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Formal Reports

Report of results of completed projects or major milestones either in scientific terms or in terms acceptable to a wider audience. Note: Unless linked to the full text, reports are only available to NATO member nations from designated distribution centres. 

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Comparison between predictions made by the SUPREMO sonar performance model and measured data Comparison between predictions made by the SUPREMO sonar performance model and measured data

Date added: 07/01/2005
Date modified: 08/14/2012
Filesize: Unknown

Comparison between predictions made by the SUPREMO sonar performance model and measured data. Prior, M.K. : Baldacci, A. SR-429. July 2005.

Predictions of acoustic reverberation and target echo intensity, made by the SUPREMO sonar performance model, are compared with measured data gathered in the Malta Plateau region of the Mediterranean Sea. The model’s ability to predict these quantities is assessed and used as a measure of model performance. It is shown that the model, when given satisfactory input descriptions of the ocean environment, is capable of predicting reverberation intensity to modal errors of magnitude around -1dB with a spread of ±4dB around the modal values. Furthermore, the model is shown to be able to predict trends in signal-to-background intensity ratio as pulse centre frequency is varied between values of 1100Hz, and 1700Hz with bandwidths of 200Hz and 800Hz. The model-measured agreement observed indicates the suitability of the SUPREMO model for use within an environmentally adaptive, low-frequency, active sonar system.

Probabilistic modelling of sidescan sonar texture using pairwise pixel interactions Probabilistic modelling of sidescan sonar texture using pairwise pixel interactions

Date added: 06/01/2005
Date modified: 08/14/2012
Filesize: Unknown

Probabilistic modelling of sidescan sonar texture using pairwise pixel interactions. Myers, Vincent L. SR-424. June 2005.

Probability models used to describe sidescan sonar background pixels are usually represented using Rayleigh or K-distributions. While useful for many purposes, these distributions do not capture information which characterises non-independent pixel amplitudes which typify many textures found in modern high-resolution sidescan sonar data. Such models are necessary if we are to compute sensor and mission performance metrics such as probabilities of detection and classification. This report describes a flexible technique for describing sidescan sonar image textures using Markov Random Fields. Pairwise pixel interactions are expressed using gray level difference histograms and, due to the equivalence of Markov and Gibbs random fields, are used as sufficient statistics to a Gibbs probability distribution. The parameters of the Gibbs distribution are found using stochastic approximation and complex textures such as sand ripples and sea grass are successfully represented using this method. The Gibbs model is also shown to be capable of seabed segmentation based on texture, as well as capable of designing a simple constant-false-alarm-rate detector.

RADARSAT mapping of BORA winds in the Adriatic Sea RADARSAT mapping of BORA winds in the Adriatic Sea

Date added: 03/01/2005
Date modified: 08/14/2012
Filesize: Unknown

RADARSAT mapping of BORA winds in the Adriatic Sea. Askari, Farid ; Signell, Richard P. SR-422. March 2005.

We examine the meteorological phenomena associated with the BORA wind system over the Adriatic Sea using RADARSAT synthetic aperture radar (SAR) imagery. A Bora wind event spins off a number of related atmospheric phenomena which manifest in SAR imagery as: high- intensity jets in the Gulfs of Trieste and Kvarneric, low-intensity shadow regions and island wakes along the Croatian coasts, and high-intensity atmospheric barrier jet along the western Adriatic coast. A key element in this study is the high resolution wind mapping from SAR imagery using scatterometer inversion algorithms and characterizing the Bora morphology. The high-resolution imaging allows for identification of topographically-controlled features and diagnosis of regions of strong vorticity and 3D motion.

Using a vertical line array and ambient noise to obtain measurements of seafloor reflection loss Using a vertical line array and ambient noise to obtain measurements of seafloor reflection loss

Date added: 03/01/2005
Date modified: 08/14/2012
Filesize: Unknown

Using a vertical line array and ambient noise to obtain measurements of seafloor reflection loss. Victor Young. SR-410. March 2005.

Bottom reflection properties can be obtained from ambient noise directionality. The results obtained at six sites in 2002 with NURC’S 62m VLA are summarised. At four of the sites the VLA was moored, in order to study a single seafloor environment, while at the other two sites the VLA was allowed to drift, in order to study geographical changes in the seafloor environment. Several variants of the measurement and processing techniques are investigated here. Firstly, rather than using only the uniformly spaced central section of a nested vertical array (VLA), one can expand the useful size of the array to about three quarters (rather than one half) of the full array length by padding out the array’s correlation matrix. The padding makes the apparently good assumption that the noise crossspectral-density matrix is Toeplitz. Thus the initially sparse matrix for the outer sections of the array (with wider hydrophone separations) can be filled out with values from elsewhere in the true matrix. This provides better angular resolution which is beneficial for the method. Secondly, the possibility of synthesising a VLA with a pair of hydrophones is investigated. The benefit of a synthetic aperture would be the lower cost of the equipment in an operational context. For the process to work (without extremely long integration times) the noise source spatial distribution needs to be stationary.

Seabed characterization by inversion of acoustic propagation and reverberation data received on a towed horizontal array Seabed characterization by inversion of acoustic propagation and reverberation data received on a towed horizontal array

Date added: 02/01/2005
Date modified: 08/14/2012
Filesize: Unknown

Seabed characterization by inversion of acoustic propagation and reverberation data received on a towed horizontal array. Nielsen, Peter L. ; Fallat, Mark ; Marconi, Valter. SR-417. February 2005.

Sonar performance prediction in shallow-water is highly dependent on the char-acteristics of the underwater environment. The bottom properties are critical input parameters to prediction tools and these parameters are probably the most dicult to estimate. Inversion of measured acoustic signals (direct propagation and diffuse reverberation) provides a technique to infer these bottom parameters. An ad-hoc approach using the sensors of the sonar system is proposed where the local geoacoustic properties are determined by inverting the data received on a horizontal array. The local geoacoustic properties are used as initial or guidance inputs to a reverberation model to estimate range-averaged geoacoustic and scattering properties from long-range reverberation data.

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