Chapter 10
Lidar Measurements: Atmospheric
Constituents, Clouds, and Ground
Reflectance
Claus Weitkamp
Institut fiir Physikalische und Chemische Analytik
GKSS-Forschungszentrum Geesthacht GmbH
D-21494 Geesthacht
Germany
10.1 Introduction
Vision is a capability developed to a high degree of perfection in many zoological species
including man. In addition to shape, color, texture and movement, distance can also be inferred.
Distance measurement schemes, however, tend to get increasingly inaccurate as soon as objects
are farther away than 10 2 to 10 4 m, depending on circumstances. As, with very few exceptions,
vision is a passive process that relies on external sources of illumination, the straightforward
and accurate time-of-flight determination of distance is not available to us naturally.
To actively illuminate an object with a short pulse of light and determine its distance from a
measurement of the arrival time of its image, an instrument is required. We call instruments of
this kind lidars. The time-of-flight scheme is not limited to the use of light, but works equally
well with other kinds of radiation (cf. Table 10.1). Also !idar is not just one technique; lidar
instruments vary greatly according to which physical process of interaction of light with the
atmospheric constituents is used. These processes are chiefly absorption and scattering.
Although in principle all gaseous constituents of the atmosphere that are present in concentrations 2: 10- 9 parts by volume and all physical parameters can be measured, lidar techniques are
still in a state of rapid development. In this contribution lidar techniques are presented that
serve the measurement of gaseous and particulate constituents of the atmosphere; techniques
for pressure, temperature, wind, gravity and radiation fields are not considered.
Section 10.2 is a survey of the processes that lead to a lidar return signal and of the algorithms
used for the retrieval of the interesting atmospheric properties from the lidar returns. It also
summarizes what problems must be solved to extract information on the earth's surface from
an optical return signal to an airborne or spaceborne remote sensor; these ground-reflectance
measurements are not depth-resolving as are lidars. Other non-depth resolving techniques such
NATO AS! Series. Vol. ! 45
Radiation and Water in the Climate System:
Remote Measurements
Edited by Ehrhard Raschke
© Springer· Verlag Berlin Heidelberg 1996
Lidar Measurements: Atmospheric
Constituents, Clouds, and Ground
Reflectance
Claus Weitkamp
Institut fiir Physikalische und Chemische Analytik
GKSS-Forschungszentrum Geesthacht GmbH
D-21494 Geesthacht
Germany
10.1 Introduction
Vision is a capability developed to a high degree of perfection in many zoological species
including man. In addition to shape, color, texture and movement, distance can also be inferred.
Distance measurement schemes, however, tend to get increasingly inaccurate as soon as objects
are farther away than 10 2 to 10 4 m, depending on circumstances. As, with very few exceptions,
vision is a passive process that relies on external sources of illumination, the straightforward
and accurate time-of-flight determination of distance is not available to us naturally.
To actively illuminate an object with a short pulse of light and determine its distance from a
measurement of the arrival time of its image, an instrument is required. We call instruments of
this kind lidars. The time-of-flight scheme is not limited to the use of light, but works equally
well with other kinds of radiation (cf. Table 10.1). Also !idar is not just one technique; lidar
instruments vary greatly according to which physical process of interaction of light with the
atmospheric constituents is used. These processes are chiefly absorption and scattering.
Although in principle all gaseous constituents of the atmosphere that are present in concentrations 2: 10- 9 parts by volume and all physical parameters can be measured, lidar techniques are
still in a state of rapid development. In this contribution lidar techniques are presented that
serve the measurement of gaseous and particulate constituents of the atmosphere; techniques
for pressure, temperature, wind, gravity and radiation fields are not considered.
Section 10.2 is a survey of the processes that lead to a lidar return signal and of the algorithms
used for the retrieval of the interesting atmospheric properties from the lidar returns. It also
summarizes what problems must be solved to extract information on the earth's surface from
an optical return signal to an airborne or spaceborne remote sensor; these ground-reflectance
measurements are not depth-resolving as are lidars. Other non-depth resolving techniques such
NATO AS! Series. Vol. ! 45
Radiation and Water in the Climate System:
Remote Measurements
Edited by Ehrhard Raschke
© Springer· Verlag Berlin Heidelberg 1996