Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 36

for an overview). If both scattering and absorbing processes are present, inversion
algorithms have to be used to differentiate between the two processes.
If emission and detection takes place at the same site, the technique is called
monostatic; if the detection is made at a different site than the emission, the tech-
nique is called bistatic. Monostatic remote sensing is relying on radiation that is
scattered at an angle of 180 ◦ , bistatic techniques are used if other scattering angles
are possible and maybe more favourable than the 180 ◦ angle.
Active remote sensing with pulsed signals usually allows for a range determina-
tion based on the travel time of the emitted signal.
Passive remote sensing is based on the detection of naturally emitted radiation
from the atmosphere or radiation from an external radiation source (often from the
sun), which has travelled through the atmosphere. The range determination usually
requires an inversion algorithm.

3.2 RADAR

RADAR (radio detection and ranging) is an active remote-sensing technique
that analyses the Rayleigh backscatter of electromagnetic waves at hydromete-
ors (Wexler and Swingle 1946 ). Occasionally, swarms of insects and birds lead
to considerable echoes, too. Bragg backscatter from clear air is not usable unless
strong moisture gradients - mainly in clouds are present (Knight and Miller 1998 ).
Therefore, the RADAR technique has been developed to measure the location and
intensity of precipitation (weather RADAR). If the Doppler shift of the backscat-
tered radiation is analysed, it can also be used for horizontal wind measurements
(Doppler-RADAR). RADAR is not a specific boundary layer observation tool. But
it can give valuable information and it is presented here because it is the oldest
ground-based remote-sensing technique. Recently, a special X band RADAR for
boundary layer research has been constructed (Al-Sakka et al. 2009 ).
Typically, eight wavelength bands are differentiated in RADAR meteorology (see
Table 3.2 ). L and P band and VHF RADAR devices are called windprofiler (see
Section 3.2.1 ). The K band is sometimes divided into a Ka and Ku band.
A RADAR consists of a parabolic antenna with an emitter and receiver in its focal
point. The antenna serves for the beam emission as well as for the signal detection.
Because the aperture of the antenna and the RADAR wavelength are nearly of equal
size, beam refraction is important and the RADAR beam cannot be focussed per-
fectly. During the emission side lobes are produced, leading to backscatter from the
ground (ground clutter) and from obstacles away from the focal line (fixed echoes),
which can disturb the measurements considerably. Known fixed echoes can be elim-
inated a posteriori. Strong electromagnetic radiation from a RADAR is dangerous
to animals and human beings because it heats up the cells and can damage them.
The ratio between emitted and received power is described by the RADAR
equation

r 2 c

τ A ε

2 Ze − 2 σ r

P R = P 0 κ + P bg , (3.1)

2