Glossary - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 160

photograph. This is often done by scanning in azimuth and elevation or by a system
of optical or electromagnetic lenses.
Incoherent Direct or incoherent determination means that the frequency of
received backscattered light is analyzed directly with a high-resolution spectrom-
eter or with a interferometer. This is one of two principal techniques used to analyse
the Doppler shift of backscattered light. It is constructionally simple, but it requires
the use of narrow-band optical filters through which the radiation must pass. See
also “coherent”.
Inelastic scattering Raman scattering is inelastic scattering. The wavelength of
the backscattered signal is different from the wavelength of the incoming signal.
See also “elastic scattering”.
In-situ measurements Measurements which record the state of the analyzed
object in the direct vicinity of the instrument are called in situ measurements.
Usually, a direct contact between the measured object and the sensor of the instru-
ments occurs. In situ measurements may be influenced by a change in the state of
the measured object due to the presence of the instrument. The opposite method is
remote sensing.
Interferometer An interferometer records an interferogram by mixing two beams
which had different travel distances. It often needs laborious techniques such as
Fast Fourier Transforms to derive ordinary wavelength or frequency spectra from
interferograms. The advantage is the much higher signal intensity of the received
information. Grates or prisms in normal spectrometers allow only for much less
signal intensities.
Mie scattering Mie scattering is a form of elastic scattering that occurs if the
size of the scattering particle (molecule, aerosol, droplet, insect) has about the same
wave length as the scattered radiation. Ground-based active remote sensing is usu-
ally relying on Mie scattering, because obeying the Bragg condition means to have
comparable dimensions of the wavelength of the emitted radiation and the scattering
object. Light scattering in mist or fog is usually Mie scattering.
Monostatic A remote-sensing method, which is based on having emitter and
receiver in the same place is called a monostatic method. This method is simpler
than the bistatic method, because power supply is required in one place only. The
backscattering of the emitted signal to the receiver may either be performed by
natural scattering processes or by special mirrors mounted in some distance. The
opposite method is called bistatic.
Passive remote sensing Remote-sensing methods based on the receiving of nat-
urally emitted radiation are called passive remote-sensing methods. The opposite
method is active remote sensing.
Path-averaging Path-averaging remote sensing delivers a mean signal, which is
representative for the whole path length over which the signal has travelled. No
range resolution is possible. Usually, a continuous signal is analyzed. There are