CHARACTERIZATION AND DECOMPOSITION OF WAVEFORMS 
FOR LARSEN 500 AIRBORNE SYSTEM 
H. Wong and A. Antoniou 
Department of Electrical and Computer Engineering 
University of Victoria 
Victoria, B.C., Canada V8W 2Y2 
ISPRS Commission VII 
Abstract 
The LARSEN 500 airborne laser ranging system devel 
oped in Canada is an active remote sensor for the mea 
surement of sea depths in shallow coastal waters. This 
system transmits a series of laser pulses from an air 
craft into the ocean and receives the reflections from the 
ocean for post-mission processing. The nature of the 
LARSEN waveforms received varies dramatically de 
pending on the turbidity, depth, and the surface rough 
ness of the sea, and the shape and texture of the sea bot 
tom. In many cases, the separation between the surface 
and bottom reflections, which is essential in estimating 
the sea depths, is lost and as a result, unreliable depth 
estimates are obtained. 
This contribution is concerned with a method that can 
accurately decompose a LARSEN waveform into two 
signal components, namely the surface and bottom re 
flections, independently of the degree of their overlap. 
First, a mathematical model that can be used to char 
acterize the LARSEN waveforms received under diverse 
circumstances is established. This model is then used in 
conjunction with an optimization technique to facilitate 
the decomposition of each LARSEN waveform into sur 
face and bottom reflections. Optimized parameters of 
the mathematical model are then used to give accurate 
estimation of the sea depths. 
The use of optimization techniques in the estimation of 
sea depths offers several advantages. First, the overlap 
ping surface and bottom reflections in the waveform are 
mathematically resolved into two separate components. 
Therefore, both the detection and resolution problems 
can be solved simultaneously. Furthermore, the method 
is insensitive to changes in the degree of overlap and, as 
a result, accurate sea-depth estimates can be obtained 
under a diverse range of circumstances. 
Key Words: laser bathymetry, mathematical modeling 
of waveforms, nonlinear optimization. 
1 INTRODUCTION 
Airborne laser ranging systems have been used exten 
sively in shallow-water bathymetry during the last two 
decades. Both high flexibility of operation and high 
rate of coverage are their prime advantages of use. The 
feasibility of using airborne laser techniques in survey 
ing shallow coastal waters was first demonstrated with a 
system constructed at the Syracuse University Research 
Corporation in 1968 (Hickman and Hogg, 1969). Since 
then, a number of countries such as the United States, 
Australia, and Canada have been actively involved in 
the development of their own airborne laser bathymet 
ric systems. (Bressel et al., 1977; Casey, 19S4; Banic et 
al., 1984; Penny et al., 19S6). 
In Canada, the use of airborne laser methods in the 
field of hydrography was initiated by the development 
of the MK I low-power neon laser bathymeter. Testing 
of the MK I over Kingston Harbour in Lake Ontario was 
carried out by the Canada Centre for Remote Sensing 
(CCRS) in 1976 (O’Neil et al., 1978). A second genera 
tion of the system, MIv II, is a nonscanning light detec 
tion and ranging (LIDAR) system and initially served as 
a complement to aerial and satellite remote sensing tech 
niques (O’Neil, 1980). In order to increase speed, extent 
of coverage and flexibility, a more advanced system, the 
LARSEN 500, was developed by CCRS and the Cana 
dian Hydrographic Service (CHS) (Casey, 1984). This is 
an advanced airborne scanning LIDAR system designed 
to measure water depths in shallow coastal waters and 
meets the standards of the Canadian Hydrographic Ser 
vice. The LARSEN 500 is capable of measuring depths 
from 1.5 to 40 m to an accuracy of 0.3 m. The concept 
and operation of this system is briefly described in the 
next section. 
1.1 Concept and Operation 
The LARSEN 500 laser beam geometry is illustrated 
in Fig. 1 and the principles of operation are as follows. 
Blue-green and infrared (IR) laser pulses are transmit 
ted simultaneously from the aircraft into the ocean in a 
quasi-circular fashion. The IR pulse is scattered by the 
water surface and its reflection permits accurate deter 
mination of the aircraft height. On the other hand, the 
blue-green pulse is reflected back both from the surface 
as well as the bottom of the ocean. The surface echo 
is much stronger than the bottom return since signal 
attenuation in water is much stronger than in air. The 
time interval between the surface and bottom reflections 
serves as a measure of water depth. 
The reflections from each set of IR and blue-green laser 
soundings constitute a LARSEN waveform. This wave 
form is sampled at 2-ns intervals and 256 consecutive 
samples are digitized to 6-bit accuracy, and are then 
recorded. The waveform starts with the detection of 
the reflection of the IR pulse from the sea surface and 
the waveform is a record of the reflections of the blue- 
green pulse from the surface, volume, and bot tom of the 
sea to a depth of approximately 40 meters. Figure 2 il 
lustrates the received waveform and various parameters 
of interest. 
1.2 Problems Encountered 
Water quality is an important factor in the depth pene 
tration of a laser pulse as it affects the range and accu 
racy of depth measurement. As absorption and scat 
tering of laser light are very strong in water (Svan- 
berg, 1981), particle content in water strongly influences 
the laser-signal attenuation. This problem is partic 
ularly serious in coastal waters (prime areas for laser 
bathymetry) since microscopic marine life is abundant 
in these areas. 
On the other hand, backscatter from suspended and dis 
solved particles in water weaken the laser-signal reflec-