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Mapping without the sun
Zhang, Jixian

windows are close each other and have no overlap. Both
requirements of high coherence contrast and high spatial
resolution for image are satisfied in this work using 3><15
window for computing coherence.
4.2 Decorrelation Factors Analyzing
Since the signal itself consists of both correlated and de
correlated components. The degree of coherence y that is
calculated from a complex SAR image pair can be considered
as the product of different correlating factors as long as the
sources of correlation are statistically independent (Zebker,
Y Y systemSNR 7 baselineY registration Y temporal
In this work, the first three terms on the right hand side of Eq. 2
are factors that one would desire to minimize, so that the
measured coherence in an area is main corresponding to the
temporal correlation r,em p° ral caused by the ground surface
change. The influence of system noise on the interferometric
phase can be derived theoretically by determining the signal-to-
noise ratio of a specific system (Bamler, 1998). This factor will
contribute very little to the overall decorrelation when using
SAR data that are processed with a high-performance processor.
The influence of decorrelation from the baseline and the error
of co-registration are reduced by common spectral filtering and
fine co-registration of two images. Finally, the temporal
decorrelation is the main factor affecting the total coherence in
this study.
Total 16 interferograms are produced by the method of data
processing presented in the section 4.1. In these image pairs,
there are different season change pairs from 35 to 346 days,
different spatial perpendicular baseline pairs from 14.4 to
1035.8 meters.
5.1 Coherence Characteristics at Different Season Interval
for Different Surface
5.1.1 Coherence characteristics for different ground
objects: In the time interval of the two images acquired, the
change are taken place in scattering geometry, physical
property of scattering mechanism for different ground objects,
which leads to the coherence difference of ground surface. We
chooses 7 interferograms, which have relative short temporal
baseline, for comparing characteristics of coherence of different
ground surface, and the result is showed in Fig. 3.
Comparing the change for different ground surface, it can be
found that ground objects’ coherence reduces with the elevation
drop of ground objects position, which is bare rock, grassland,
pebbles and water body from high elevation to low. Bare rock is
stable scatter and change of physical property is slow with time,
so it has the highest coherence. The alpine aim and the alpine
meadow in Beiluhe area appear some conglomerate on ground
surface due to thawing and freezing circle, they have relative
high coherence because of the conglomerate’s stable character.
At same time, the aim and meadow vegetation is sparse and its
growth cycle is relative short, their coherence show relative
big change for different time interval. The pebbles are mainly
located in the stream valley, and always appear petty gains.
They show relative low coherence, because their pattern is easy
to change. The water body is easy to be influenced by wind and
its surface change very quickly, so it shows the lowest
coherence. In summary, there are widely various rang of
coherence for different ground surface in Beiluhe area, the bare
rock has the highest coherence and the water body has the
lowest, and the coherence reduces in turn according to their
elevation from high to low order.
Figure 3. Comparison of different ground objects’ coherence
with temporal baseline.
5.1.2 Influence of thawing and freezing in the region of
permafrost: The microwave backscatter signature of a
landscape is controlled by the landscape's structure and
dielectric properties. The interaction of an electric field with a
dielectric material has its origin in the response of charged
particles to the applied field. Liquid water exhibits a dielectric
constant that dominates the microwave response of natural
landscapes. As water freezes, the molecules become bound in a
crystalline lattice, and the dielectric constant decreases
substantially. For vegetated landscapes that undergo
thawing/freezing transitions, this drop in dielectric constant
results in a large backscatter shift. During this time period of
thawing/freezing cycle, temperatures of permafrost surface
ranged from warm to well below freezing, it change the ground
objects’ dielectric constants, where liquid water in the
permafrost froze, resulting in a change of ground objects’
The cycle of permafrost’s thawing and freezing are controlled
by soil, climatic and surface conditions, is related to the surface
energy and mass balance, which includes solar and long-wave
radiation exchange, evaporation, and sensible and latent heat
transfer. There are obvious differences in the thawing and
freezing time for different object. We compare coherence of 4
pairs of interferomety, which are made up of two images
acquired at the process of temperature falling in winter and the
process of thawing in summer. The result is showed in Fig. 4.
Comparing different profile change, it can be found that the
difference of alpine aim’s and alpine meadow’s coherence is
more obvious, the coherence of alpine aim is higher than of
alpine meadow in the pair A and C, is lower in the pair C and D.
The two images, composing pair A, are all acquired at the
process of temperature falling in winter, when the frozen
ground is stable. At that time, the coherence is mainly