LIDAR

Light detection and ranging (lidar) is a technique in which
a beam of light is used to make range-resolved remote
measurements. A lidar emits a beam of light, that interacts
with the medium or object under study. Some of this light
is scattered back toward the lidar. The backscattered light
captured by the lidar’s receiver is used to determine some
property or properties of the medium in which the beam
propagated or the object that caused the scattering.
The lidar technique operates on the same principle
as radar; in fact, it is sometimes called laser radar.
The principal difference between lidar and radar is the
wavelength of the radiation used. Radar uses wavelengths
in the radio band whereas lidar uses light, that is
usually generated by lasers in modern lidar systems. The
wavelength or wavelengths of the light used by a lidar
depend on the type of measurements being made and may
be anywhere from the infrared through the visible and into
the ultraviolet. The different wavelengths used by radar
and lidar lead to the very different forms that the actual
instruments take.
The major scientific use of lidar is for measuring
properties of the earth’s atmosphere, and the major commercial
use of lidar is in aerial surveying and bathymetry
(water depthmeasurement). Lidar is also used extensively
in ocean research (1–5) and has several military applications,
including chemical (6–8) and biological (9–12)
agent detection. Lidar can also be used to locate, identify,
and measure the speed of vehicles (13). Hunters
and golfers use lidar-equipped binoculars for range finding
(14,15).
Atmospheric lidar relies on the interactions, scattering,
and absorption, of a beam of light with the constituents
of the atmosphere. Depending on the design of the lidar,
a variety of atmospheric parameters may be measured,
including aerosol and cloud properties, temperature, wind
velocity, and species concentration.
This article covers most aspects of lidar as it relates to
atmospheric monitoring. Particular emphasis is placed on
lidar system design and on the Rayleigh lidar technique.
There are several excellent reviews of atmospheric lidar
available, including the following:
Lidar for Atmospheric Remote Sensing (16) gives
a general introduction to lidar; it derives the lidar
equation for various forms of lidar including Raman
and differential absorption lidar (DIAL). This work
includes details of a Raman and a DIAL system
operated at NASA’s Goddard Space Flight Center.
Lidar Measurements: Atmospheric Constituents, Clouds,
and Ground Reflectance (17) focuses on the differential
absorption and DIAL techniques as well as their
application to monitoring aerosols, water vapor, and
minor species in the troposphere and lower stratosphere.
Descriptions of several systems are given, including the
results of measurement programs using these systems.
Optical and Laser Remote Sensing (18) is a compilation
of papers that review a variety of lidar techniques
and applications. Lidar Methods and Applications (19)
gives an overview of lidar that covers all areas of
atmospheric monitoring and research, and emphasizes