Features of the Heard Island signals received at Ascension

The Heard Island transmissions were received 9140 km away at Ascension Island by an irregular array of bottom‐mounted hydrophones. The single‐hydrophone signal‐to‐noise ratio sometimes exceeded 30 dB in a 1‐Hz band, confirming the detectability of 57‐Hz underwater sound at global distances. The arrival‐time pattern consists of a single broad pulse about 10 s long, whose fine structure decorrelates in about 12 min, in sharp contrast with the stable, discrete sequences observed over shorter, midlatitude paths. The amplitude fluctuations of both the fine arrival structure and the unmodulated receptions are uncorrelated between hydrophones as little as 3.4 km apart. Phase varies less than one cycle during a 1‐h transmission after correcting for source motion, and the rms phase difference between hydrophones is about 3 rad averaged over the array. Phasor diagrams suggest that the effects of both source motion and ocean dynamics vary over the array. The probability density functions of the real and imaginary parts of a downshifted cw transmission are nearly Gaussian.