Analysis and Processing of Signals in Laser Bathymetry

摘要

Airborne laser bathymetry is a remote sensing technique for the mapping of underwater topography. Compared to topographic airborne laser scanning, light propagation in two media must be considered. An important difference also is that for use in bathymetry, only wavelengths of the laser in the visible range can be used, since light in the infrared range is practically unable to penetrate the water. The first generation of scanners for bathymetry came into use shortly after the invention of the LASER. Although in the beginning mainly only analogue electronics was available for the evaluation of the signals, later the entire trace of the backscattered echo was recorded digitally. The advent of affordable computers finally opened the way to more complex signal processing. For the determination of the elevation of the underwater bottom it is necessary to identify two significant time instants in the waveform. The first is when the light impulse enters the water and the second is when it hits the bottom. It is especially important to know the first moment, because from this moment on the impulse moves slower and in a different direction. The standard method to identify an instant of time in a signal is by gaussian decomposition of the signal. Underwater, however, the method suffers from the problem that a lot of distributed small particles cause clutter that is hindering the exact decomposition. For a tenuous distribution of such particles the waveform is of exponential character.