342
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
where δρ n /ρ n corresponds to the random error, and Δx and ΔR are the horizontal
and vertical resolutions of the DIAL measurement, respectively. It is worthwhile to
note that signal averaging in the vertical direction is more efficient than averaging
horizontally. This averaging of signals also results in a reduction in the vertical and
horizontal resolutions. Thus, a tradeoff is necessary to achieve a balance between
spatial resolution and measurement precision (typically, a vertical range resolution of
200–300 m and an average of 300–600 laser shot pairs are used in data reduction).
14.3 AIRBORNE H 2 O DIAL SYSTEM
Since the first application of the DIAL technique in 1966 (Schotland 1966), a number
of systems for water vapor profiling have been demonstrated. The basic setup of a
DIAL system is shown in Figure 14.2. It consists of a transmitter (containing a laser
emitting on online/offline wavelength) and a receiver. Short laser pulses with a few
to hundreds of nanoseconds duration and specific spectral properties are generated
by the laser. In most cases, a beam expander within the transmitter unit is used to
reduce the divergence of the light beam before it is sent out into the atmosphere. At
the receiver end, a telescope is used to collect the photons backscattered from the
atmosphere. It is usually followed by an optical analyzing system, which, dependfollowed by an optical analyzing system, which, dependfollowed by an optical analyzing system, which, depending on the application, selects specific wavelengths or polarization states out of the
collected light. The selected radiation is directed onto a detector, where the received
optical signal is converted into an electrical signal. The intensity of this signal in its
dependence on the time elapsed after the transmission of the laser pulse is detertransmission of the laser pulse is detertransmission of the laser pulse is determined electronically and stored in a computer.
The applicability of most systems before 1996 was severely limited by imperfections of the laser systems. For an overview of DIAL developments before 2000, see
the work of Weitkamp (2005). During the last 10 years, rapid progress has been
made by H 2 O DIAL practitioners. With the application of the injection seeding technique, frequency stability and spectral purity were dramatically improved (Chyba
et al. 1997; Wulfmeyer et al. 1995; Ehret et al. 1998). With the availability of reliable and affordable pump lasers (high-brightness diode lasers), simplified resonator designs, ultrastable mechanical setups for the resonator and the coupling of the
Transmitter
Receiver
Beam
expander
Laser
Telescope
Field
stop
Optical
analyzer/
detector
Data
acquisition/
computer
FIGURE 14.2 Principle setup of the DIAL system.
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
where δρ n /ρ n corresponds to the random error, and Δx and ΔR are the horizontal
and vertical resolutions of the DIAL measurement, respectively. It is worthwhile to
note that signal averaging in the vertical direction is more efficient than averaging
horizontally. This averaging of signals also results in a reduction in the vertical and
horizontal resolutions. Thus, a tradeoff is necessary to achieve a balance between
spatial resolution and measurement precision (typically, a vertical range resolution of
200–300 m and an average of 300–600 laser shot pairs are used in data reduction).
14.3 AIRBORNE H 2 O DIAL SYSTEM
Since the first application of the DIAL technique in 1966 (Schotland 1966), a number
of systems for water vapor profiling have been demonstrated. The basic setup of a
DIAL system is shown in Figure 14.2. It consists of a transmitter (containing a laser
emitting on online/offline wavelength) and a receiver. Short laser pulses with a few
to hundreds of nanoseconds duration and specific spectral properties are generated
by the laser. In most cases, a beam expander within the transmitter unit is used to
reduce the divergence of the light beam before it is sent out into the atmosphere. At
the receiver end, a telescope is used to collect the photons backscattered from the
atmosphere. It is usually followed by an optical analyzing system, which, dependfollowed by an optical analyzing system, which, dependfollowed by an optical analyzing system, which, depending on the application, selects specific wavelengths or polarization states out of the
collected light. The selected radiation is directed onto a detector, where the received
optical signal is converted into an electrical signal. The intensity of this signal in its
dependence on the time elapsed after the transmission of the laser pulse is detertransmission of the laser pulse is detertransmission of the laser pulse is determined electronically and stored in a computer.
The applicability of most systems before 1996 was severely limited by imperfections of the laser systems. For an overview of DIAL developments before 2000, see
the work of Weitkamp (2005). During the last 10 years, rapid progress has been
made by H 2 O DIAL practitioners. With the application of the injection seeding technique, frequency stability and spectral purity were dramatically improved (Chyba
et al. 1997; Wulfmeyer et al. 1995; Ehret et al. 1998). With the availability of reliable and affordable pump lasers (high-brightness diode lasers), simplified resonator designs, ultrastable mechanical setups for the resonator and the coupling of the
Transmitter
Receiver
Beam
expander
Laser
Telescope
Field
stop
Optical
analyzer/
detector
Data
acquisition/
computer
FIGURE 14.2 Principle setup of the DIAL system.