J. Pihl

Position Correction Using Echoes From a Navigation Fix for Synthetic Aperture Sonar Imaging

In synthetic aperture sonar (SAS), the platform position must be known sufficiently accurately for signals to be added coherently along the synthetic aperture. Often, the onboard navigation system is insufficiently accurate by itself, so corrections are needed. A well-known method is the displaced phase center antenna (DPCA) procedure for correcting platform position using seabed echoes. DPCA methods have the advantage of insensitivity to changing interference patterns, moving specular reflection, and changing occlusion, with aspect. However, when seabed echoes are unusable, either because they are too weak, or because they are corrupted by multipath, the seabed DCPA method may fail. Therefore, we present an alternative DPCA method using sonar echoes from a suitable navigation fix, based on an object detected after standard beamforming. In our proposed system, look angle is obtained by tracking the centroid of the rectified image of the fix object. When the standard DPCA correction equations are modified for a fixed reflector, it turns out that they provide incremental range and look-angle errors, precisely the values required when the target itself is used as the navigation fix. Moreover, the values obtained are then self-compensating for errors in estimating seabed depth or forward motion of the platform. The navigation fix is selected by bracketing in range, and beamforming overlapping subsets of the receiver array. In this paper, we present experimental results at transmitter frequencies of 25 and 100 kHz where our method enabled well-focused SAS images to be generated with little recourse to other navigation information. Hence, SAS can be carried out, even when a sophisticated inertial navigation system (INS) is not available.

Improved Active Sonar Tactical Support by Through-the-Sensor Estimation of Acoustic Seabed Properties

Accurate environmental information is required for obtaining confident sonar performance predictions. This environmental information is, however, often unreliable or unavailable. To support antisubmarine warfare (ASW) operations, a through-the-sensor approach has been developed in which relevant acoustic seabed properties are derived from reverberation data, and a demonstrator system has been installed on a Royal Norwegian Navy frigate. It determines relevant acoustic seabed parameters from the reverberation data near real time. This demonstrator system has been validated in several sea trials conducted off the coast of Bergen, Norway. The acoustic seabed parameters derived in these trials have a good correspondence with the available prior information. Furthermore, the results show that acoustic seabed parameters derived from reverberation data in previous trials can be used to improve reverberation prediction for subsequent trials, even when environmental conditions, i.e., sound-speed profiles, are different. Because the demonstrator makes information on acoustic seabed properties directly available for in situ sonar performance prediction, it can be used as a tactical decision aid.

Stabilizing reverberation inversion by regression relations involving a grain size parameter

Within the European Defence Agency (EDA) project Rumble-2, an operational low-frequency active sonar system has been used to collect reverberation data at several sea trials in the North Sea. A global optimization method is used to determine the bottom parameters that provide the best match between measured and modeled time traces. A fast ray model is used for the forward computations. The bottom parameters are the Lambert back-scattering parameter and the sound speed c, density ρ, absorption α, and thickness of the sediment. The reverberation data do not constrain all these parameters to unique values, however, and different approaches have been tried in the project to reduce the ambiguity problems. The approach reported here is to use the mean grain size Mz as a common descriptive parameter. From regression relations by Hamilton and Bachman, c, ρ, and α can be set as functions of Mz. More ambitiously, the regression relations could be applied as a priori constraints, with uncertainties, in a Bayesian fr...

Bayesian reverberation inversion incorporating grain-size dependent regression relations as a priori information

An operational low-frequency active sonar system has been used in the EDA (European Defence Agency) project Rumble-2 to collect reverberation data at sea trials in the North Sea. Using a fast ray model for the forward computations, the reverberation data are inverted to determine bottom parameters: Lambert back-scattering parameter μ and sound speed c, density ρ, absorption α, and sediment thickness h. Hamilton-Bachman regression relations for c, ρ, and α, are incorporated with uncertainties and with the mean grain size Mz as a common descriptive parameter. This fits nicely into a Bayesian inversion framework, but with a non-uniform prior pdf (probability density function). Markov-chain Monte-Carlo techniques are applied to produce an ensemble of models distributed in model space according to the PPD (posterior probability density), which is thereby assessed. Laplace distributions are chosen to model the data residuals, or data errors, and particular attention is given to estimation of the corresponding covariance matrix in the non-stationary case. There appears to be a risk of finding a point in model space for which the reverberation trace matches the noisy data trace better than the “true“ model. This could lead to too small data error estimates and a too narrow estimate of the PPD. Application to the actual Rumble-2 reverberation data pings indicates that the mean grain size Mz and the Lambert parameter μ are reasonably well determined, whereas there are uncertainties concerning the sediment thickness. The pings involve bottom areas mainly covered by sand and silt, respectively, and the inversion results reflect this difference in the anticipated way

Mid Frequency Bottom-Interacting Sound Propagation and Reverberation in the Baltic

Abstract: The experiment BAROC (Baltic Acoustics on Rocky Outcrops) was performed in May 2002. Analysis and modelling of part of the received data are reported here, concerning transmission loss and reverberation in a shallow-water area south of Stockholm. LFM pulses were transmitted in two directions with frequency content between 500 and 5500 Hz. In both directions a clear sound channel was observed, for which the optimum propagation frequency was about 4 kHz. Strong bottom reverberation was measured. A ray-trace model has been used as forward model for inversion of bottom parameters with a genetic algorithm. The inversion results were subsequently verified with a parabolic-equation model, and they were used to assess the level of reverberation.