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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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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) • Numerical prediction of transmission loss at different frequencies and experimental results • Possible outline of the network • Navigation • Communication • Ocean Observation • 4. Problems ?
TAP (blue) and ACOUS (red) experiment paths in the Arctic Ocean
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
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
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!
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
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
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
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
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
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.
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
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
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,… ?