30
H. Rott
Fig. 2.12. Geometry of the principle radar scattering mechanisms for bare surface
and vegetated areas. The individual backscatter contributions are described in the
text
where Bi is the incidence angle of the radar beam. Figure 2.13 shows examples of angular backscatter functions. The bare soil data (curves 1 and 2)
clearly demonstrate the decrease of the angular dependence of the backscattering coefficient 1 , a O , with increasing roughness, in both cases for wet soil.
For dense vegetation (curve 3) the signal contribution from the soil below the
canopy is small at C-band and higher frequencies, the diffuse backscattering
of the vegetation canopy shows comparatively small angular variations. This
is different for the wet snowpack (curve 4) with smooth surface and high
dielectric losses, for which a O is high only at and near vertical incidence due
to specular reflection.
In addition to roughness, radar measurements are sensitive to soil moisture, because the dielectric constant, and consequently the reflectivity, depend on the soil water content. If the soil is covered by vegetation, the capability to derive soil moisture depends on the transmissivity of the canopy
and decreases towards higher frequencies. L-band shows the best capability
for sensing soil moisture below a vegetation canopy. However, also in this
frequency band the structure and water content of the vegetation influence
the scattering properties and consequently affect the analysis of soil moisture
(Dubois et al., 1995).
2.5 Sensor Principles
Categories of spaceborne instruments for earth observation are listed in Table 2.2. For surface observations mainly imaging sensors are used, but also
non-imaging sensors such as altimeters, measuring along the nadir-track. For
1 a O is commonly expressed in decibels (dB), which is 10 times the logarithm to
the base 10 of the ratio of two quantities (in case of (J"0 the ratio of reflected to
incident power).
H. Rott
Fig. 2.12. Geometry of the principle radar scattering mechanisms for bare surface
and vegetated areas. The individual backscatter contributions are described in the
text
where Bi is the incidence angle of the radar beam. Figure 2.13 shows examples of angular backscatter functions. The bare soil data (curves 1 and 2)
clearly demonstrate the decrease of the angular dependence of the backscattering coefficient 1 , a O , with increasing roughness, in both cases for wet soil.
For dense vegetation (curve 3) the signal contribution from the soil below the
canopy is small at C-band and higher frequencies, the diffuse backscattering
of the vegetation canopy shows comparatively small angular variations. This
is different for the wet snowpack (curve 4) with smooth surface and high
dielectric losses, for which a O is high only at and near vertical incidence due
to specular reflection.
In addition to roughness, radar measurements are sensitive to soil moisture, because the dielectric constant, and consequently the reflectivity, depend on the soil water content. If the soil is covered by vegetation, the capability to derive soil moisture depends on the transmissivity of the canopy
and decreases towards higher frequencies. L-band shows the best capability
for sensing soil moisture below a vegetation canopy. However, also in this
frequency band the structure and water content of the vegetation influence
the scattering properties and consequently affect the analysis of soil moisture
(Dubois et al., 1995).
2.5 Sensor Principles
Categories of spaceborne instruments for earth observation are listed in Table 2.2. For surface observations mainly imaging sensors are used, but also
non-imaging sensors such as altimeters, measuring along the nadir-track. For
1 a O is commonly expressed in decibels (dB), which is 10 times the logarithm to
the base 10 of the ratio of two quantities (in case of (J"0 the ratio of reflected to
incident power).