Experimental Results in Acoustic Communications Under Shore-Fast Greenland Ice

A small experimental network of ocean sensors linked by acoustic communications was tested in March-April of 2017 in shore-fast ice near Thule Air Base in northern Greenland. The objective was to simultaneously test a real-time communications system for linking together under-ice sensors, and also to gather synoptic oceanographic and acoustic data to be used for improving under-ice propagation modeling. The acoustics portion of the experiment included tests at ranges from 3 to 35 km at a carrier frequency of 3.5 kHz at data rates from approximately 120-1200 bits per second. Initial results from the Micro-Modem real-time hardware used in the Thule test showed reliable links at 20-25 km at most data rates, and moderate connectivity from 25-35 km, depending on the specific path and data rate. The communications performance is governed by several factors including: the relative depth of source and receiver, the location within the fiords and proximity to glacial fronts with varying sound-speed profiles, pinniped (seal) vocalizations, pier-side machinery noise and occasional random impulsive noise events attributed to iceberg movement. The results show good link stability despite a spatially-variant sound speed field resulting from mixing of glacial and ocean waters.