Active Microwave Systems for Monitoring Drought Stress - Monitoring and Predicting Agricultural Drought: A Global Study - page 108

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polarization of the transmitting antenna is the same as for reception of
the signal, one speaks of like-polarization (HH or VV), and otherwise of
cross-polarization (HV or VH). Until the launch of ENVISAT, all satellite
systems used linear combinations in either the horizontal (HH in the case
of RadarSat and JERS-1) or vertical directions (VV in the case of ERS-1
and ERS-2).
Crop structure has a strong effect on the polarization of the backscat-
tered waves. Generally, if the target has a strong vertical structure (e.g.,
grain crops), scattering is dominated by the returns from the underlying
ground surface. Modified by vegetation density and radar frequency, HH
polarization tends to interact to a lesser extent with the crop than VV and
penetrates more effectively to the underlying soil. Cross-polarized signals
(HV or VH) depend on multiple reflections between the canopy and the
soil to depolarize the transmitted signal. Experimental and modeling stud-
ies suggest that the ability to estimate soil and vegetation parameters will
be enhanced significantly with the launch of fully-polarized SAR systems
such as RadarSat-2 and ALOS (Bindlish and Barros, 2000).

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Frequency The effect of radar frequency on backscatter is largely a func-
tion of the size and dielectric properties of the target. The dielectric con-
stant of water, which constitutes 80-90% of plant matter and is present
in the soil, differs markedly depending on radar frequency. As frequency
decreases and wavelength increases, the size of the target components be-
come smaller relative to the sensor, and the scattering surfaces appear
“smoother,” making scattering by vegetation elements less efficient.
In the presence of dense vegetation, at high frequencies or smaller wave-
lengths, radar backscatter is primarily a function of canopy scattering, and
the soil has a minor impact. At lower frequencies or longer wavelengths,
the impact of soil becomes greater as the radar signal penetrates farther
into the canopy. It is usually believed that radars operating in X or K band
(table 8.1) are not sensitive to soil moisture when crops are fully estab-
lished, whereas L- and P-band radars penetrate crop and grass canopies
even at the height of the growing season.
As a result of differential backscatter from cotton and alfalfa in Ku and
C band, Moran et al. (1998) suggested that dual frequency would be useful
in estimating vegetation status and soil moisture. Unfortunately, no space-
borne SAR system has yet been approved that would allow instantaneous
measurements at two different frequencies.

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Incident Angle The incident angle is defined as the angle between the inci-
dent radar beam and the vertical to the intercepting surface. In early studies
using single-channel C-band radar, steeper incident angles (15-25°) were
generally found to be more useful in estimating soil moisture in presence or
absence of vegetation. In absence of vegetation and at steeper angles, the
radar backscatter is affected to a lesser extent by surface roughness, but
in presence of vegetation, the path length through and attenuation by the