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-