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, 
1994): 
Y Y systemSNR 7 baselineY registration Y temporal 
(2) 
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. 
5. CHARACTERISTICS OF COHERENCE 
ANALYZING 
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. 
Interferogram 
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’ 
coherence 
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