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2.2 Data processing 
2.2.1 Calculation ground point coordinates 
In this part we calculate the target point position from the 
external DEM in Cartesian coordinates. The external DEM data 
with latitude (p , longitude \j/ and height h are generally defined 
in geodetic coordinates. In our simulation, the external DEM 
data should be firstly converted to the corresponding (P x , P y , P z ) 
in the Earth Center Cartesian coordinates system. The 
transformation from geodetic coordinates to Cartesian 
coordinates is given as(Kun Ren et al., 2003): 
P x =(R + h) cos(<^) cos(y) 
< P y = (R + h) cos(<p) sin(y) (2) 
P z =[R(l-e 2 ) + h]sm(<p) 
where P x , P , P z are coordinates of ground point P in the 
Earth center Cartesian coordinate system; e = yj(a 2 +b 2 )/a 2 , 
are the numeric eccentricity; a are the semi-major axe of the 
Earth; b are the semi-minor axe of the Earth; 
R = a/y]1 — a 2 sin 2 (<£>) are local Earth’s radius of curvature. 
2.2.2 Interpolation satellite positions 
In this part we use the DEOS fortran program getorb for 
obtaining the precise orbits. Firstly we obtain the precise 
satellite orbits data from Delft Institute of Earth Oriented Space 
Research. Then we download the precise orbits from the DEOS 
ftp (ftp://dutlru2.lr.tudelft.nl/pub/orbits/), which includes the 
Orbital Data Records (ODR) and the list of ODR files arclist. 
The Orbital Data Records (ODR) are binary files containing the 
orbital positions of a satellite as a function of time. The position 
of the satellite's nominal centre-of-mass is given in the Earth 
Centred Fixed coordinate system. The accuracy of the precise 
orbits data provided by Delft is less than 8cm in radial direction. 
Secondly we call the program getorb to interpolate the orbits 
data. We know the satellite positions is a function of azimuth 
time, namely rows number. The time of first state vector and 
azimuth line time can be obtained from the head file of the SLC 
data. So we let the time of first state vector to be the reference 
time origin and the azimuth line time to be the interval time 
between the rows. 
Then the satellite position corresponding to the external DEM 
in the Earth center Cartesian coordinates system can be 
obtained. From the known positions of a target and the satellites 
in the Cartesian coordinates system, the distance of the target to 
each of the two satellites can be derived. Thus, the 
interferogram can be simulated. The processing diagram is 
shown in Fig. 2. 
Fig. 2 the simulation of interferogram processing diagram 
3. DATA AND RESULTS 
In this study case, all the SLC data presented were acquired by 
the ERS-1/2 satellite system. The important ERS operating 
parameters and the external DEM data parameters for our 
simulation are listed in Table 1. The interferogram is simulated 
according to the processing diagram shown in Fige.2. The 
geolocation errors are about in a resolution cell of ERS-1/2 
image, namely 3.9m in azimuth direction and 7.9m in range 
direction. Here the space of pixels is 3.0 arc-second namely 
about 90 meter in ground. The difference of the elevation can 
change arbitrarily. The external DEM is shown in Fig. 3. 
Parameter 
Value