ice Localization and Communication

Abstract

This paper proposes a localization method for underwater vehicles operating below sea ice and describe experimental research results comprising the basis of the method. The method integrates a hybrid navigation system of Doppler velocity log (DVL) - inertial navigation system (INS) and an under-the-ice electromagnetic localization (UEML) system. If ice-relative dead reckoning would achieve positioning error comparable to bottom-relative one, the hybrid system has position error of a few meters for a kilometer. An underwater part of the UEML system installed on an underwater robot transmits EM beacon. A part in air of The UEML system on which is set a drone above or a vehicle on ice surface measures its absolute position with a GNSS (Global Navigation Satellite System) and EM field strength. It thus makes a 2D-EM field map as an absolute reference for underwater robot. We carried out experimental research of EM propagation among air, sea ice, and seawater. It is recognized that attenuation of EM wave of 10 kHz between seawater and ice-surface (in air) vertically is about 80 dB at depth of 5 m when a 1 m-class-small loop antenna and an underwater 2 m-half sheath antenna are used. We estimated that vertical coverage of the UEML is 17 m assuming a link budget of 134 dB from measured data with fitting curve calculated by Sommerfeld equation. In contrast, at distance between the antennas of 80 m, decay becomes 134 dB when depth of the 2 m-HSA is 4 m due to lateral waves. From these results, beacon signal will be received in relatively wide area in air when an underwater robot approaches to less than 17 m. The communication test in horizontally shows that service range of PSK modulated waves of 1 kbps is over 40 m. From these results we can estimate that communication between the drone and the underwater robot can be also established in depth of less than 17 m. In this depth range we can perform position update of the navigation system installed in the robot. This limitation would not be crucial problem since one of purpose of under-the-ice robot is observation of and survey near the ice bottom. If deep sea survey is required, taking two underwater vehicles configuration may be better.