An underwater navigation algorithm that provides a "cold start" (CSA) geographic position, geo-position, underwater while submerged using travel times measured from a constellation of acoustic sources is described in Mikhalevsky, Sperry, Woolfe, Dzieciuch, and Worcester [J. Acoust. Soc. Am. 147(4), 2365 - 2382 (2020)]. The CSA geo-position is used as the receive position in the ocean for acoustic modeling runs using an ocean general circulation model (GCM). A different geo-position is calculated using adjusted ranges from the travel time offsets between the data and modeled arrival times for each source. Because the CSA geo-position is close to the true position, the source to CSA position propagation model path and the source to true vehicle position data path of the acoustic arrivals are nearly coincident, enabling accurate measurement of travel time offsets. The cold start with model (CSAM) processing reduced the CSA geo-position errors from a mean of 58 to 25 m. A simulation is developed to estimate CSA and CSAM performances as a function of group speed variability between the source paths. The CSAM geolocation accuracy can be calculated from and is controlled by the accuracy of the GCM.

A long range Underwater Navigation Algorithm (UNA) is described that provides a geolocation underwater while submerged without having to surface for a Global Navigation Satellite System (GNSS) position. The UNA only uses measured acoustic travel times from a constellation of underwater acoustic sources analogous to the constellation of satellites in GNSS. The UNA positions are calculated without any a priori track, position or sound speed information, and thus provide a "Cold Start" capability. The algorithm was tested using data from the 2010-2011 Philippine Sea Experiment in which six sources were deployed in a pentagon ∼400 km on a side. 502 positions of hydrophones in a bottom-moored vertical line array at depths of 485-3037 m drifting in a tidal watch circle up to 600 m in diameter were computed. The sources were 129-450 km from the hydrophone receivers. The mean UNA position error from ground truth was 58 m with a standard deviation of 32 m. The UNA Cold Start Algorithm position can be used as the point in the ocean for calculating acoustic model runs from the source positions with a four-dimensional sound speed field from a general circulation model to improve the accuracy.

The Arctic Program Office of the Office of Naval Research ten Arctic field programs from 1978–1994 under the visionary leadership of program managers Dr. G. Leonard Johnson and Dr. Tom Curtin. During this period, over ten ice camps in both the western Arctic (Beaufort Sea) and the eastern Arctic (Nansen and Pole Abyssal Plains were manned and four ice breakers served as platforms in the marginal ice zone (Fram Straits). Since the cost of the support logistics for Arctic field programs is so very high, these experiments were multidisciplinary and almost all had an acoustic component. Some of the highlights were transoceanic reverberation, seismic reflection and refraction, random channels for time and Doppler spreading, target detection, matched field processing,ocean acoustic tomography, seismicity, and ambient noise were among the many topics examined. There were also robust efforts advancing data acquisition. Large, two dimensional horizontal arrays with both cabled and “WIFI” telemetry, large vertical arrays, precision sensor navigation, and sophisticated remote instrumentation buoys were deployed. With the end of the “Cold War” the last field program was in 1994 and the ONR Arctic program eventually was disestablished. Now, the retreat Arctic ice cover and Arctic Ocean warming has reinvigorated ONR's interest in the Arctic and after two decades ONR field programs are planned for the near future.

The dramatic reduction of sea ice in the Arctic Ocean will increase human activities in the coming years. This activity will be driven by increased demand for energy and the marine resources of an Arctic Ocean accessible to ships. Oil and gas exploration, fisheries, mineral extraction, marine transportation, research and development, tourism, and search and rescue will increase the pressure on the vulnerable Arctic environment. Technologies that allow synoptic in situ observations year-round are needed to monitor and forecast changes in the Arctic atmosphere-ice-ocean system at daily, seasonal, annual, and decadal scales. These data can inform and enable both sustainable development and enforcement of international Arctic agreements and treaties, while protecting this critical environment. In this paper, we discuss multipurpose acoustic networks, including subsea cable components, in the Arctic. These networks provide communication, power, underwater, and under-ice navigation, passive monitoring of ambient sound (ice, seismic, biologic, and anthropogenic), and acoustic remote sensing (tomography and thermometry), supporting and complementing data collection from platforms, moorings, and vehicles. We support the development and implementation of regional to basin-wide acoustic networks as an integral component of a multidisciplinary in situ Arctic Ocean observatory.

On April 9 and 13, 1999, two Arctic Climate Observation Using Underwater Sound tomography signals were transmitted from a 20.5-Hz acoustic source moored at the Franz Victoria Strait to an eight-element, 525-m vertical array at ice camp APLIS in the Chukchi Sea at a distance of ∼2720 km. The transmitted signal was a 20-min long, 255-digit m-sequence that can be treated as a binary-phase shift-keying communication signal with a data rate of 2 bits/s. The almost error-free performance using either spatial diversity (three elements) for a single transmission or temporal diversity (two transmissions) with a single element demonstrates the feasibility of ice-covered trans-Arctic acoustic communications.

The waters of the Arctic Ocean have been warming since the early 1990s. Average maximum temperatures have risen by more than 1 °C. In the last 20 years submarine measurements of sea ice draft have shown a 40% reduction in average sea ice thickness while satellite remote sensing has shown a 14% reduction in sea-ice extent over the same period decreasing at a rate of 3%–5% per decade with thicker multi-year ice at 7%–10% per decade. Forecasts indicate that if these trends continue the Arctic Ocean could be ice-free in the summer before the end of this century. Significant effort is needed to expand our observational capabilities in the Arctic Ocean to support better modeling, forecasting, and improve our understanding of this critical ocean and the linkages to global climate. One technique acoustic thermometry has been shown to be a very effective for monitoring average heat content and average temperature in the Arctic Ocean and, in particular, in the Arctic Intermediate Water layer. Two experiments conducted in 1994 and 1999 measured the warming and demonstrated the feasibility of long term observations. Plans are in process to incorporate acoustic thermometry and tomography in in-situ Arctic Ocean observatories.

Operational monitoring and forecasting system for global and regional oceans, including the Arctic, combines observations from different satellite remote sensing techniques and in-situ open ocean measurements with ocean circulation models through advanced assimilation techniques. Satellites can sufficiently monitor changes in surface properties of the polar oceans, while the interior of the ocean is poorly observed since the water mass is opaque to electromagnetic waves and Argo floats cannot yet be used in the Arctic. Correspondingly, the internal of the Arctic Ocean is not monitored on a systematic basis, and this represents a significant gap in the Global Ocean Observing System. It is recommended to design and implement a cost-efficient, multi-purpose acoustic infrastructure for ocean acoustic tomography, navigation/positioning of gliders and floats under ice, and monitoring of ambient noise and marine mammals.

Acoustic data from the Arctic climate observations using underwater sound (ACOUS) experiment are analyzed to determine the correlation between acoustic propagation loss and the seasonal variability of sea ice thickness. The objective of this research is to provide long-term synoptic monitoring of sea ice thickness, an important global climate variable, using acoustic remote sensing. As part of the ACOUS program an autonomous acoustic source deployed northwest of Franz Josef Land transmitted tomographic signals at 20.5Hz once every four days from October 1998 until December 1999. These signals were received on a vertical array in the Lincoln Sea 1250km away. Two of the signals transmitted in April 1999 were received on a vertical array at ice camp APLIS in the Chukchi Sea north of Point Barrow, Alaska, at a distance of approximately 2720km from the source. Temporal variations of the modal propagation loss are examined. The influence of ice parameters, variations of the sound speed profile, and mode-coupling effects on the propagation losses of individual modes is studied. The experimental results are compared to the results of the earlier experiments and the theoretical prediction using numerical modeling.

One can exploit the broadband acoustic striation patterns produced by loud merchant ships in shallow water to obtain geoacoustic parameters. One measures these patterns as a function of range and frequency, calculates their two dimensional Fourier transform to produce intensity as a function of wavenumber and delay, then performs an inverse Radon transform to obtain the distribution of the interference invariant. With this observable, one can perform a global inversion for parameters of interest, such as sediment and bottom sound speed, density, and attenuation. If the parameter estimate is unbiased and the observable vector Gaussian has high signal-to-noise ratio, then one can obtain the theoretical minimum variance and covariance associated with the parameters by solving for the Cramer–Rao lower bound (CRLB). In this presentation, the parameters of interest are assumed to be deterministic and observed ‘‘noise’’ is a function not only of additive Gaussian noise at the hydrophone, but also due to uncertainty in other model parameters, such as hydrophone depth and range bias. This talk will present numerically calculated CRLB as a method for determining the performance of this inversion method.

S. D. Chuprov’s interference invariant, or β parameter, characterized broadband striation patterns as a function of range and freuencey. Puchenkina and Salin proposed using the β parameter to estimate geoacoustic bottom parameters. Further work by Baggeroer, Rousseff, and Spindel recommended characterizing β as a probability distribution instead of a discrete quantity. the authors of this presentation will illustrate use of the β distribution to perform geoacoustic parameter inversion in shallow water, using simulated annealing methods. This inherently broadband process utilizes ships of opportunity combined with a priori range information to provide high-SNR input signals to the inversino process. [Work sponsored by DARPA Advanced Technology Office, Contract N00024-01-C-6319.]

Remote acoustic measurements of large-scale variations in the Arctic Ocean water temperature were taken along the first stationary transarctic path since 1998 within the framework of the joint Russian-American project Arctic Climate Observation using Underwater Sound (ACOUS). The measurement period was longer than a year. The acoustic path was formed with the aid of an autonomous source located at a distance of about 200 km to the northwest of Franz Josef Land and an autonomous receiving array located in the Lincoln Sea at a distance of 1250 km from the source. The acoustic signal records (obtained after the receiving array was lifted in March 2001) showed that the source transmitted signals regularly during 14 months from October 1998 to December 1999. During this period, 107 signals (each of them with a duration of about 20 minutes) were transmitted and recorded on the array with a 4-day interval. The filtering of individual acoustic modes in the signals received by the vertical array yielded the time dependences of the times of arrival, amplitudes, and phases of these modes. The experimental data were compared to the results of the numerical simulation of signal propagation under the experimental conditions. An analysis of the experimental acoustic data revealed a number of unexpected phenomena, such as significant fluctuations in the amplitudes and forms of the pulses of both the first and second modes and also a rapid decrease (since August 1999) in the time of propagation of mode 1. The numerical simulation showed that the fluctuations in the mode amplitudes are due to the effect of mode coupling within the horizontally inhomogeneous portions of the path, and the decrease in the time of propagation of mode I is due to the occurrence of unusually warm water masses of Atlantic origin in a vast zone to the north of the Eurasian continental slope. The experiment also showed that consideration for the mode coupling effects allows one to localize variations in the ocean environment along the path, i.e., to obtain a horizontal resolution for the inverse problem of acoustic ocean thermometry.

The first stationary transarctic path for year-round acoustic observations of temperature changes in the Arctic Ocean was established in October 1998 as part of the U.S./Russian ACOUS project. The path was created with an autonomous acoustic source deployed 200 km north-west of Franz Josef Land and an autonomous vertical receiving array in the Lincoln Sea 1250 km away. The source was operating until December 1999. The obtained acoustic thermometry time series consists of 107 20-minute tomographic signals transmitted at a 4-day interval. Acoustic mode filtering of the received signals was performed and temporal variations of the modal amplitudes, travel times and phase were analyzed. The analysis of experimental data revealed several acoustic phenomena such as strong long-term variations of the modal amplitudes and pulse shapes and considerable decrease of the travel time of mode I starting from August 1999. Numerical modeling showed that the amplitude variations of modes were due to mode coupling and substantial change in the travel time of mode I was a result of unexpectedly large crosswise extent of the warm Atlantic water circulation core north of the Eurasia continental slope. It is also demonstrated that consideration of the mode coupling effects in the modal arrival patterns gives a way for locating changes in the ocean environment along the acoustic path.

Acoustic thermometry has been shown to be a very effective technique for monitoring average heat content and average temperature in the Arctic Ocean and in particular in the Arctic Intermediate Water (AIW) layer. As part of the U.S./Russian Arctic Climate Observations using Underwater Sound (ACOUS) program a 14-month time series of acoustic transmissions were analyzed along a 1250 km propagation path that extended from the Franz Victoria Strait to the Lincoln Sea from Oct. 1998 through Dec. 1999. The receive array mooring in the Lincoln Sea was recovered in April 2001. Modal travel times were estimated after pulse compression processing and mode filtering of the vertical line array. The interarrival time between mode 1 and modes 2 and 3 show net cooling during the first several months followed by a dramatic warming of the AIW along the propagation path. This warming is consistent with direct CTD measurements made along a central Arctic transect performed by the USS Hawkbill during the Scientific Ice Exercise (SCICEX) 2000. [Work supported by ONR, NSF, the Civilian Research and Development Foundation, and the Ministry of Industry, Science and Technology of the Russian Federation.]

In 1997, the Naval Studies Board recommended a focus on exploitation of the intrinsic coherence of the ocean to achieve 20 dB greater detectability by sonar systems. That same year, the JASONs suggested achieving these gains by elimination of shipping discretes. The Acoustic Testbed Working Group evaluated adaptive beamforming technology used in conjunction with various passive acoustic array topologies to suppress noise from discrete shipping and achieve the desired performance gains. These studies indicate that acoustic arrays of limited spatial extent, with both horizontal and vertical aperture make the most efficient use of large numbers of phones. This is because the motion of discrete shipping in typical littoral environments limits the amount of time integration that can be applied when using large horizontal apertures.