The subject of this thesis is an investigation of the potential for detecting small targets (size about 1 m³) with airborne scanning laser depth sounding. Examples of other optical techniques of interest for small target detection are: range gated imaging, polarized detection and passive multispectral imaging. We have attempted to carry out a comprehensive study concerning the influence of environmental and sensor parameters on the capability for target detection on a flat sea bottom. In particular, the effects of sea surface waves, target height and shape are studied. Our results concern targets on bottom with depths between 3 m and 10 m, which, from our experience in Swedish waters, is an interesting range for detection of 1-m³ targets. Floating and submerged targets are not studied explicitly, but the detection of such targets is discussed briefly. We have also developed a simple and preliminary signal processing algorithm for small bottom target detection. Such detection is important for charting purposes, safe navigation and different military applications. The airborne sensor has advantages over shipborne ones, since it is rapid and functional in shallow waters. The high mobility of the airborne platform is useful for many types of operations. Our results are intended for supporting the design of future lidar systems and decisions of their operational use.

Our results are based on both experimental and theoretical methods. The experimental data were collected in the Baltic Sea with the Swedish Hawk Eye system mounted on a helicopter. We have developed theoretical simulation models for the parts of the studies that were impossible or too costly to study by experimental evaluations. The simulation results correspond well to the experimental data. Our results show that a laser system designed for this purpose can be used for detection of small objects down to depths of approximately 10 m in waters representative to the Baltic Sea. Although the water clarity is the main limitation, sea surface waves can significantly affect the detectability.

The typical horizontal sounding spacing for small target detection should be around 1 m, which sets a requirement on the laser pulse repetition frequency to 1000 Hz or higher for reasonable area survey rates. The laser system we have studied uses a single pixel detector. A receiver with an array of detectors can increase the horizontal resolution and might provide possibilities for signal processing that corrects for effects caused by transmission through the sea surface. Small target detection requires that the laser system uses short laser pulses and fast photodetectors. With our preliminary detection algorithm, the experimental false alarm rate could be held at zero level for flight passages over areas of 100 m² corresponding to a false alarm rate less than 1 %. In practical applications the false alarm rate should be much smaller than 1 %, and more advanced detection algorithms are required.