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... of the operation of the Page test in detecting signals of varying duration in noise: the test statistic is shown as a function of time sample (picture from [18]). time-spread channels. The Page test =-=[16]-=- is used here as the energy detector which integrates different segments that belong to the same echo [17]. Besides signal detection, this test indicates the start-stop times of a received signal in a...

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...ency resolution but poor range resolution and poor background reverberation suppression. FM pulses are characterized by poor Doppler resolution but high range resolution and reverberation suppression =-=[4, 5]-=-. Other waveforms with different properties exist, like Newhall, Pseudo Random Noise and Costas pulses [6, 7, 8]. In this paper only linearly frequency modulated (LFM) pulses are considered. Receiving...

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...h likely belong to the same target, are clustered in time and angle to form sonar objects. The closing morphological filter (dilation followed by erosion), derived from binary image processing theory =-=[19]-=-, is used for this purpose and to fill holes within clusters. As a result of clustering, a fine bearing estimation can also be obtained as the weighted average of bearing values of the cells belonging...

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...create three detection outputs: the beam collapse detection output, the SNR (as local measure of performance), and the Receiver Operating Characteristics (ROC) curve (as global measure of performance =-=[10]-=-). In the next sections the main processing steps are described in more detail, starting from the beamformer. 3.1 Beamforming One way of solving the port/starboard ambiguity problem is a special kind ...

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...terized by poor Doppler resolution but high range resolution and reverberation suppression [4, 5]. Other waveforms with different properties exist, like Newhall, Pseudo Random Noise and Costas pulses =-=[6, 7, 8]-=-. In this paper only linearly frequency modulated (LFM) pulses are considered. Receiving arrays are linear assembles of hydrophones designed to increment signal-to-noise ratio (SNR) and directionality...

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... the trimmed mean normalizer that calculates the local mean reverberation and background noise power after discarding some of the largest and some of the smallest value samples of each sliding window =-=[14]-=-. 3.4 The Page Test Detector In shallow water environments, propagation to the target, reflection off the target and propagation to the receiver spread an active sonar transmitted signal in time and f...

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... In this paper only linearly frequency modulated (LFM) pulses are considered. Receiving arrays are linear assembles of hydrophones designed to increment signal-to-noise ratio (SNR) and directionality =-=[4, 9]-=-. Traditional towed line arrays suffer from ambiguous returns in which the sonar operator is unable to ascertain whether a target is to port (left) or to starboard (right). Although operational method...

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...terized by poor Doppler resolution but high range resolution and reverberation suppression [4, 5]. Other waveforms with different properties exist, like Newhall, Pseudo Random Noise and Costas pulses =-=[6, 7, 8]-=-. In this paper only linearly frequency modulated (LFM) pulses are considered. Receiving arrays are linear assembles of hydrophones designed to increment signal-to-noise ratio (SNR) and directionality...

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...is utilized in the 2000-3500Hz band (may operate up to 5000Hz). In LFAS applications the transmission is usually azimuthally omnidirectional, with beamwidth in the vertical plane from 3 to 30 degrees =-=[4]-=-. The two main types of signals utilized are continuous wave (CW) and frequency modulated (FM) pulses. CW pulses are characterized by good Doppler frequency resolution but poor range resolution and po...

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...s shown as a function of time sample (picture from [18]). time-spread channels. The Page test [16] is used here as the energy detector which integrates different segments that belong to the same echo =-=[17]-=-. Besides signal detection, this test indicates the start-stop times of a received signal in a normalized MF output time series. The Page test is the optimal detector in the sense of minimizing the wo...

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...lizer. in section 3.3) according to the channel time spread estimate. Closed-form expressions for time channel spread have been derived for isovelocity water by using ray invariants and acoustic flux =-=[21]-=-. According to this theory, it is possible to predict the two-way channel impulse response as a function of sound speed in the water, the water depth and the geoacoustic bottom properties. In [22] the...

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...terized by poor Doppler resolution but high range resolution and reverberation suppression [4, 5]. Other waveforms with different properties exist, like Newhall, Pseudo Random Noise and Costas pulses =-=[6, 7, 8]-=-. In this paper only linearly frequency modulated (LFM) pulses are considered. Receiving arrays are linear assembles of hydrophones designed to increment signal-to-noise ratio (SNR) and directionality...

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...steps are described in more detail, starting from the beamformer. 3.1 Beamforming One way of solving the port/starboard ambiguity problem is a special kind of beamforming, called cardioid beamforming =-=[11]-=-. The beam pattern obtained from cardioid beamforming corresponds to the beam pattern for a normal line array [9] weighted by the cardioid-shaped beam pattern obtained by combining the data of the thr...

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... according to the multipath structure and the target reflection properties. Since in real applications this information is not available, sub-optimum detectors have to be searched for. It is shown in =-=[15]-=- that integrating the magnitude-squared matched filter output (i.e. incoherent combination) can improve detection performance in Figure 5: Example of the operation of the Page test in detecting signal...

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...njunction with environmentally adaptive processing, to enable the sonar system to adjust not only to different types of background interference but also to the characteristics of the acoustic channel =-=[1, 2]-=-. The Centre’s active sonar system has been upgraded to include broadband capabilities with the procurement of a two-octave modular towed source in conjunction with a new cardioid array with left/righ...

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... detection performance in Figure 5: Example of the operation of the Page test in detecting signals of varying duration in noise: the test statistic is shown as a function of time sample (picture from =-=[18]-=-). time-spread channels. The Page test [16] is used here as the energy detector which integrates different segments that belong to the same echo [17]. Besides signal detection, this test indicates the...

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...lux [21]. According to this theory, it is possible to predict the two-way channel impulse response as a function of sound speed in the water, the water depth and the geoacoustic bottom properties. In =-=[22]-=- the analytical formulae have been validated by numerical simulations using a sound propagation ray tracing model. The results are shown in fig. 8, where the black lines are the analytical calculation...

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...njunction with environmentally adaptive processing, to enable the sonar system to adjust not only to different types of background interference but also to the characteristics of the acoustic channel =-=[1, 2]-=-. The Centre’s active sonar system has been upgraded to include broadband capabilities with the procurement of a two-octave modular towed source in conjunction with a new cardioid array with left/righ...

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...e noise estimate. The magnitude of the central point is normalized with the mean noise power. This process is repeated for each point of the MF output as the window slides over the entire time series =-=[13]-=-. This normalizer is called the mean normalizer, as it calculates the mean power of the background noise. Variations on the normalizer may be used. A frequently adopted alternative is the trimmed mean...

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..., Figure 6: Example of beam collapse plot. which is a key requirement in multistatic operations where the different units involved may want to exchange the detection data via radio or satellite links =-=[20]-=-. Similarly, these files can be transmitted to cooperating underwater platforms through low data rate acoustic transmissions. 3.6 Displays and Performance Analysis Sonar contacts are used for data vis...

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... optimal tuning of some of the key sonar parameters such as the source and receiver depths and the transmitted waveform. The tuning is driven by predictions made by a suitable sonar performance model =-=[23]-=-. 5. CONCLUSIONS A modern experimental sonar system suitable for active detection in the littoral is presented. This system consists of two broadband sources and a cardioid (triplet) receive array wit...