Active Microwave Systems for Monitoring Drought Stress - Monitoring and Predicting Agricultural Drought: A Global Study - page 109

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Table 8.1 Radar frequencies and corresponding wavelengths

Band Frequency (GHz) Wavelength (cm)

Ka 27-40 0.8-1.1

K 18-27 1.1-1.7

Ku 12-18 1.7-2.4

X 8-12.5 2.4-3.8

C 4-8 3.8-7.5

L 1-2 15-30

P 0.3-1 30-100

vegetation decrease. Conversely, at shallow angles (40-45°) the path length
through the canopy increases, and so does the vegetation response. Con-
sequently, backscatter measurements at different incident angles provide
useful information about the relative contributions of soil and vegetation to
total backscatter. Although SAR sensors have only one antenna that looks
at the earth's surface from one direction, the scatterometers onboard ERS-
1/2 and METOP have three and six antennas, respectively, that acquire
instantaneous measurements at different incidence angles. This multiangle
capability of scatterometers enables soil moisture and vegetation effects to
be separated quite efficiently (Wagner et al., 1999a).

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PgEn

Ta rget Parameters

The important target characteristics influencing radar backscatter are ge-
ometry and dielectric properties. The dielectric constant, which is a mea-
sure of the relative effectiveness of a substance as an electrical insulator,
ranges from as high as 80 for liquid water at low microwave frequencies to
< 4 for dry matter. Thus, in an agricultural environment, radar backscatter
is a function of soil surface roughness and moisture content and, where
vegetation exists, of crop type, phenological state, and vegetation water
content.

[109

Soil Radar measurements of bare soil surfaces are very sensitive to the
water content in the soil surface layer due to the pronounced increase
in the soil dielectric constant with increasing water content. For longer
wavelengths, the backscatter coefficient may increase up to about 10-fold
from dry to wet soil conditions. Although there is a very strong relationship
between the backscatter measurements and the soil moisture content, the
retrieval of soil moisture is a challenging task due to the confounding
influence of surface roughness.
There are several empirical and theoretical models that describe back-
scatter from bare soil in terms of the soil moisture and surface rough-
ness (Fung, 1994; Dubois et al., 1995). Unfortunately, many studies found
relatively poor agreement between modeled and experimentally observed
radar responses. It has been suggested that the inadequate representation