Exponential Decomposition with Implicit Deconvolution of Lidar Backscatter from the Water Column

  • Roland Schwarz
  • Norbert Pfeifer
  • Martin Pfennigbauer
  • Andreas Ullrich
Original Article

Abstract

Bathymetric laser scanning is a powerful tool to obtain information about the morphology of coastal, river, and inland waters. Laser scanning in general is a method to sense the shape of remote objects by sweeping a laser beam across the objects while measuring the distance to every surface point. In bathymetric applications, the electromagnetic light wave also needs to penetrate the water column resulting in a spread reflection from below the surface of the water body complicating the interpretation of the received wave. As the signal seen by the sensor’s receiver is the result of a convolution of the system waveform with the differential backscatter cross section, one approach is to use a deconvolution method to recover the object shape. An alternative approach is to fit a parameterised model to the measured receiver signal. While deconvolution methods are not capable to directly deliver object parameters such as distance to water surface or bottom, modelling methods suffer from neglecting the system waveform. We present a new waveform decomposition method that avoids current shortcomings. The proposed method uses a model composed of segments of exponential functions, which is motivated by the physics of the backscatter process in the water column, and a record of the system waveform which is stored as part of the sensor’s calibration data. The method further consists of an algorithm which evaluates the parameters of the exponential model while, at the same time, performing a deconvolution from the system waveform in an implicit manner. The effectiveness of the method is exemplified using real data from a nearshore airborne LIDAR data acquisition.

Keywords

Laser radar Sea surface Bathymetry Transient response Deconvolution 

Zusammenfassung

Zerlegung von rückgestreuten Lasersignalen mittels Exponentialfunktionen mit impliziter Dekonvolution. Bathymetrisches Laserscanning dient zur Erfassung der Morphologie küstennaher Gewässer, von Binnengewässern und von Flüssen. Laser Scanning ist im Allgemeinen eine Methode zur Erfassung von Objekten mit Hilfe eines abtastenden Laserstrahls bei gleichzeitiger Distanzmessung zu den Oberflächenpunkten. In der Bathymetrie wird die Auswertung erschwert, da die elektromagnetische Lichtwelle auch (obzwar erwünscht) in den Wasserkörper eindringt und dadurch eine auseinanderlaufende Rückstreuung von Bereichen unter der Wasseroberflöche verursacht. Da das vom Sensor gemessene Signal der Faltung der System-Wellenform mit dem differentiellen Rückstreuquerschnitt entspricht, kann die Objektform mittels Entfaltung (Dekonvolution) erhalten werden. Eine alternative Methode ist es, ein parametrisches Modell an die Empfängerdaten anzupassen. Während die Entfaltungsmethode nicht direkt Parameter wie die Distanz zur Wasseroberfläche oder zum Grund liefert, vernachlässigen parametrische Modelle den Einfluss der System-Wellenform. Wir präsentieren eine verbesserte Methode zur Zerlegung für Wellenformen, die diese Nachteile vermeidet. Motiviert durch die Physik der Rückstreuung in der Wassersäule kommt ein Modell aus Segmenten von Exponentialfunktionen ebenso zur Anwendung, wie die System-Wellenform, die aus einer Kalibrierung bekannt ist. Weiters beschreiben wir einen Algorithmus, der die Parameter des Exponentialmodells ermittelt sowie zugleich eine implizite Entfaltung der System-Wellenform bewirkt. Die Wirksamkeit der Methode wird exemplarisch anhand von Daten veranschaulicht, die aus einer luftgestützten LIDAR Aufnahme stammen.

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Copyright information

© Deutsche Gesellschaft für Photogrammetrie, Fernerkundung und Geoinformation (DGPF) e.V. 2017

Authors and Affiliations

  • Roland Schwarz
    • 1
  • Norbert Pfeifer
    • 2
  • Martin Pfennigbauer
    • 1
  • Andreas Ullrich
    • 1
  1. 1.Riegl Research Forschungs Gesellschaft mbHHornAustria
  2. 2.Department of Geodesy and GeoinformationTechnische Universität WienViennaAustria

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