HEARD ISLAND FEASIBILITY TEST - LONG-RANGE SOUND-TRANSMISSION FROM HEARD ISLAND TO KRYLOV UNDERWATER MOUNTAIN

The Russian research vessel AKADEMIK NIKOLAI ANDREYEV participated in the HIFT experiment while anchored above the Krylov Seamount (17.5 degrees N, 30 degrees W). The propagation path length from Heard Island to Krylov was 12.5 Mm, and was located just between the Ascension station (9 Mm) and the more distant American and Canadian stations (16 to 17 Mm). All 35 successful transmissions were received by two omnidirectional hydrophones and a 30-m-long array. Receptions were sampled at 228 Hz and processed in both time and frequency domains. The main result of the processing was quite unexpected: the signal exhibited strong amplitude variability (fading as deep as 20 dB in less than 5 min) and simultaneously exhibited an incredible phase stability. All the phase variance up to the accuracy of positioning was due to the mutual motion of the transmit and receive ships. The minimal propagation loss for cw transmissions was 120 dB. M-sequence processing indicated an 18- to 20-s arrival spread estimate. However, the arrivals were very unstable with a lifetime of about 5 min.