The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Voi XXXVII. Part B7. Beijing 2008 
84 
. 4tt ZWD 
Ac? = 
A cosd inc 
(2) 
where A (p = the phase changes of two SAR images 
A = the RADAR wave length 
ZWD = the zenith wet delay 
COS 0 inc = incline angle of radar wave 
According to the experiments, 1 mm precipatable water vapour 
(PWV) may cause 6.2 mm ZWD delay. Equation (2) showed 
that water vapour can cause phase changes on interferograms. 
On the other side with special data processing, fringes on 
interferograms can also derive water vapour changes between 
two SAR image acquiring. 
3. INSAR AND GPS MONITORING TIANJIN AREA 
3.1 Interferogram of Tianjin area 
A differential interferogram is derived by tandem image (2 
Mar. 1996 and 3 Mar. 1996). Precise orbit is used to remove the 
flat earth effects, and SRTM3 DEM is used to remove the 
topographic effects. Displayed in Figure 1 are the interferogram 
and the coherence map. 
3.2 GPS continuous operation stations in Tianjin 
The location of the 13 GPS continuous operating stations, as 
well as Tianjin district boundary and SRTM-3 DEM are 
displayed in Figure 3. 
115*43- 116*06' 06*5*' t W4Z urCJOf Hr« 11735 117*54' 11*1* 1HT30* 
~Z 0 2 4 6 8 to 12 U 16 18 20 22 2* 
Figure 3. Map of Tianjin district boundary, SRTM-3 DEM and 
,. GPS station distribution 
According to the Tandem SAR image acquiring season and 
epoch, the zenith delay over each station is calculated, as is 
shown in-Figure 4. 
Figure 1. Tandem differential interferogram (left) and 
coherence (right) in Tianjin region 
Figure 4. GPS zenith tropospheric delay variation at Tianjin 
region in Feb 2007 
The phases are unwrapped with branch-cut methods, as are 
showed in Figure 2(A). From the unwrapped map, we can see 
that there is a colour ramp form the low left to the top right. 
Obviously this ramp is not caused by tropospheric delay but 
baseline errors. After the baseline errors are removed with 
curvature surface fitting method, the residual phases are 
decreased down to below 5mm (Figure2 B) 
The total tropospheric delay on 2 and 3 Mar 2007 at 13 stations 
are interpolated over the whole tandem image, which is shown 
in Figure 5. 
« 
f~* '** ' 1 
gP* /' ' 
j? •* 
• 
• ill ^ j 
«« 
f ' j. " 
A 
« 
n 
■M-r : -> ' ’ ' J 
Figure 5. Interpolation map of GPS total tropospheric delay at 
Tianjin region on 2 and 3 Mar 2007. 
Figure 2. Comparison of tropospheric delay along radar line of 
sight between before and after removing baseline errors with 
curvature surface fitting method 
The differential interpolated tropospheric delay between 2 and 3 
Mar 2007 is shown in Figure 6 (A),.on which a colour ramp 
exits from the low left to the top right, same to Figure 2 (A).