Geoscience Reference
In-Depth Information
medical examinations (Fercher et al. 2003 ). This research (Bennett and Christie
2007 ; Bennett et al. 2006 ), which has led only to single case studies but not
to a viable method for meteorological purposes so far, uses a fibre-optic based
continuous-wave Doppler LIDAR. Infra red radiation at
mfroma
distributed feedback laser diode is split between a reference path and a signal
path. Radiation following the signal path is amplified and transmitted towards a
scattering target, whence some tiny proportion is returned to the instrument. This
is then mixed in a coupler with delayed radiation that has followed the reference
path. A narrow-band beat signal can only be expected when the length of the delay
line matches the path length in the atmosphere to within the coherence length of
the source. Scattering from other distances will give a beat signal widened by twice
the linewidth of the source (Harris et al. 1998 ). In the given instrument, the line
width of the source is
λ =
1.55
μ
ν =
1.4 MHz; the coherence length for the beats signal is
34 m, and this should be the effective range resolution of the
system. The range of the system is between 42 and 217 m. For a reliable operation
the atmospheric visibility should be below 30 km.
r
=
c
/
(2
π ν
)
=
3.5.5 Other Optical Flow Measurement Techniques
A further method to measure wind flow that is based on aerosol backscatter is laser
Doppler anemometry (LDA, Durst et al. 1976 ), also sometimes called laser Doppler
velocimetry (LDV), which relies on the interference of two coherent laser beams.
Using a beam splitter, the instrument emits two coherent beams, which cross each
other at some distance (probably much less than 100 m) from the device. At the posi-
tion of the cross-over, an interference pattern is formed (the frequency of the result-
ing beats is also called Doppler frequency which explains the name of this measure-
ment technique). Aerosol particles floating in the air passing through this cross-over
region are alternately illuminated or in darkness. From the frequency of this inten-
sity oscillation, the air speed perpendicular to the fringes of the interference pattern
can be determined. Both velocity components can be derived with this technique by
producing a second pattern turned by 90 with respect to the first one. Successful
boundary layer application of this technique, which has been designed primarily for
wind tunnel flow measurements, has not been reported. Most probably, the sensitiv-
ity at longer ranges is not sufficient for this purpose in the unseeded atmosphere.
Particle imaging velocimetry (PIV, Adrian 1991 ) is another method commonly
used for flow measurement in wind tunnels. Like LDA, it requires careful seeding
of the flow with particles or oil droplets and is therefore unsuitable for remote sens-
ing in the atmosphere. The same comments also apply to the technique of Doppler
Global Velocimetry (DGV, Meyers 1995 ).
3.6 Radiometers
Radiometers record passively the intensity and sometimes additionally the polar-
isation of the incident radiation. The use of radiometers in remote sensing aims
to determine the spectral and physical characteristics of incoming radiation. By an
Search WWH ::




Custom Search