Geoscience Reference
In-Depth Information
terms of time spans. Besides, observation directions are usually fixed both
horizontally and vertically. These limitations originated from the system
stability as well as the complication of system maintenance including the
laser device.
A micro-pulse lidar (MPL), developed by Spinhirne in 1993, is
a compact lidar system that provides easy operation and long-term
observation.
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Using a laser-diode pumped laser of micro-joule output
energy, MPL ensures the eye-safety features. Signal-to-noise ratio was
improved by narrowing the receiver field-of-view (FOV). However, this
makes it dicult to adjust the laser beam within the receiver's FOV. Since
the same telescope is used to both transmit and receive the laser beam, a
small amount of the emitted beam back-reflected from the beam splitter
often damages the detector.
In this chapter, we describe a portable automated lidar (PAL) system,
which we have developed to conduct full-time operation and all-weather
observation through the laboratory window.
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The PAL system has an
automated correction mechanism for misalignment of the overlap between
the transmitted laser beam and the receiver FOV. Hence the system is able
to operate in a stable and stand-alone way. In addition, we have recently
installed the scanning mechanism by attaching a horizontal stage to the
PAL system. This improvement contributes greatly to monitoring the two-
dimensional structure of the atmosphere nearly instantaneously.
2. PAL System
The PAL system is a variation of MPL system. The system configuration is
shown in Fig. 1 and its specifications are summarized in Table 1. Since the
transmitted energy is 15
J, the system is nearly eye-safe at the expense
of weak signals (lidar echo). To attain enough signal-to-noise ratio, the
background light due to sky radiance must be eliminated with a narrow-
bandwidth filter (0.5 nm) and a narrow FOV of 0.2 mrad. At the same time
it is essential to keep the good overlap between the laser beam and the
telescope FOV. Misalignment of the overlap, however, sometimes occurs
from changes in the ambient temperature and accidental disturbances.
The system has the auto-alignment mechanism, in which the laser beam
is scanned vertically and then horizontally within the receiver's FOV and
the maximum in the return signal (a certain range near the peak of the
A-scope) is sought every 15 min.
The detector is a photo-multiplier operated in the photon counting
mode (Hamamatsu photonics K.K. R1924P). The lidar echoes are
µ
