## 4 Experimental Results and Discussion
### The Experiments in the Sea  
In this paper, we consider the use of an active hull-mounted sonar system to detect a moving underwater target. Two schematics of several experiment situations considered are described in Figure 5. In conventional algorithms, the target detection and classification are done by comparing the matched filter signal to a preset threshold. In this case, target and clutter signals with SNRs above the threshold are labeled. In long-range detection, several classification schemes are applied with appropriately high SNR signals that have enough feature information to separate target and clutter components. These schemes can increase the false alarm rate when the clutter signals are stronger than the target signals and also decrease the classification rate for weak target signals. It is essential to detect targets with continuing false alarms because any target can be a threat. Thus, it is very important to constantly attempt target detection even if the probability of false alarm sometimes goes up. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & & **Target** & **Clutter** \\ \hline \multirow{4}{*}{In-layer feature} & A & 24.1 & 23.0 \\  & B & 20.5/49.5/\(-\)0.87 & 19.9/5.7/\(-\)0.48 \\  & C & 10.5/70.3/\(-\)0.02 & 15.4/10.6/\(-\)0.78 \\  & D & 2.75/59.7/0.14 & 10.3/31.4/\(-\)0.19 \\  & E & 1.10/19.9/0.06 & 3.11/70.4/0.26 \\ \hline \multirow{4}{*}{Cross-layer feature} & A, B & 1.18/4.86 & 1.16/40.3 \\  & B, C & 1.95/\(-\)1.52 & 1.29/\(-\)2.69 \\ \cline{1-1}  & C, D & 3.82/0.70 & 1.49/\(-\)4.71 \\ \cline{1-1}  & D, E & 2.50/4.77 & 3.31/\(-\)3.50 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Feature data in target and clutter according to Figure 4.  
Figure 4: 2-D display samples with segmented layers of the target and clutter in the experiment to show feature information. The experimental data were obtained in the East Sea of South Korea in 2006, 2008, 2009, 2011, and 2012 with hull-mounted active sonar systems (DSQS21 BZ and SQS-240K) operating on Korean Navy ships. The experiments were designed to cover a variety of underwater environments and general target maneuvers. We included both summer and winter data with their opposing sonic propagation characteristics and data from the beam to the bow direction to obtain a variety of target echo strengths as a function of the incident angle. The feature component analysis experiments were continuously conducted until the target signal was lower than the ambient noise. The target size was less than 100 m long; the East Sea of Korea is approximately 500 to 3000 m deep. The range to the target from ownship was about 4 to 15 kyds (approximately 3.6 to 13.7 km). The active sonar was operated with LFM and CW signals having a center frequency 5 to 10 kHz and a pulse length under 500 ms. Figure 5 shows examples of the experimental ownship and target movement paths. Matched filter outputs were acquired from the signal processing unit of the experimental active sonar system. First, we extracted all echoes having a peak signal above the noise level. Typically, most extracted patches are clutter because the real target is a single return, and it is especially difficult to separate low SNR target signals from clutter. Echo characteristics are determined by the target size, target shape, ping incident angle, multipath effect, and elongation effect. In this paper, we focus on an adaptive approach for continuous detection. Normally, if the reflected signal level is low, it is harder to separate the target from clutter because sufficient feature vectors cannot be extracted, decreasing the classification rate. In order to process low SNR target echoes, we continuously checked and acquire data in situations where the target echo is both lower and higher than the noise level. The orientation of the target, the active sonar PRI, and the GPS position and orientation of ownship are used to confirm the target echo. Figure 6 shows one example from several experiments of a normalized matched filter output in the bearing-range domain.