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During the Acoustic Engineering Test (AET) of the Acoustic Thermometry of Ocean Climate (ATOC) program, acoustic signals were transmitted from a broadband source with 75-Hz center frequency to a 700-m-long vertical array of 20 hydrophones at a distance of 3252 km; receptions occurred over a period of six days. Each received pulse showed early identifiable timefronts, followed by about 2 s of highly variable energy. For the identifiable timefronts, observations of travel-time variance, average pulse shape, and the probability density function (PDF) of intensity are presented, and calculations of internal-wave contributions to those fluctuations are compared to the observations. Individual timefronts have rms travel time fluctuations of 11 to 19 ms, with time scales of less than 2 h. The pulse time spreads are between 0 and 5.3 ms rms, which suggest that internal-wave-induced travel-time biases are of the same magnitude. The PDFs of intensity for individual ray arrivals are compared to log-normal and exponential distributions. The observed PDFs are closer to the log-normal distribution, and variances of log intensity are between (3.1 dB)2 (with a scintillation index of 0.74) for late-arriving timefronts and (2.0 dB)2 (with a scintillation index of 0.2) for the earliest timefronts. Fluctuations of the pulse termination time of the transmissions are observed to be 22 ms rms. The intensity PDF of nonidentified peaks in the pulse crescendo are closer to a log-normal distribution than an exponential distribution, but a Kolmogorov–Smirnov test rejects both distributions. The variance of the nonidentified peaks is (3.5 dB)2 and the scintillation index is 0.92. As a group, the observations suggest that the propagation is on the border of the unsaturated and partially saturated regimes. After improving the specification of the ray weighting function, predictions of travel-time variance using the Garrett–Munk (GM) internal-wave spectrum at one-half the reference energy are in good agreement with the observations, and the one-half GM energy level compares well with XBT data taken along the transmission path. Predictions of pulse spread and wave propagation regime are in strong disagreement with the observations. Pulse time spread estimates are nearly two orders of magnitude too large, and Λ–Φ methods for predicting the wave propagation regime predict full saturation.

Broadband acoustic signals were transmitted during November 1994 from a 75-Hz source suspended near the depth of the sound-channel axis to a 700-m long vertical receiving array approximately 3250 km distant in the eastern North Pacific Ocean. The early part of the arrival pattern consists of raylike wave fronts that are resolvable, identifiable, and stable. The later part of the arrival pattern does not contain identifiable raylike arrivals, due to scattering from internal-wave-induced sound-speed fluctuations. The observed ray travel times differ from ray predictions based on the sound-speed field constructed using nearly concurrent temperature and salinity measurements by more than a priori variability estimates, suggesting that the equation used to compute sound speed requires refinement. The range-averaged ocean sound speed can be determined with an uncertainty of about 0.05 m/s from the observed ray travel times together with the time at which the near-axial acoustic reception ends, used as a surroga...

Acoustic path lengths in the Heard Island Feasibility Test ranged from under 1 Mm to 18 Mm (1 Mm is 1000 km). The signal set consisted of three basic waveforms: cw, pentaline, and M‐sequence‐modulated carrier. This set offered the opportunity for successful measurements given the large uncertainty in prior estimates of propagation loss, stability, and arrival spread. Receivers ranged from simple sonobuoy systems to elaborate horizontal and vertical arrays. International collaborators acquired data at a variety of sites worldwide. The resulting data has been collected and subjected to a summary form of frequency domain processing. Variations in the recorded spectral phases are largely the result of nonuniformity in the speed of the source ship as determined by GPS comparison. Time domain processing has shown that at all ranges the receptions exhibit exceptional stability.

Various types of phase measurements have been introduced into ocean acoustic measurements since the early 1960s, many utilizing the increased precision possible with digital processing. These are usually considered natural and basic by their developers, but are frequently misunderstood and mistrusted by those they were designed to benefit. This paper explains a recent phase technique which was used in the Heard Island Feasibility Test data analysis of the Ascension Island receptions. It revealed that the common phase fluctuations of the wideband M‐sequence receptions were not random, but were caused by minor deviations of the source ship track 9200 km away.

Data are presented from an acoustic experiment designed to detect climatic trends of temperature in the ocean with basin‐scale resolution. These data are presented as an intriguing new way to recognize changes in spatially averaged temperature. In 1983, travel times of acoustic signals (133 Hz, 60‐ms resolution) were measured over 4000 km between a source and receiver mounted near Oahu and northern California, respectively. In 1987, measurement was begun on the travel times along six additional sections in the northeast Pacific, each at a distance of 3000 to 4000 km. Travel times changed by about ±0.2 s at each receiver at interannual periods. Changes in acoustic travel time exceeding about ±0.03 s are due to changes in the spatially averaged temperature along each section. A change of ±0.03 s is equivalent to a change in spatially averaged temperature of only about ∓0.02 °C in the upper kilometer of the ocean. The dynamical processes responsible for the temperature variability along the acoustic sections are not yet identified. No evidence is seen for a long term change in climate from intermittent data between 1983 and 1989 along a 4000‐km section.

Measured travel times of acoustic pulses, propagated across a 3000-km section of the North Pacific, are inconsistent with travel times predicted with the internationally accepted algorithm for the speed of sound in water. The sound-speed algorithm predicts a speed that is too fast at oceanic pressures found below about 1-km depth. An accurate algorithm of sound speed is important for deriving temperature from measurements of acoustic travel time across oceans. Accurate estimates of the temperature field are probably important for better understanding the ocean’s role in determining weather, climate, and the distribution of marine organisms.

Large‐scale temperatures in the North Pacific were measured by long‐range acoustic transmissions from 1996–2006. Acoustic sources off California and Kauai transmitted to receivers distributed throughout the North Pacific from 1996–1999. Kauai transmissions continued from 2002–2006. Acoustic travel‐time data are inherently integrating. This averaging suppresses mesoscale variability and provides an accurate measure of large‐scale temperature, subject to the limitations of the ray path sampling. At basin scales, the ocean is highly variable, with significant changes occurring at time scales from weeks to years. The interannual variability is large compared to trends in the data. Willis, et al. used objective mapping techniques applied to satellite altimetry and hydrography to derive 0–750 m temperature fields for the global ocean. Travel times equivalent to the measured travel times can be calculated using these fields. The measured and calculated travel times are similar, but also show significant differences. Similar comparisions using travel times derived from the ‘‘Estimating the Circulation and Climate of the Ocean’’ (ECCO) model and a high‐resolution Parallel Ocean Program (POP) model also show similarities and differences. The ECCO model was constrained by altimetric and profile data by data assimilation, suggesting that the acoustic travel times provide meaningful additional constraints on model behavior.

A shallow water propagation experiment was performed in conjunction with a Saclant Undersea Research Center project known as the Malta Plateau Experiment (or MAPEX 2000) in early March 2000. A small, inexpensive, prototype 800‐Hz source broadcasting 128‐digit M‐sequences was deployed off the southern coast of Sicily in approximately 135 m of water at a distance of 5 km from a fixed 64‐element vertical array of hydrophones. This array relayed received signals to the R/V Alliance via radio telemetry. The Alliance then deployed a 256‐element towed array and proceeded away from the source at a speed of 5 kts for about 3 h. The experiment was repeated once more using a second prototype with a different length M‐sequence. The maximum distance from source to receiver was 30 km. The data were processed to remove Doppler and channel pulse responses were generated. The data will be compared with model results and the utility of the new projector will be discussed. [Work supported by ONR Code 321SS.]

Moving ship tomography seeks to synthesize a high‐resolution three‐dimensional snapshot of the ocean sound speed field from a few tomographic instruments by moving them during the survey. A moored array of six acoustic transceivers was deployed in the Greenland Sea from September 1988 to August 1989 as part of the Greenland Sea Tomography Experiment. A prototype ship‐deployed array of ten hydrophones was used to receive about 30 transmissions after mooring deployment and about 40 prior to recovery, distributed at locations encircling the moored sources. The positions of the sources and receivers are imperfectly known at this time, but it is possible to compare the observed arrivals with ray theory and identify many of the observed arrivals with numerical ray paths. The second survey yielded enough identified rays to produce estimates of the ocean sound‐speed field in spite of the need to solve for the unknown instrument positions. The resolution of the sound‐speed maps will improve as the ship and mooring...

The quality of ocean acoustic travel‐time measurements depends on the coherence as well as the bandwidth of the signal. Ocean internal‐wave fields are thought to be responsible for the loss of coherence in low‐frequency acoustic signals. If the coherence bandwidth is less than the signal bandwidth, it is possible to consider sub‐bands of the signal as separate measurements. The separate measurements can then be combined incoherently to improve the quality of the travel‐time measurement. Theoretical work and computer simulations also predict that the travel‐time of acoustic signals is biased by the ocean internal wave field. The path‐integral theory for scattering predicts that the size of the travel‐time bias depends on the logarithm of the center frequency of the acoustic signal, thus separate sub‐bands would have different travel times and incoherent recombination would not be an optimal procedure. A recent experiment, in which two acoustic signals with different center frequencies were transmitted simultaneously, makes it possible to measure the bias experimentally and to determine the efficiency of sub‐band averaging. Preliminary results suggest that the bias is about 50 ms at 3500‐km range, which is roughly as large as expected.

The Acoustic Thermometry of Ocean Climate (ATOC) sound sources are located 8 miles north of Kauai, Hawaii and 50 miles west of Half Moon Bay, California. The sources are connected to shore‐based electronics by undersea coaxial cables. Recent modifications in the shore‐based electronics have permitted the ATOC transducers to serve as hydrophones as well as transmitters. Data collected over this 4‐Mm reciprocal path will be presented and discussed in terms of current and internal wave effects. [Work supported by ONR.]

Approximately 15 months of data are now available from the 75‐Hz ATOC transmissions from the Pioneer Seamount off Half Moon Bay, CA to horizontal arrays at 11 U.S. Navy horizontal arrays in the northeast Pacific, two vertical arrays, one near Hawaii and the other near Kiritimati (Christmas) Island. The data demonstrate that ocean basin temperatures can be determined with millidegree precision. Travel time variations at tidal frequencies agree well with predicted values. Low‐frequency travel time variations provide range and depth averaged temperatures that are comparable to seasonal temperature variations derived from historical oceanography (Levitus94). They are also comparable (but not identical) to temperature variations derived from contemporaneous sea surface height variations measured by satellite altimetry. Preliminary results of combining the acoustic and altimetric data in a numerical general circulation model of the North Pacific are also available.

The ATOC AST measurements in July 1996 used simultaneous broadband waveforms in bands near 28 and 84 Hz to study frequency‐dependent differences in long‐range propagation. The low band was below the normal operating range of the Hydroacoustics HLF6 transducer. The nonlinearity of the source, driven exceedingly hard in the low band, generated the high band waveform. Both m‐sequence based and prescription FM waveforms were used. The desired level in each band was 135 W. This paper describes the procedures used to generate the transmitted waveforms and to process the receptions for multipath travel times and modal analysis. [Work supported by the Strategic Environmental Research and Development Program through ARPA.]

Time series of reciprocal ray travel times were obtained at 350‐, 410‐, and 670‐km ranges in the western North Atlantic during the 1991–1992 Acoustic Mid‐Ocean Dynamics Experiment (AMODE). Transmissions were recorded for approximately 300 days between six transceivers in a pentagonal array. Barotropic current along each of the 15 propagation paths is derived from the difference of reciprocal ray travel times, while ten independent estimates of areal‐averaged relative vorticity are found by integrating current around triangles in the pentagonal array. The estimated tidal currents are highly accurate, and tidal relative vorticity at the M2 frequency is detected. This vorticity is induced primarily by the stretching of vortex lines by tidal elevation. Harmonic constants (amplitude, phase) of M2 tidal vorticity are about (4–8±2×10−9 s−1, 270°−320° ±20°), while harmonic constants of about (2–3×10−9 s−1, 300°−340°) are predicted using the shallow‐water equations. The measured tidal harmonic constants are compar...

The ATOC acoustic feasibility network became functional in December 1995 following the installation of an acoustic source on Pioneer Seamount approximately 50 nm off the coast of Northern California. The current effort is part of the Marine Mammal Research Program Pilot Study. This paper describes the acoustic network of sources and receivers, including the augmentations to be made in summer 1996. The network was designed to take advantage of existing hydrophone arrays, while providing for maximum coverage of the North Pacific by the acoustic paths. The acoustical receptions currently being obtained are processed to obtain ‘‘dot plots’’ which show the arrival times and arrival angles of acoustical energy above a chosen signal‐to‐noise ratio. The ‘‘dot plot’’ histories demonstrate stable ray arrivals (albeit with significant amplitude fluctuations) for even the longest propagation ranges of 5 Mm. [Work supported by the Strategic Environmental Research and Development Program through ARPA.]

ATOC signals transmitted from the Pioneer Seamount acoustic source have been received on hydrophone arrays located throughout the North Pacific Basin since the beginning of 1996. Propagation times can be used to infer average temperature (or heat content) variability because the speed of sound changes with temperature. The time series of resolved‐ray travel times show both an annual cycle with amplitudes up to 0.5 s, and other higher frequency fluctuations of comparable magnitude caused by natural oceanic variability. The range‐averaged temperature is derived from the low‐pass filtered (

The waveforms transmitted during the November 1994 Acoustic Thermometry of Ocean Climate (ATOC) project R/P Flip measurement used an m‐sequence phase modulated 75‐Hz carrier, two carrier cycles per digit. Because the source had an inherent Q of 5, the drive waveform was modified in order to produce a two cycle per digit output. This paper describes the basic characteristics of the source, the procedure used to broaden the source bandwidth, the enforcement of a source peak power limit, and shows the resulting waveforms. The received waveforms possessed small amounts of Doppler ‘‘shift’’ caused by the motion of Flip on its mooring. Doppler limits the amount of time over which receptions can be integrated. Doppler normally must be treated as time compression/expansion. However, if the Doppler is sufficiently small it can be treated as simple time shift. This paper compares the results of using both approaches to account for the motion of Flip. [Work supported by the Strategic Environmental Research and Development Program through ARPA.]

A low‐frequency acoustic source suspended from R/P FLIP approximately 340 nautical miles WSW of San Diego transmitted to receivers 90 to 10 000 km distant during the Acoustic Engineering Test of the Acoustic Thermometry of Ocean Climate (ATOC) Program. The source was sus‐pended for 7 days during November 1994 near the depth of the sound channel axis (about 650 m) in water over 4000 m deep, in order to avoid near‐source bottom interactions. The source transmitted a phase‐coded m‐sequence with a center frequency of 75 Hz and a digit length of 27 ms [Metzger et al., this meeting]. Measured receptions on five bottom‐mounted SOSUS receivers at ranges from 300–4000 km, on two vertical line array receivers at ranges of 90 and 3300 km, and on a sonobuoy modified to have the hydrophone on the sound channel axis at about 10 000‐km range, are compared with ray theoretic, adiabatic normal mode, and broadband parabolic equation predictions. [Work supported by the Strategic Environmental Research and Development Program through ARPA.]

The acoustic thermometry of ocean climate‐feasibility ascension Cape Town (ATOC‐FACT) propagation experiment took place during November and December of 1992. Explosive charges were detonated near the oceanic sound‐channel axis at five locations in the South Atlantic off Cape Town. The resulting signals were recorded on bottom‐mounted hydrophones at Ascension, Navy horizontal arrays off Bermuda, bottom‐mounted hydrophones off Kaneohe and Wake and from a hydrophone array deployed from a research ship off Southern Tasmania. The primary goal of the experiment was to determine the extent that the acoustic path between Ascension and Cape Town is blocked by Bonaparte’s Seamount and other bathymetric features. Secondary goals were to determine the horizontal arrival structure at Bermuda and the temporal arrival structure at Ascension, Hawaii, and off Tasmania. After giving an overview of the experiment, we use recorded data to discuss the question of how precisely arrival‐time differences of explosive signals can be measured. Detonations separated both temporally and spatially are considered. It is hoped this work will contribute to an understanding of the role explosive signals can play in the design of a global ATOC network.

The ultimate goal of acoustic monitoring of ocean processes is to provide quality measurements of the right type to be assimilated into worldwide climate models. The immediate goals are to achieve the travel time resolution and accuracy necessary to provide the oceanographer and climate modeler with identifiable receptions of multipath or multimode arrivals. The economic antagonists are the hardware costs. The natural antagonists are signal spreading, attenuation, dispersion, scattering, continual change, human uncertainty, and noise. This paper illustrates how a signal processor can help organize the balances, lay out the interactions, and provide workable solutions. Examples from current measurements will be used to illustrate effective signal handling.

A signal processing technique is described that provides precise estimates of the change in travel time of acoustic pulses using an optimal combination of multipath phase and amplitude [J. L. Spiesberger et al., J. Oceanogr. Eng. 14, 108–119 (1989)]. Limits to the precision of the estimator for basin‐scale transmissions (0.135 ms at 2‐min interval) are imposed by internal waves rather than acoustic noise. The technique does not require any understanding of the spatial coordinates of acoustic paths between the source and receiver and is thus easy to use. The technique was used in the Heard Island feasibility experiment. Applications of this signal processing technique to new observations of mode 1 internal waves in the north Pacific Basin are also discussed.

The most remarkable feature of the HIFE receptions is their phase stability in both time and frequency. Examples include (1) tracking of the peak‐energy bin of FFT’s, (2) refinement of the carrier estimate to a precision adequate to estimate the transmission launch angle, and the small residual phase deviation track over a full hour, (3) the small phase swing of the integrated autocorrelation phase (IAP1) over one full hour, (4) the phase stability of arrivals, after removal of the broadband phase deviations measured by IAP1, and (5) comparison of the measured distributions of amplitude and phase of the individual frequency lines and individual time arrivals. The implications for phase coherent signal processing, in particular for more frequent transmissions in fixed–fixed tomography and for Doppler utilization in moving source tomography, will be discussed. Last but not least, the utilization of the arrival phase along the several sheets of propagating timefronts by passive broadband channel matching wil...

HIFE transmitted for 28 full hours and 8 half hours, repeating the same signal continuously for each transmission. HIFE had a repertoire of cw, Pentaline, and four different phase‐coded time‐resolving signals based on m sequences. The several purposes of each signal will be explained. Receptions were processed on line at 14 sites at ranges of 200 to 18 000 km; to date, the digital tapes from 8 sites have been processed at CSPL. The feasibility questions concerning power, stability, time resolution, and dispersion were answered, and a thorough study of the receptions begun. Examples of the on‐line processing and selected frequency domain and travel‐time‐domain time series will be presented and discussed. An extraordinary look at the phase stability of the propagation was revealed through comparison of (1) the diagonal ship surge deviation from a uniform path measured at the transmitter, and (2) the integrated autocorrelation phase (IAP1) after constant Doppler correction measured at a 9000‐km distant receiver.

From July to October of 1990, an applied tomography demonstration experiment was conducted in the North Atlantic in the area between Bermuda and North America. The highly variable eastward extension of the Gulf Stream Current was of particular interest. Acoustic sources placed on the bottom near Bermuda transmitted every 4 h, and at times every 40 min, to bottom hydrophones north of the Gulf Stream. The signals were replica processed and the results relayed to the Applied Physics Laboratory in an unattended near real‐time mode of operation. In addition, a ship towing a hydrophone array on the northern side of the Gulf Stream received the tomography transmissions during a 1 week period. Sample data results for a single slice through the Gulf Stream are presented. The results show consistent and trackable multipaths that are used to monitor the position of the Gulf Sream. Furthermore, a high‐resolution image of the Gulf Stream between 60 and 70 deg west longitude was obtained from the towed array data. Independent comparison data are also presented. [Work supported by the Oceanographer of the Navy and ONR.]

Measurements of acoustic travel time across the northeast Pacific Ocean are used to detect large‐scale (>500 km) thermal trends in the ocean interior. The trends are novel observations, and there is probably no method to verify them with other data sets. The results demonstrate the practicality of a new and powerful means to observe ocean temperature and to investigate its relation to weather, climate, and greenhouse warming.

Measured travel times of acoustic pulses, propagated across a 3000‐km section of the north Pacific Ocean, are inconsistent with travel times predicted with the internationally accepted algorithm for the speed of sound in water. The sound‐speed algorithm predicts a speed that is too fast at oceanic pressures found below about 1‐km depth. An accurate algorithm of sound speed is important for deriving large‐scale temperatures from measurements of acoustic travel time across oceans. Accurate estimates of the large‐scale thermal field are important for better understanding the ocean's role in determining weather, climate, and the distributions of marine organisms.

The travel time measurements used in ocean acoustic tomography are based on the use of wideband waveforms. Because of this, the effects of source or receiver motion must be modeled as time compression/expansion and not as simple frequency shifts. Equivalent situations occur when there is a frequency error between the clocks used to generate an ocean acoustic tomography wideband signal and the clock used to digitize the associated reception, and when the clock used to digitize the reception is not “properly” matched to the structure of the transmitted waveform. The common thread is that you have uniformly spaced samples at one set of times but you really need them at a different uniform spacing at a different set of times. The obvious solution is to interpolate. This paper describes the effects of some simple interpolaton techniques on the processing of the wideband waveforms used in making ocean acoustic tomography travel time measurements.

A phase‐coded signal with 64‐ms resolution was transmitted at 10‐min intervals for a 48‐day period between an acoustic source moored at 2000‐m depth and a bottom mounted receiver at ∠3000‐m depth and at ∠900‐km range. About 16 multipaths were resolved. They were stable in the presence of ocean fluctuations and could be identified (with some exceptions) from ray theory. The precision to which daily travel‐time fluctuations along multipaths could be measured was better than 10 ms. The resolution, stability, identification, and precision is adequate for acoustic monitoring of mesoscale ocean variability by measuring travel‐time variations along ray paths.

Factor inverse matched filtering (FIMF) and factor inverse filtering (FIF) are signal processing techniques used to obtain desired signal responses. Both are especially useful procedures for ‘‘pulse‐compression’’ processing and channel measurements. The theory is developed for a simple channel and known noise power spectral density so that comparison may be made with matched filtering. Expressions for the pulse‐compression energy gain, nonflatness loss NFL, and total performance are derived. The NFL is useful in selecting the best among practical pulse‐compression modulations, and with FIMF and FIF, has been used extensively since 1974 by the authors and their co‐workers in underwater acoustic propagation measurements and ocean acoustic tomography.

If the classical differential equations defining rays are rewritten in terms of travel time instead of ray path distance, it is straightforward to trace families of rays from a point with time as the independent variable. Points on rays having the same time coordinate define a timefront surface, thus designated to emphasize its high‐frequency nature. Timefronts are spherical at short ranges, slowly evolving into an accordian structure. The shape of the timefront changes relatively slower over time scales of seconds, enabling a physical picture of long distance arrival times to be seen by inspection. A simple algorithm for deducing accurate travel times of eigenrays to a given receiver steps the timefront iteratively until its branches cross the receiver coordinates. Several features characteristic of long distance arrival structure are demonstrated. Numerical results from typical profiles, some including surface ducts, are presented.

A procedure for remote sensing of mesoscale ocean dynamic processes has been proposed by Munk and Wunsch [Deep Sea Res., 26A, 123–161 (1979)]. It is based on the measurement of acoustic travel time variations between fixed sources and receivers distributed on the boundary of a large ocean area. Inverse theory is invoked to estimate the structure of the interior sound velocity field from travel time perturbations along individual ray paths. In this paper we present the results of a 48‐day multipath stability experiment that demonstrates sufficient path stability to enable ocean acoustic remote sensing using this technique. A 220‐Hz phase encoded signal with 64‐ms resolution was transmitted at 10‐min intervals between a moored source and a receiver at a 900‐km range. Ray paths are clearly resolved, unambiguous, and identifiable. [Work supported by ONR.]