Sean Pecknold

Improved Active Sonar Performance Using Costas Waveforms

Costas waveforms are a class of waveforms having the form of frequency-hopped pulse trains. When used as a transmit waveform in an active sonar system they may provide superior performance to conventional waveforms such as continuous-wave (CW) and frequency-modulated (FM) pulses, as each Costas waveform provides both range and speed information for a target echo. Matched-filtering identifies individual Costas waveforms from among a set of Costas waveforms that are received. This characteristic allows for more frequent ensonification of a water mass for targets at moderate ranges, giving a higher rate of return echoes for improved target tracking. Alternatively, it offers the potential for overlapping bandwidths and waveform types to be used in a multisonar environment. In this work, the Costas wideband ambiguity, cross- and autocorrelation functions are derived. The range and speed resolutions of Costas waveforms are compared to CW and FM pulse resolutions both via a broadband pulse propagation model and using data obtained during the Defence Research And Development Canada-Atlantic (DRDC Atlantic) Towed Integrated Active-Passive Sonar (TIAPS) deep sea trials in September 2003. Costas waveforms are shown to provide simultaneous range and speed resolution in deep- and shallow-water environments, and Costas waveforms are shown to be individually identifiable at the receiver. The performance of Costas waveforms in reverberation is also briefly examined, and found to be similar to that of FM pulses.

Time-series modeling using the waveform transmission through a channel program

DRDC Atlantic has developed a coherent transmission loss model that simulates the propagation of acoustic pulses through an ocean environment. Using a set of input eigenrays and an input waveform, the WATTCH (waveform transmission through a channel) model can generate a set of output time series. Each time series represents the expected signal corresponding to a given range and depth. Assuming the required eigenrays can be generated, WATTCH can simulate the effects of complex environments. This is illustrated by examples comparing measured data to modeled data in two environments. In adddition, an example is provided using WATTCH to model data for testing a target detection algorithm.

Improved torpedo range estimation using the fast orthogonal search

A modified torpedo detection algorithm (MTDA) that improves upon the range estimates of an earlier torpedo detection algorithm (TDA) is presented in this work. The original TDA used the fast orthogonal search (FOS) algorithm to estimate the frequency of direct-path and a surface-reflected-path signal emitted by a torpedo. This technique assumed that the torpedo was traveling directly at the receiver and modeled the signals using sinusoidal functions. The range to the torpedo was estimated using the frequency estimates of the direct and reflected path and a trigonometric description of the approaching signals. The TDA was successful at detecting approaching torpedoes but had an average range error of 680 m. The modified torpedo detection addresses these deficiencies by: 1) estimating the angle between the approaching torpedo and the receiver and 2) using chirp signals as the candidates for FOS that model the theoretical direct- and reflected-path signals. In addition, the FOS algorithm is modified to fit the direct and reflected paths in pairs to eliminate the need to iterate the FOS algorithm. Signals of approaching torpedoes were simulated for several initial angles of approach using the generic sonar model (GSM) and waveform transmission through a channel (WATTCH) programs. The average error of the range estimates for sea state (SS0) 0 and 1 for the MTDA were 153 m as compared to 680 m for the TDA. In addition, the MTDA was shown to be resistant to false alarms.

Directivity measurements of the horizontal line array projector

Defence Research and Development Canada - Atlantic (DRDC Atlantic) has completed a technology demonstration project that integrated active and passive shipborne towed array capabilities. One of the source arrays developed by the project was a 32-element horizontal projector array (HPA). HPA directivity data were recorded during a number of sea trials. This paper presents the system configuration, experimental geometries, and directivity measurements.

Autonomy and networking challenges of future underwater systems

In the last decade, the use of underwater systems of all kinds has seen a steady increase. In this publication, we advocate the use of such systems in a complementary, collective, and networked manner. Even though todays achievements of such systems look impressive, the level of autonomy and networking required for tomorrows needs remains deficient. After reviewing current trends and limitations, we formulate challenges pertaining to research areas that are key for future employments of underwater systems. We conclude by discussing future research efforts, especially those at the DRDC Atlantic Research Centre.

Beamforming shallow water propagation of mid-frequency acoustics on the Chukchi Shelf

The shallow water Canadian Basin Acoustic Propagation Experiment (CANAPE 2016–2017) was designed to study the effect of oceanographic variability on the acoustic field in the Arctic. The physics of the acoustic waveguide on the Northeastern edge of the Chukchi Shelf are influenced by dynamic boundary conditions and spatio-temporal fluctuations in temperature/salinity profiles, including the upwelling of Atlantic bottom water, sinking Bering Sea surface water and sub-mesoscale eddy formation. These fluctuations influence the acoustic waveguide characteristics of a persistent sound speed channel centered at 150 m depth. The University of Delaware (UDel) deployed seven oceanographic moorings (OM) perpendicular to the isobaths on the Chukchi Shelf (145–700 m depth). A Naval Research Lab source (S1) was deployed in-line with the OMs, generating Linear Frequency Modulated (LFM) signals in alternating 700–1100 Hz and 1400–4000 Hz bands. A Vertical Line Array (VLA) was deployed in-line and within the span of the OMs. The array aperture spanned the depth of the sound speed channel. Beamforming measurements from the VLA in combination with environmental measurements and 2D PE model output will determine the degree of influence of individual physical oceanography processes on the spatio-temporal structure of the sound channel and internal acoustic wave propagation.The shallow water Canadian Basin Acoustic Propagation Experiment (CANAPE 2016–2017) was designed to study the effect of oceanographic variability on the acoustic field in the Arctic. The physics of the acoustic waveguide on the Northeastern edge of the Chukchi Shelf are influenced by dynamic boundary conditions and spatio-temporal fluctuations in temperature/salinity profiles, including the upwelling of Atlantic bottom water, sinking Bering Sea surface water and sub-mesoscale eddy formation. These fluctuations influence the acoustic waveguide characteristics of a persistent sound speed channel centered at 150 m depth. The University of Delaware (UDel) deployed seven oceanographic moorings (OM) perpendicular to the isobaths on the Chukchi Shelf (145–700 m depth). A Naval Research Lab source (S1) was deployed in-line with the OMs, generating Linear Frequency Modulated (LFM) signals in alternating 700–1100 Hz and 1400–4000 Hz bands. A Vertical Line Array (VLA) was deployed in-line and within the span of the ...

The effects of ice cover and oceanography on medium-frequency acoustic propagation on the Chukchi Shelf

The Canada Basin Acoustic Propagation Experiment (CANAPE) was a year-long experiment exploring the changing nature of sound propagation and ambient noise in the Arctic ocean. As part of this experiment, medium-frequency signals at 0.7–1 kHz and 1–4 kHz were transmitted by two sources on the Chukchi Shelf. One of these sources was located in an area of 150 m of water depth, approximately 350 m from a directional receiver array and 50 km from an 8-element vertical line array in a water depth of about 125 m. Oceanographic sensors were located both on the arrays and on a set of moorings on the shelf, and an ice-profiling sonar was located between the arrays about 15 km from the source. In this talk, we will focus on using the measured environmental data and propagation modeling to characterize the variability observed in the short-range and long-range received acoustic signals over the course of CANAPE.The Canada Basin Acoustic Propagation Experiment (CANAPE) was a year-long experiment exploring the changing nature of sound propagation and ambient noise in the Arctic ocean. As part of this experiment, medium-frequency signals at 0.7–1 kHz and 1–4 kHz were transmitted by two sources on the Chukchi Shelf. One of these sources was located in an area of 150 m of water depth, approximately 350 m from a directional receiver array and 50 km from an 8-element vertical line array in a water depth of about 125 m. Oceanographic sensors were located both on the arrays and on a set of moorings on the shelf, and an ice-profiling sonar was located between the arrays about 15 km from the source. In this talk, we will focus on using the measured environmental data and propagation modeling to characterize the variability observed in the short-range and long-range received acoustic signals over the course of CANAPE.

Measured acoustic intensity fluctuations on the Chukchi continental shelf during Canada Basin Acoustic Propagation Experiment 2017

One of the main objectives of the Shallow Water (SW) CANAPE experiment was to gain a thorough understanding of a yearlong propagation of broadband signals from deep to shallow water with simultaneous oceanographic and acoustic measurements together along the and across the shelf break region. Using more than eleven acoustic arrays and seven oceanographic moorings in a 30 km2 region on the Chukchi shelf this task is being done by assessing both deep water sound signatures and shallow water source transmissions. In this paper with present analysis of acoustic signals from both shallow and deep water sources on the Chukchi continental shelf for a specific time period between June and August 2017 where a 20 dB intensity drop from along-the-shelf source (S2) at 150 m water depth was observed for more than few weeks. This intensity drop is strongly correlated with occurrence of a large oceanographic event spanning the top 150 m water column due to Pacific Water outflow from Bering sea and retreat of Marginal Ice Zone (MIZ). During the same period, cross-the-shelf source (S1) was not transmitting signal but the reception from the deep water acoustic transmitters also show variability that could be correlated with the basin scale water column variability and the ice coverage. [Work supported by ONR.] One of the main objectives of the Shallow Water (SW) CANAPE experiment was to gain a thorough understanding of a yearlong propagation of broadband signals from deep to shallow water with simultaneous oceanographic and acoustic measurements together along the and across the shelf break region. Using more than eleven acoustic arrays and seven oceanographic moorings in a 30 km2 region on the Chukchi shelf this task is being done by assessing both deep water sound signatures and shallow water source transmissions. In this paper with present analysis of acoustic signals from both shallow and deep water sources on the Chukchi continental shelf for a specific time period between June and August 2017 where a 20 dB intensity drop from along-the-shelf source (S2) at 150 m water depth was observed for more than few weeks. This intensity drop is strongly correlated with occurrence of a large oceanographic event spanning the top 150 m water column due to Pacific Water outflow from Bering sea and retreat of Marginal I...

Arctic ambient noise statistics at the edge of the Canada Basin

Knowledge of ambient noise is important for the design and operation of acoustic observation and communication systems. Several concurrent year-long acoustic datasets from the Canada Basin Acoustic Propagation Experiment (CANAPE) were collected in 2016–2017. Selected data were analyzed to investigate soundscape temporal and spatial characteristics in an area on the western slope of the Chukchi shelf and Canada Basin, north of Barrow Alaska. From October 2016 to October 2017, four 8-element vertical arrays were deployed in water depths between 100 and 300 m. Each array recorded at approximately 15% duty cycle at several sample rates between 4000 Hz and 64000 Hz and at 24-bit resolution. One-minute, 10-second, and 1-second root-mean-square sound pressure levels were computed to generate ambient noise statistics and summarized in empirical probability density functions (PDF) for various bands. Fit functions for ambient noise were determined for each empirical PDF. Impulse detection was used to investigate potential correlation of ice cracking with ambient noise. A multivariate correlation of ambient noise levels was performed with several environmental parameter covariates, including ice cover, wind speed, and air temperature. These correlations may form the basis for predictive models for ambient noise modeling in the arctic.Knowledge of ambient noise is important for the design and operation of acoustic observation and communication systems. Several concurrent year-long acoustic datasets from the Canada Basin Acoustic Propagation Experiment (CANAPE) were collected in 2016–2017. Selected data were analyzed to investigate soundscape temporal and spatial characteristics in an area on the western slope of the Chukchi shelf and Canada Basin, north of Barrow Alaska. From October 2016 to October 2017, four 8-element vertical arrays were deployed in water depths between 100 and 300 m. Each array recorded at approximately 15% duty cycle at several sample rates between 4000 Hz and 64000 Hz and at 24-bit resolution. One-minute, 10-second, and 1-second root-mean-square sound pressure levels were computed to generate ambient noise statistics and summarized in empirical probability density functions (PDF) for various bands. Fit functions for ambient noise were determined for each empirical PDF. Impulse detection was used to investigate po...

Underwater noise measurements during a year long Shallow Water Canada Basin Acoustic Propagation experiment from 2016 to 2017

A year-long, multi-institution, acoustical oceanographic measurement on the Chukchi Continental Shelf region of the Canada Basin started on October 2016 is reported. Ten vertical receiver line arrays and a horizontal receiver line array were deployed together with oceanographic sensors to measure the sound speed profiles, ice formation, and currents over an area of approximately 30 km by 50 km. Various aspects of the noise including spectral fluctuations, directionality, seasonal dependence, and intensity fluctuations are studied over time. In this paper, we present preliminary measurement results depicting spatial and temporal distribution of underwater background noise at the experiment site. [Work supported by ONR321 OA.]