The Heard Island Feasibility Test

In January 1991, the Heard Island Feasibility Test (HIFT) was carried out to establish the limits of usable, long‐range acoustic transmissions. Coded acoustic signals transmitted from a source near Heard Island in the southern Indian Ocean were monitored at 16 sites in the North and South Atlantic, the North and South Pacific, the Indian Ocean, and the Southern Ocean. The question posed by HIFT, whether at such global ranges the signals would permit phase‐coherent processing and thus yield favorable signal‐to‐noise levels, was answered in the affirmative. There was no evidence of distress by the local marine mammal population in response to the acoustic transmissions. HIFT was prerequisite to a program for Acoustic Thermometry of Ocean Climate (ATOC). The principal challenges to such a program are discussed.