The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
266 
document of RSI Co. Thus, the pixel location using R-D model 
is effective. 
4. CONCLUSIONS 
Error models in pixel positioning base on R-D model have 
been derived for the effect of orbit determination error, earth 
model error, Doppler center frequency error and atmospheric 
propagation delay error. By the computing and analysis in 
three test sites, the following results can be concluded 
1. The earth model error can introduce remarkable error, 
especially in high elevation area as Tibet Zhang 
autonomous region, thus the exact earth model must be 
used. 
2. In most cases, the zero-Doppler condition can not be 
satisfied, thus its true-value must be computed. 
3. Once the exact earth model and Doppler center 
frequency true-value have been used, the uncertainty in 
the orbit determination becomes the dominant factor. If 
the precise orbit were not adopted, the maximum error of 
three groups of test is 704 meters which is still permitted 
in the technical document of RSI Co.. When the precise 
orbit is used, the maximum error can be improved to 19m. 
Thus, the pixel location using R-D model is effective. 
4. Compared with the orbit error, the atmospheric delay 
error is smaller, but it also needs to be considered in low 
latitude areas. 
5. The positioning errors are nonlinear over the globe. 
They decrease gradually with decreasing latitude as well 
as increasing longitude. 
From the equations (1)~(3), we can see that the R-D model can 
also be used to improve the SAR satellite ephemeris data when 
accurate reference points, such as radar comer reflectors, exist. 
Accurate satellite ephemeris data are very important to the 
many applications of SAR images, such as differential 
interferometric synthetic aperture radar (DInSAR) technique. 
Analying on this will be discussed in the future. 
Acknowledgements 
The work in the paper was supported by Nature Science 
Foundation, China (No. 50579013). The authors wish to thank 
the European Space Agency for providing ERS-2 SAR data. 
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