Search

During the False Bay trial (June 2007), the performance of a set of three optical sensors was tested against several small surface targets in a coastal area: a hyperspectral camera, a camera with a rotating polarization filter and a high resolution camera. One objective was the validation of a contrast and clutter model for the visual spectral band in this type of scenarios. Another issue was to test the benefit of using a polarisation filter and a hyperspectral unit in front of standard TV cameras. Finally the loss in identification capability of a high resolution camera at long range due to atmospheric blur was investigated. Data were collected of targets in near-sun direction at ranges up to seven kilometers, in all cases for down looking angles (targets below the horizon). Environmental parameters such as solar irradiance and windspeed were measured as input for the contrast and clutter models. Spatial, spectral and temporal effects of the target contrast and of the background clutter behaviour in the visual spectral band were determined as function of range and compared with model predictions. Samples of data and predictions are presented in this paper. The spatial and temporal target characteristics are of key importance for the development of algorithms for target detection, classification and tracking. Finally, rules of thumb, based on the measurements and model predictions, on the detection and identification range performances of specific optical sensors against small surface targets in a maritime environment are presented.

During the False Bay trial (June 2007), the performance of a set of three optical sensors was tested against several small surface targets in a coastal area: a hyperspectral camera, a camera with a rotating polarization filter and a high resolution camera. One objective was the validation of a contrast and clutter model for the visual spectral band in this type of scenarios. Another issue was to test the benefit of using a polarisation filter and a hyperspectral unit in front of standard TV cameras. Finally the loss in identification capability of a high resolution camera at long range due to atmospheric blur was investigated. Data were collected of targets in near-sun direction at ranges up to seven kilometers, in all cases for down looking angles (targets below the horizon). Environmental parameters such as solar irradiance and windspeed were measured as input for the contrast and clutter models. Spatial, spectral and temporal effects of the target contrast and of the background clutter behaviour in the visual spectral band were determined as function of range and compared with model predictions. Samples of data and predictions are presented in this paper. The spatial and temporal target characteristics are of key importance for the development of algorithms for target detection, classification and tracking. Finally, rules of thumb, based on the measurements and model predictions, on the detection and identification range performances of specific optical sensors against small surface targets in a maritime environment are presented.

Coastal surveillance and naval operations in the littoral both have to deal with the threat of small sea-surface targets. These targets have a low radar cross-section and a low velocity that makes them hard to detect by radar. Typical threats include jet skis, FIAC's, and speedboats. Previous lidar measurements at the coast of the Netherlands have shown a very good signal to clutter ratio with respect to buoys located up to 10 km from the shore where the lidar system was situated. The lidar clutter is much smaller than the radar clutter due to the smoothness of the sea surface for optical wavelengths, thus almost all laser light is scattered away from the receiver. These results show that due to the low clutter a search lidar is feasible that can detect small sea-surface targets. Based on these promising results a search-lidar demonstrator project has started end of year 2008. The system set-up of the search lidar demonstrator is presented and experimental results near the coast of Holland are presented. By using a high rep-rate laser the search time is limited in order to be useful in the operational context of coastal surveillance and naval surface surveillance. The realization of a search lidar based on a commercially available high power and high rep-rate laser is presented. This demonstrator is used to validate the system modeling, determine the critical issues, and demonstrate the feasibility.

Coastal surveillance and naval operations in the littoral both have to deal with the threat of small sea-surface targets. These targets have a low radar cross-section and a low velocity that makes them hard to detect by radar. Typical threats include jet skis, FIAC's, and speedboats. Previous lidar measurements at the coast of the Netherlands have shown a very good signal to clutter ratio with respect to buoys located up to 10 km from the shore where the lidar system was situated. The lidar clutter is much smaller than the radar clutter due to the smoothness of the sea surface for optical wavelengths, thus almost all laser light is scattered away from the receiver. These results show that due to the low clutter a search lidar is feasible that can detect small sea-surface targets. Based on these promising results a search-lidar demonstrator project has started end of year 2008. The system set-up of the search lidar demonstrator is presented and experimental results near the coast of Holland are presented. By using a high rep-rate laser the search time is limited in order to be useful in the operational context of coastal surveillance and naval surface surveillance. The realization of a search lidar based on a commercially available high power and high rep-rate laser is presented. This demonstrator is used to validate the system modeling, determine the critical issues, and demonstrate the feasibility.

We present an algorithm that produces a fused false color representation of a combined multiband IR and visual imaging system for maritime applications. Multispectral IR imaging techniques are increasingly deployed in maritime operations, to detect floating mines or to find small dinghies and swimmers during search and rescue operations. However, maritime backgrounds usually contain a large amount of clutter that severely hampers the detection of small targets. Our new algorithm deploys the correlation between the target signatures in two different IR frequency bands (3-5 and 8-12 ìm) to construct a fused IR image with a reduced amount of clutter. The fused IR image is then combined with a visual image in a false color RGB representation for display to a human operator. The algorithm works as follows. First, both individual IR bands are filtered with a morphological opening top-hat transform to extract small details. Second, a common image is extracted from the two filtered IR bands, and assigned to the red channel of an RGB image. Regions of interest that appear in both IR bands remain in this common image, while most uncorrelated noise details are filtered out. Third, the visual band is assigned to the green channel and, after multiplication with a constant (typically 1.6) also to the blue channel. Fourth, the brightness and colors of this intermediate false color image are renormalized by adjusting its first order statistics to those of a representative reference scene. The result of these four steps is a fused color image, with naturalistic colors (bluish sky and grayish water), in which small targets are clearly visible. Keywords: target detection; false color fusion, small targets, clutter, image fusion

We present an algorithm that produces a fused false color representation of a combined multiband IR and visual imaging system for maritime applications. Multispectral IR imaging techniques are increasingly deployed in maritime operations, to detect floating mines or to find small dinghies and swimmers during search and rescue operations. However, maritime backgrounds usually contain a large amount of clutter that severely hampers the detection of small targets. Our new algorithm deploys the correlation between the target signatures in two different IR frequency bands (3-5 and 8-12 im) to construct a fused IR image with a reduced amount of clutter. The fused IR image is then combined with a visual image in a false color RGB representation for display to a human operator. The algorithm works as follows. First, both individual IR bands are filtered with a morphological opening top-hat transform to extract small details. Second, a common image is extracted from the two filtered IR bands, and assigned to the red channel of an RGB image. Regions of interest that appear in both IR bands remain in this common image, while most uncorrelated noise details are filtered out. Third, the visual band is assigned to the green channel and, after multiplication with a constant (typically 1.6) also to the blue channel. Fourth, the brightness and colors of this intermediate false color image are renormalized by adjusting its first order statistics to those of a representative reference scene. The result of these four steps is a fused color image, with naturalistic colors (bluish sky and grayish water), in which small targets are clearly visible. Keywords: target detection; false color fusion, small targets, clutter, image fusion

Small maritime targets, e.g., periscope tubes, jet skies, swimmers and small boats, are potential threats for naval ships under many conditions, but are difficult to detect with current radar systems due to their limited radar cross section and the presence of sea clutter. On the other hand, applications of lidar systems have shown that the reflections from small targets are significantly stronger than reflections from the sea surface. As a result, dedicated lidar systems are potential tools for the detection of small maritime targets. A geometric approach is used to compare the diffuse reflection properties of cylinders and spheres with flat surfaces, which is used to estimate the maximum detectable range of such objects for a given lidar system. Experimental results using lasers operating at 1.06 μm and 1.57 μm confirm this theory and are discussed. Small buoys near Scheveningen harbor could be detected under adverse weather over more than 9 km. Extrapolation of these results indicates that small targets can be detected out to ranges of approximately 20 km.

Small maritime targets, e.g., periscope tubes, jet skies, swimmers and small boats, are potential threats for naval ships under many conditions, but are difficult to detect with current radar systems due to their limited radar cross section and the presence of sea clutter. On the other hand, applications of lidar systems have shown that the reflections from small targets are significantly stronger than reflections from the sea surface. As a result, dedicated lidar systems are potential tools for the detection of small maritime targets. A geometric approach is used to compare the diffuse reflection properties of cylinders and spheres with flat surfaces, which is used to estimate the maximum detectable range of such objects for a given lidar system. Experimental results using lasers operating at 1.06 μm and 1.57 μm confirm this theory and are discussed. Small buoys near Scheveningen harbor could be detected under adverse weather over more than 9 km. Extrapolation of these results indicates that small targets can be detected out to ranges of approximately 20 km.

Low Frequency Active Sonar (LFAS) systems are originally designed for ASW purposes. Although their main purpose is detection of submarines, they can, if adjusted waveforms are transmitted, also be used for detection of small fast moving targets (torpedoes). In this study the Detection, Classification and Localisation performance of classical HFM and CW pulses is compared to that of three new wideband Doppler sensitive pulses. The PTFM (Pulse Train of Frequency Modulated signals) has already proven effective in suppression of reverberation in ASW trials. Two mutants, the DTFM and TTFM (a double train and a triple train) combine this property with high range resolution, at the cost of little lower Doppler resolution. This improves localisation and even allows for hard-kill solutions. Pulse properties of the 5 candidate pulses have been analysed theoretically and experimentally during a sea trial. The first analysis results of this sea trial are compliant with theory.

Low Frequency Active Sonar (LFAS) systems are originally designed for ASW purposes. Although their main purpose is detection of submarines, they can, if adjusted waveforms are transmitted, also be used for detection of small fast moving targets (torpedoes). In this study the Detection, Classification and Localisation performance of classical HFM and CW pulses is compared to that of three new wideband Doppler sensitive pulses. The PTFM (Pulse Train of Frequency Modulated signals) has already proven effective in suppression of reverberation in ASW trials. Two mutants, the DTFM and TTFM (a double train and a triple train) combine this property with high range resolution, at the cost of little lower Doppler resolution. This improves localisation and even allows for hard-kill solutions. Pulse properties of the 5 candidate pulses have been analysed theoretically and experimentally during a sea trial. The first analysis results of this sea trial are compliant with theory.

The detection of small surface targets at sea is an increasing requirement for warships. The present sensors on board do not provide the required detection probabilities for these low observable targets like small rubber boats, floating mines, periscopes, people etc. The reason for the low probability of detection is not only the low contrast but also the severe clutter problem and occlusion behind waves in higher sea states and light level problems at night. Analysis of the features of the various sources of false alarms like birds, whitecaps and sunglints in the spectral bands from the UV to the thermal IR has learned about possibilities for algorithms for discrimination of the red targets. These algorithms are based on the ratios of spectral reflectance/emission in different bands and spatial and temporal effects. These algorithms are weather and direction dependent (wind, sun and observation direction). The knowledge of sea background characteristics for target detection (surface targets as well as low altitude airborne targets) has been increased in recent years thanks to measurement campaigns with advanced sensors such as LAPTEX (Greece) and thanks to the model improvements. Examples of imagery will be shown as well as results of various processing algorithms.

The detection of small surface targets at sea is an increasing requirement for warships. The present sensors on board do not provide the required detection probabilities for these low observable targets like small rubber boats, floating mines, periscopes, people etc. The reason for the low probability of detection is not only the low contrast but also the severe clutter problem and occlusion behind waves in higher sea states and light level problems at night. Analysis of the features of the various sources of false alarms like birds, whitecaps and sunglints in the spectral bands from the UV to the thermal IR has learned about possibilities for algorithms for discrimination of the red targets. These algorithms are based on the ratios of spectral reflectance/emission in different bands and spatial and temporal effects. These algorithms are weather and direction dependent (wind, sun and observation direction). The knowledge of sea background characteristics for target detection (surface targets as well as low altitude airborne targets) has been increased in recent years thanks to measurement campaigns with advanced sensors such as LAPTEX (Greece) and thanks to the model improvements. Examples of imagery will be shown as well as results of various processing algorithms.