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process of temperature falling begins. In this process,
temperature falling very quickly. This process ends in the
next January.
(4) From the end of January, active layer’s temperature
begins rising. At the end of March, thawing at day
occasionally takes place near the surface of the ground. At
the beginning of April, freezing at night and thawing at
day take place every day
(4) Pebbles: the stream valley is wide and shallow, and there
are a lot of pebbles in Beiluhe area. The stream spreads
over the area, and there are many sand dunes at the bank.
(5) Lake: Some lakes locate at the southwest part of the study
area, and it appears blue in the composite winter image
because of lake ice.
3.2 Data Sets
3. THE STUDY AREA AND DATASETS
3.1 The Study Area in Beiluhe
The Beluhe area is located at the center of Qinghai-Tibetan
Plateau (34047'N, 9205l'E), is typical of permafrost area
according to the standard of classification of frozen ground
(Nan, 2002). The climate in this area is cold and dry. In winter,
the climate of this area is controlled by high-altitude westerly
airstreams, which are fine, dry and windiness. In summer, the
warm moist air from over the Indian Ocean permeates along
stream valley, and rain-fall is in plentiful in the area. Annual
mean temperature is -4.4°C, the mean temperature is-16.5°C in
January and the mean temperature is 7.5°Cin July over the area.
The highest temperature is 32.6°C, and the annual temperature
range is 24 °C. Precipitation mainly occurs in June, July and
August, and it usually takes place in the form of snow and hail
based on the meteorological data of the Tuotuohe
meteorological station near study area.
Figure 1. The optical composite image with DEM at Beluhe
experiment area in Qinghai-Tibetan Plateau.
The mean elevation is above 4,300 meters, but the relative
height of mountains is not very high, and slopes of the
mountains are smooth in the study area as shown in the CBERS
image (Fig. 1). The main types of ground objects in the study
area are listed as the following:
(1) Bare rock: there is a huge area of exposed rock at the top
of mountain, whose elevation is above 4,500 meters.
(2) Alpine aim: aim mainly appears in the hillside of the
mountains in the mid of the study area and alpine cushion
vegetation (i.e. androsace of cushion morphology) is
distributed on the aim.
Total 13 scenes ENVISAT ASAR imageries are acquired on the
dates between Jan. 8, 2004 and June 1, 2006 in this study. All
the imageries are descending pass, VV polarization SAR data in
SLC format. The temporal and spatial distribution of the 13
scene ASAR data are shown in Fig. 2, where the horizontal axis
stands for spatial perpendicular baseline relating to the data
acquired on Jan. 27, 2005, and the vertical axis indicate the date
of ASAR data acquired. .All ASAR data is acquired at the UTC
03:59, which is in the morning at local time.
Figure 2. The temporal and spatial distribution of 13 scenes
ASAR data acquired in study area
4. DATA PROCESSING
4.1 Coherence Image Processing
The coherence is one of the important characteristics for repeat
pass interferometry, which represents the local correlation of
the radar reflection characteristics of the surface target between
the two observations. It can be calculated by ensemble
averaging N neighboring pixels of complex SAR data as
(Bamler, 1998):
(v;)
hh
(l)
where, y = degree of coherence
( ) = ensemble averaging
/j, 1 2 = SAR data acquired in complex format
* = the conjugate of a complex.
(3) Alpine meadow: there is alpine meadow in the gentle
wide valley, where stipa purpurea is the constructive
specie. The vegetative cycle of stipa purpurea is relative
short, and generally stipa purpurea wilt in August every
year.
In practice, the estimation of coherence magnitude can be done
over a finite size window by coherently averaging the complex
values. A window of 3><15 single-look SAR pixels is selected to
compute coherence in the process of coherence computation. To
avoid the co-correlation of coherence in neighbour area,
