Glossary - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 162

Range gate Height interval to which a received signal will be attributed. The size
of the range gate depends on the signal propagation speed and pulse length (pulsed
measurements) or on the frequency resolution (continuous-wave measurements).

Rayleigh scattering Rayleigh scattering is a form of elastic scattering that occurs
if the size of the scattering object is much smaller than the wave length of the scat-
tered radiation. The intensity of Rayleigh scattering is proportional to

-4 . Rayleigh
scattering at atmospheric molecules is responsible for the blue colour of the clear
sky.

λ

Refraction Refraction can take place when a beam or pulse travels through a
medium with changing properties (i.e. changing refractive index) or when it passes
the edge of a solid structure ( “side lobes”). The refractive index of air for electro-
magnetic as well as acoustic waves changes with temperature and humidity. Gradual
changes in refractive index (e.g. due to vertical temperature gradients) can lead to
considerable bending of acoustic and electromagnetic beams in the atmosphere.
Sudden refractive index changes (e.g. at inversions or between different turbulence
elements) may lead to backscattering of part of the signal. See also “scintillation”

Remote sensing Most general, remote sensing is measuring from a distance. It
relies on receiving incoming radiation, be it acoustic, optical, or electromagnetic
signals. Distinction is made between active and passive remote sensing. Passive
remote sensing does not influence the state of the measured object. Active remote
sensing in some cases provokes specific changes of the measured object (see, e.g.
Raman scattering). The opposite technique to remote sensing is in situ measurement.

Scanning Scanning is remote sensing with turnable emitters and receivers. Several
scanning techniques can be distinguished. If the azimuth of the instrument is fixed
and only the elevation is changed, cross section in vertically oriented planes are
obtained. In RADAR meteorology this mode is called range-height-indicator (RHI).
If the elevation is fixed and only the azimuth is changed periodically, spatially
resolved information from the circumference of the instrument are obtained. In
RADAR meteorology this mode is used with low elevation angles and is called
plane-position-indicator (PPI). In wind LIDAR applications this mode is used with
larger elevation angles and is called conical scanning or VAD (velocity-azimuth-
display). Conical scanning is an alternative method to the Doppler beam swinging
method. Volume scanning requires the periodical change of both elevation and
azimuth.

Scattering When propagating radiation interacts with physical objects (this
includes gas molecules, aerosols, and liquid and solid water particles in the atmo-
sphere), or when the refractive index is changing along the path, part of the radiation
is scattered. Distinction is made between single scattering (the radiation inter-
acts with only one object) and multiple scattering (several scattering processes
take place). See “Mie scattering”, “Raman scattering”, “Rayleigh scattering” and
“Stokes scattering” for further details. Scattering at an angle of 180 ◦

is called

backscattering.