5.1 Introduction
Laser systems used in measuring distances are called LiDAR (Light Detection and
Ranging). As with radar, LiDAR is an ‘active’ remote sensing technology since it
provides its own energy. The sensors considered in Chaps. 1–4 are ‘passive’,
because they measure natural electromagnetic energy, typically provided by the
sun. LiDAR instruments are unique in that they confine the coherent light energy
within a very narrow beam, providing pulses of very high peak intensity. This
enables blue-green wavelength LiDAR to penetrate clear to moderately turbid
coastal waters for bathymetric measurements and allows mainly near-infrared
LiDAR to probe gaps in forest canopies to provide topographic data for digital
elevation models (DEMs) (Brock and Sallenger 2000; Brock and Purkis 2009). For
these measurements, LiDAR systems employ an accurate clock that records the
round-trip travel time between the transmission and reception of a reflected laser
pulse.
The development of LiDAR technology commenced in the 1970s, with early
systems built in the USA and Canada (Ackermann 1999). However, the technology was not implemented aboard aircraft until the late 1980s, at which point it
started to be used for the accurate determination of terrain models (Baltsavias
1999). The application in these early years was limited by its complexity, cost
effectiveness, and poor georeferencing. These limitations were gradually overcome following the availability of more reliable electronics, more advanced lasers,
and the increased geo-accuracy afforded by the Global Positioning System (GPS).
In 1988, the U.S. Army Corps of Engineers constructed an operational LiDAR
system which was subsequently developed commercially by Optech Inc. (LaRocque and West 1990; Irish and Lillycrop 1999). Around this time the Australian
Laser Airborne Depth Sounder (LADS) system was also developed and later
commercialized by the Tenix LADS Corporation (Irish and White 1998). In the
last decade, LiDAR has overcome many of the hurdles that plagued early setups.
Current systems use commercial lasers that can pulse at rates as high as
200 kHz and inertial measurement units (IMUs) that provide pointing precision
that enables georeferencing accuracy to the sub-metre level. An IMU is an electronic device that measures and reports on an aircraft‘s velocity, orientation, and
gravitational forces using a combination of digital accelerometers and gyroscopes.
Good positional accuracy of LiDAR postings demands precise information on the
aircraft location at both the time the laser sounding is dispatched groundward, and
the time the reflection is subsequently received (Latypov 2002). GPS is only
sufficiently accurate for this purpose when differentially corrected against a basestation (DGPS). Here, the position of the aircraft is acquired by GPS, and differential-GPS on-the-fly algorithms are applied at post processing (not real time).
The GPS reference station should be positioned within the survey area and the
rover unit (aboard the survey-aircraft) should not depart by more than 25 km. Note
that at extremely stable (but rare) conditions of the troposphere and ionosphere this
distance might exceed 100 km. With the differential correction robustly applied to
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S. J. Purkis and J. C. Brock
Laser systems used in measuring distances are called LiDAR (Light Detection and
Ranging). As with radar, LiDAR is an ‘active’ remote sensing technology since it
provides its own energy. The sensors considered in Chaps. 1–4 are ‘passive’,
because they measure natural electromagnetic energy, typically provided by the
sun. LiDAR instruments are unique in that they confine the coherent light energy
within a very narrow beam, providing pulses of very high peak intensity. This
enables blue-green wavelength LiDAR to penetrate clear to moderately turbid
coastal waters for bathymetric measurements and allows mainly near-infrared
LiDAR to probe gaps in forest canopies to provide topographic data for digital
elevation models (DEMs) (Brock and Sallenger 2000; Brock and Purkis 2009). For
these measurements, LiDAR systems employ an accurate clock that records the
round-trip travel time between the transmission and reception of a reflected laser
pulse.
The development of LiDAR technology commenced in the 1970s, with early
systems built in the USA and Canada (Ackermann 1999). However, the technology was not implemented aboard aircraft until the late 1980s, at which point it
started to be used for the accurate determination of terrain models (Baltsavias
1999). The application in these early years was limited by its complexity, cost
effectiveness, and poor georeferencing. These limitations were gradually overcome following the availability of more reliable electronics, more advanced lasers,
and the increased geo-accuracy afforded by the Global Positioning System (GPS).
In 1988, the U.S. Army Corps of Engineers constructed an operational LiDAR
system which was subsequently developed commercially by Optech Inc. (LaRocque and West 1990; Irish and Lillycrop 1999). Around this time the Australian
Laser Airborne Depth Sounder (LADS) system was also developed and later
commercialized by the Tenix LADS Corporation (Irish and White 1998). In the
last decade, LiDAR has overcome many of the hurdles that plagued early setups.
Current systems use commercial lasers that can pulse at rates as high as
200 kHz and inertial measurement units (IMUs) that provide pointing precision
that enables georeferencing accuracy to the sub-metre level. An IMU is an electronic device that measures and reports on an aircraft‘s velocity, orientation, and
gravitational forces using a combination of digital accelerometers and gyroscopes.
Good positional accuracy of LiDAR postings demands precise information on the
aircraft location at both the time the laser sounding is dispatched groundward, and
the time the reflection is subsequently received (Latypov 2002). GPS is only
sufficiently accurate for this purpose when differentially corrected against a basestation (DGPS). Here, the position of the aircraft is acquired by GPS, and differential-GPS on-the-fly algorithms are applied at post processing (not real time).
The GPS reference station should be positioned within the survey area and the
rover unit (aboard the survey-aircraft) should not depart by more than 25 km. Note
that at extremely stable (but rare) conditions of the troposphere and ionosphere this
distance might exceed 100 km. With the differential correction robustly applied to
116
S. J. Purkis and J. C. Brock