Geoscience Reference
In-Depth Information
Beam focusing A method to determine the distance between an active optical
remote-sensing instrument and the backscattering air volume. The optical beam is
focused in some distance from the instrument. The strong backscatter from the range
around the focus is dominating the received signal. Thus the retrieved information is
attributed to this focal range. The focal distance can be varied within certain bounds.
This technique allows for continuous (non-pulsed) observations. Alternatives are
the measurement of travel times for pulsed active remote-sensing techniques and
techniques using frequency modulation for continuous wave remote sensing.
Bistatic A remote-sensing method, which is based on a spatial separation of emit-
ter and receiver is called a bistatic method. This often requires power supply in both
places. The opposite method is called monostatic.
Bragg condition Optimal backscatter is received, if the scattering objects in the
atmosphere have about half the size of the wavelength of the emitted optical, elec-
tromagnetic, or acoustic radiation. The Bragg condition that is essential for optimal
atmospheric remote sensing has been described for the first time in 1913 by the
British physicists William Henry Bragg (1862-1942) and William Lawrence Bragg
(1890-1971) shortly after their return from Adelaide, South Australia, where W.H.
Bragg was a professor at Adelaide University from 1885 until 1908, by referring
to the scattering of x-rays at crystals (Bragg and Bragg 1913 ). Backscatters from
different centres of action have a maximum positive interference, if this condition is
met.
Brightness temperature The brightness temperature is the temperature of a black
body (emissivity equals unity), which is in thermal equilibrium with its surround-
ings. For grey bodies (emissivity less than unity) the brightness temperature is lower
than the true temperature of the body.
Cabannes line The resulting intensity distribution of elastically scattered radia-
tion (see “Rayleigh scattering”) is called Cabannes line. It has a spectral width of
a few GHz or about 0.1 cm -1 . If detection is made with low spectral resolution,
the sum of the scattering at the Cabannes line and the Raman lines (see “Raman
scattering”) is called Rayleigh scattering. From the point of view of Raman scat-
tering, the Cabannes line is the central line between the lines from Stokes and
Anti-Stokes scattering (She 2001 , Wandinger 2005 ). The scattering process lead-
ing to this sharp central line is called Cabannes scattering in contrast to the more
general term Rayleigh scattering.
Chaff Chaff is artificial reflecting material dispersed in the air from aircraft in
order to enhance or (for military purposes) to disturb electromagnetic backscatter
from the atmosphere. Chaff can be, e.g. made from thin pieces of aluminium or
metalized glass fibres or plastic. Due to its low sedimentation rate, it may be used
as atmospheric tracer.
Coherent Coherent or heterodyne determination means that the received backscat-
tered signal is mixed with a signal from a second (local) oscillator before analysis
in the receiving instrument. The resultant beat frequency is much lower than the
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