IEEE Geoscience and Remote Sensing Magazine - March 2013 - 20

Table 6. POSSIBLE RADAR OBSERVABLES ESTIMATED FROM
THE SCATTERING MECHANISMS MODEL-BASED DECOMPOSITION AND THEIR RELATION TO THE ENVIRONMENTAL
PARAMETERS/APPLICATION EXAMPLES, SEE (22) [45], [46].
Scattering Mechanisms
Model-Based Decomposition

Application Examples

Surface scattering

Segmentation and classification
Soil moisture estimation (under
vegetation cover) of the surface

Dihedral scattering

Segmentation and classification
Soil moisture estimation (under
vegetation cover) of the surface
and trunk component

Volume scattering

Segmentation and classification
Estimation of the volume
component (random vs
oriented volumes)

displaced from that of the first antenna. As illustrated in
Fig. 11, the antenna displacement causes for each scatterer
of the imaged scene a characteristic range difference that
can be evaluated to retrieve the surface height.
From Fig. 11 it becomes apparent that for large distances
r0 and short baselines B = the one-way range difference
Tr will be proportional to the height difference Th. This
proportionality can be expressed as
Tr ,

where r0 is the slant range, i i is the local incident angle, and
B = is the baseline perpendicular to the line of sight. In a
coherent radar, this range difference Tr corresponds to a
measured phase difference T{
T{ = m 2r Tr, (28)

from all elemental scatterers that are contained within a
circular ring or torus centered on the platform path. The
radius of the ring is given by the range and its length and
width are provided by the azimuth and range resolutions,
respectively. As a result, a single SAR image does not contain information about the elevation angle and therefore
the height of the imaged scene.
The objective of across-track interferometry is to overcome this limitation and to resolve the elevation angle with
high precision. For this, the scene is mapped with a second
antenna that moves along a flight path which is laterally

where m is the carrier wavelength. The factor m takes into
account whether the range difference is only due to the
receive path or due to both the transmit and the receive
paths. Therefore, m is equal to one for a single-pass SAR
interferometer where only one antenna transmits and two
antennas receive the scattered echoes, and m is equal to two
for a repeat-pass or ping-pong scenario where each radar
image is acquired with its own illuminator.
Figure 12 shows as an example the phase differences
obtained for a pair of SAR images that were acquired
by the spaceborne single-pass SAR interferometer

0
[Vol. %]

0
[Vol. %]
Land Use

19. April

B=
$ Th, (27)
r0 sin ^i ih

7. June

0
[Vol. %]
5. July

FIGURE 10. Soil moisture maps obtained after applying a model-based polarimetric decomposition to remove the vegetation layer and

inverting the remaining ground component. The underlying soil moisture estimation accuracy was on the order of 4 vol% and 11 vol% [50].
20

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march 2013

Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - March 2013