2 Physical Principles and Technical Aspects of Remote Sensing
29
>. 1.0
-
.s;
'(j)
(/J
0.9
'E
w
~-0.8
....
....
....
....
N'
0.7
0.6
0.5
4
10
40
100
Frequency [GHz]
Fig. 2.11. Spectral emissivities at an incidence angle of 50
0
at horizontal polarization (broken lines) and vertical polarization (full lines) for (1) grass, (2) bare soil,
(3) dry alpine snow, and (4) a smooth water surface
frequency and polarization, as well as polarimetric radars are in use. Polarimetric radars transmit and receive signals at various polarizations and
monitor also the phase relationships between these channels. For specifying
the polarization state of a backscatter signal, the first index refers to the receive, the second to the transmit polarization (e.g. HV: horizontally polarized
received, vertically polarized transmitted).
Figure 2.12 illustrates the main backscattering mechanisms for a radar
beam under oblique incidence: 1) Backscatter from a rough surface; a significant part of the energy is reflected incoherently in all directions, therefore the
backscatter signal is comparatively high. 2) Backscatter from a smooth surface; the coherent (specular) contribution dominates, only a very small part
is scattered back to the illuminating antenna. 3) Double bounce reflection;
the beam is reflected at the soil surface and at vertical structures (stalks,
trunks), resulting in pronounced phase shifts between H Hand VV polarized beams. 4) Direct backscatter from a volume scattering medium (e.g. a
vegetation canopy or a snowpack) . 5) Indirect scattering contribution due to
forward scattering at the soil surface and diffuse scattering in the vegetation,
or the reverse process.
Surface roughness is an important parameter for backscattering. A criterion to classify the roughness of a surface has to consider the relation between
the standard deviation of surface height, hs, and the radar wavelength, >.. The
Fraunhofer criterion states that a surface is smooth if hs < >./(32coSOi),
29
>. 1.0
-
.s;
'(j)
(/J
0.9
'E
w
~-0.8
....
....
....
....
N'
0.7
0.6
0.5
4
10
40
100
Frequency [GHz]
Fig. 2.11. Spectral emissivities at an incidence angle of 50
0
at horizontal polarization (broken lines) and vertical polarization (full lines) for (1) grass, (2) bare soil,
(3) dry alpine snow, and (4) a smooth water surface
frequency and polarization, as well as polarimetric radars are in use. Polarimetric radars transmit and receive signals at various polarizations and
monitor also the phase relationships between these channels. For specifying
the polarization state of a backscatter signal, the first index refers to the receive, the second to the transmit polarization (e.g. HV: horizontally polarized
received, vertically polarized transmitted).
Figure 2.12 illustrates the main backscattering mechanisms for a radar
beam under oblique incidence: 1) Backscatter from a rough surface; a significant part of the energy is reflected incoherently in all directions, therefore the
backscatter signal is comparatively high. 2) Backscatter from a smooth surface; the coherent (specular) contribution dominates, only a very small part
is scattered back to the illuminating antenna. 3) Double bounce reflection;
the beam is reflected at the soil surface and at vertical structures (stalks,
trunks), resulting in pronounced phase shifts between H Hand VV polarized beams. 4) Direct backscatter from a volume scattering medium (e.g. a
vegetation canopy or a snowpack) . 5) Indirect scattering contribution due to
forward scattering at the soil surface and diffuse scattering in the vegetation,
or the reverse process.
Surface roughness is an important parameter for backscattering. A criterion to classify the roughness of a surface has to consider the relation between
the standard deviation of surface height, hs, and the radar wavelength, >.. The
Fraunhofer criterion states that a surface is smooth if hs < >./(32coSOi),