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Alexander Gavrilov, CMST

Long-range acoustic transmissions for navigation, communication, and ocean observation in the Arctic. Alexander Gavrilov, CMST. Peter Mikhalevsky, SAIC. OUTLINE. Some examples of long-range acoustic transmissions in the Arctic Ocean (TAP and ACOUS experiments)

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Alexander Gavrilov, CMST

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  1. Long-range acoustic transmissions for navigation, communication, and ocean observation in the Arctic Alexander Gavrilov, CMST Peter Mikhalevsky, SAIC

  2. OUTLINE • Some examples of long-range acoustic transmissions in the Arctic Ocean (TAP and ACOUS experiments) • Numerical prediction of transmission loss at different frequencies and experimental results • Possible outline of the network • Navigation • Communication • Ocean Observation • 4. Problems ?

  3. TAP (blue) and ACOUS (red) experiment paths in the Arctic Ocean

  4. TAP signal at ice camp SIMI after pulse compression 3500 3 3000 - numerical prediction 4 2500 2 2000 Amplitude,Pa 1500 1000 500 1 1805 1810 1815 1820 1825 1830 1835 Travel time, s Evidence of multi-path (multi-mode) propagation

  5. Before processing After pulse compression 110 120 105 110 100 95 100 90 90 Signal level, dB re. 1 Pa Signal level, dB re. 1 Pa 85 80 80 75 70 70 60 65 60 50 0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400 450 Day number Day number ACOUS signal and noise levels at individual receivers of the Lincoln Sea array (ACOUS source level: 195 dB; distance: ~ 1250 km) 450 Noise level in a 1-Hz frequency band Noise level limited by receivers’ dynamic range

  6. SNR and coherence of ACOUS signals on the Lincoln Sea array SNR before (blue) and after (red) pulse compression Cross-correlation matrix of 10 periods of ACOUS signal 1 45 1 40 0.99 2 35 0.98 3 30 0.97 4 25 0.96 5 SNR, dB 20 0.95 6 15 0.94 7 10 0.93 8 5 0.92 9 0 0.91 10 0.9 -5 1 3 5 7 9 0 50 100 150 200 250 300 350 400 450 2 4 6 8 10 Period number Day number Exceptional temporal stability of the channel at 20 Hz!

  7. Level of two ACOUS signals (blue) and noise (red) on APLIS vertical array after pulse compression (Distance: ~2720 km) 80 75 70 65 60 Signal level, dB re. 1 Pa 55 ~ 34 dB theoretical limit 50 45 40 35 100 200 300 400 500 600 700 Depth, m

  8. 105 100 95 90 85 Noise level, dB re. 1 Pa/Hz1/2 80 75 70 65 1 2 3 10 10 10 Frequency, Hz Variation of ambient noise level in the Arctic ~90% of time

  9. 0 10 -1 10 Attenuation, dB/km -2 10 1.5 F -3 10 1 2 10 10 Frequency, Hz Frequency dependence of modes 1 - 40 attenuation modeled for the Central Arctic Basin and some experimental results NUSC 1959 FRAM IV, 1982 TAP, 1994 (mode 1) TAP, 1994 (modes 2-4) ACOUS, APLIS (mode 1) ACOUS, APLIS (mode 2) Ice model parameters: mean ice thickness – 3.5 m; bottom standard deviation – 2.3 m; top standard deviation – 0.6 m; correlation length – 40 m

  10. Depth: 50 m Depth: 400 m 80 80 -80 -125 -130 -135 -120 70 70 -90 60 60 -100 -105 -115 -120 -100 Frequency, Hz -110 -105 -115 50 50 -110 -85 -95 -110 -100 40 40 -120 -95 -90 -90 30 30 -130 -140 200 400 600 800 1000 1200 200 400 600 800 1000 1200 Range, km Range, km Transmission loss along ACOUS path at 50 m and 400 m modeled for a broadband signal 0-dB SNR for a 50-Watt (~190 dB) source -20-dB SNR for a 50-Watt (~190 dB) source

  11. 90W 120 60 C C C a a a n n n a a a d d d a a a 5 00 2000 150 30 35 00 G G G r r r e e e e e e n n n l l l a a a n n n d d d 5 00 180 0 20 00 5 00 40 00 ACOUS source 35 00 20 00 5 00 150 30 R R R u u u s s s s s s i i i a a a 120 60 Notional acoustic network 90E Autonomous sources Acoustic observation paths Cabledtransceiver nodes Cable with shore terminals Cabled/autonomoustransceiver nodes

  12. Navigation: • Stationary acoustic sources are to transmit pulse-like signals for accurate measurements of travel times to moving platforms. Nav. signals should also contain certain information (at least source ID numbers, UTC time, etc.). • 2. Communication: • Two-way communication is needed to check the operational state (most important) and to track position of mobile platforms • Underwater communication of oceanographic data over long distances does seem feasible • 3. Observation (thermometry, ice monitoring) • Feasible for stationary receivers/transceivers. For mobile platforms, it requires accurate timing and complicated interpretation of travel time data.

  13. A simple method to design navigational/ communicational/observational signals Series of two signals: training (observational) signals followed by informational signal = navigational signal , where , and is the M-sequence of length N = 2M - 1 is the Hadamard code of number m < N Processing: compute the likelihood function: for each message m, using Hadamard transform

  14. 1.0 M=512 10-1 Error probability 10-2 M=1024 10-3 0.0 0.10 0.05 0.15 0.20 0.25 Signal-to-noise ratio Error probability for binary message m at different SNR for two different signal bases

  15. Most serious problems • Weight, power consumption and reliability of low-frequency sources, especially for mobile platforms 2. Doppler effect for mobile platforms 3. Slow communication rate 4. Accurate timing for mobile platforms 5. Separation of acoustic thermometry/halinometry data from navigational errors. 6,7,… ?

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