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FROM SINGLE-PULSE TO FULL-WAVEFORM AIRBORNE LASER SCANNERS:
POTENTIAL AND PRACTICAL CHALLENGES
W. Wagner * *, A. Ullrich”, T. Melzer *, C. Briese ©, K. Kraus ©
* Christian Doppler Laboratory for Spatial Data from Laser Scanning and Remote Sensing, Vienna University of
Technology, Gusshausstrasse 27-29, 1040 Vienna, Austria - (ww, tm)@ipf.tuwien.ac.at
° Riegl Research GmbH, 3580 Horn, Austria - aullrich@riegl.co.at
© Institute of Photogrammetry and Remote Sensing, Vienna University of Technology, Gusshausstrasse 27-29, 1040
Vienna, Austria - (cb, kk)@ipf.tuwien.ac.at
KEY WORDS: Laser scanning, Digitisation, Retrieval, Vegetation, Accuracy, Automation
ABSTRACT:
Airborne laser scanning, often referred to as lidar or laser altimetry, is a remote sensing technique which measures the round-trip
time of emitted laser pulses to determine the topography of the Earth's surface. While the first commercially available airborne laser
scanners recorded only the time of one backscattered pulse, state-of-the-art systems measure first and last pulse; some are able to
measure up to five pulses. This is because there may be several objects within the travel path of the laser pulse that generate multiple
echoes. Pulse detection is then used to determine the location of these individual scatterers. In this paper we discuss the physical
measurement process and explain the way how distributed targets (such as trees or inclined surfaces) transform the emitted pulse. It
is further shown through theoretical experiments that different detectors may yield quite different height information, depending on
the type of the target. For example, even in the simple case of a tilted roof (with a tilt angle of 45?) the range values obtained by
using different detectors may vary by — 0.4 m for a laser footprint size of 1 m. Airborne laser scanner systems that digitise the full
waveform of the backscattered pulse would give more control to the user in the interpretation process. It would e.g. be possible to
pre-classify the acquired data with respect to the shape of the echoes, to use different detection methods depending on surface cover
and the intended application, and to employ more physically-based retrieval methods.
1. INTRODUCTION
Airborne laser scanning is a rapidly growing technology which
has initially been conceived for topographic mapping. Airborne
laser scanners employ, with few exceptions, pulsed lasers that
repetitively emit short infrared pulses towards the Earth's
surface. Some of the energy is scattered back to the sensor
where it is measured with an optical receiver. A timer measures
the travelling time of the pulse from the laser scanner to the
Earth's surface and back. Since the round-trip time is directly
related to the distance of the sensor to the ground, the
topography of the Earth's surface can be reconstructed.
One advantage of airborne laser scanning compared to classical
photography is that laser scanners are not dependent on the sun
as a source of illumination. Consequently, the interpretation of
laser scanner data is not hampered by shadows caused by
clouds or neighbouring objects. For example, laser scanner
pulses may travel unimpeded back and forth along the same
path through small openings in a forest canopy, providing
information about the forest floor. In contrast, optical images
provide information only about the illuminated top layers of the
forest canopy, while lower canopy layers and the forest floor
constitute a dark background.
Since 1960, when Theodore Maiman demonstrated that “light
amplification by stimulated emission of radiation” (laser) is also
possible in the infrared and optical part of the electromagnetic
spectrum, lasers have been widely used for military intelligence
* Corresponding author
and civil surveying. But it took more then thirty years before
laser scanners were deployed on commercial airborne platforms
for topographic mapping purposes. There are many reasons for
the relatively late adoption of airborne laser scanner
technology: Flood (2001) mentions as critical factors the
increasing availability of commercial off-the shelf sensors in
the mid-90s, advancements in the design and capabilities of the
sensors themselves, and an increased awareness by end users
and contracting agencies. Ackermann (1999) points out the
importance of precise kinematic positioning of the airborne
platform by differential GPS (“Global Positioning System”) and
inertial attitude determination by IMU (“Internal Measurement
Unit”) for accurate referencing to an external coordinate
system. Finally, also the increasing computer power probably
played an important role, given that a large amount of data is
acquired during each laser scanner flight (0.1 - 10 points per
square meter).
The development of airborne laser scanning has been largely
technology driven (Ackermann, 1999), but advances in our
understanding of the measurement process have quickly led to
system improvements. The first commercially available
airborne laser scanners recorded the time of one backscattered
pulse. The recording of only one pulse is sufficient if there is
only one target within the laser footprint. In this case the shape
of the reflected pulse is “single mode” and straightforward to
interpret. However, even for small laser footprints (0.2 - 2 m)
there may be several objects within the travel path of the laser
pulse that generate individual backscatter pulses. Therefore
