The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Voi. XXXVII. Part Bl. Beijing 2008 
3.2 SAR technique 
Synthetic Aperture Radar technique allows radars with wide 
physical antenna beam width to obtain high angular resolution 
in azimuth thanks to the acquisition of the scenario from 
different view angles: the coherent composition of the 
acquisitions permits to obtain a synthetic antenna whose beam 
width is inverse proportional to the relative movement between 
the scenario and the radar (Ferretti et al., 2007). 
For strip-map radar as IBIS-L the obtained angular resolution is 
given by: 
reflected by the object in different time instants. 
The displacement of the investigated object is determined from 
the phase shift measured by the radar sensor at the discrete 
acquisition times. The Line-of-Sight (LoS) displacement d LOS 
(i.e. the displacement along the direction of wave propagation) 
and the phase shift zl^are related by the following: 
dios oc A cp 
Ak 
(4) 
(3) 
where k is the wavelength of the E.M. signal and L is the length 
of the relative movement of the radar head w.r.t. the target. In 
case of IBIS-L, the movement of the sensor module on the 2 m 
linear scanner permits the system to have an angular resolution 
of 4.5 mrad. 
The combination of the SF-CW technique with the SAR 
technique leads to the radar image being organized into pixels 
with dimensions of: 0.5 m in range and 4.5-10' 3 r m in cross 
range, where r is the distance from radar to target. 
The following figure 4 shows an example of a resolution grid 
for a system with a distance resolution of 5 m and angular 
resolution of 5.2 mrad. A look to this figure enable to quickly 
understand that the shape of the object under monitoring is 
fundamental to allow the discrimination between specific 
observed points. Considering for instance the range and cross 
range directions in the horizontal plane, if the object features a 
vertical surface, the number of observed scatterers will be very 
small. On the contrary, if the object presents a tilted surface, the 
number of scatter points would sharply increase. 
The first applications using the interferometric technique were 
performed by satellite to detect the displacement of large areas 
of terrain, with a resolution at ground level of a few meters; 
now the same technique can be used with radars installed on the 
ground, permitting the illumination of specific areas with a very 
high range resolution. 
It is worth underlining that the interferometric technique 
provides a measurement of the LoS displacement of all the 
pixels of the structure illuminated by the antenna beam; once 
d LoS has been evaluated, the vertical and horizontal 
displacements (or their projection on another tilted 3-D plane) 
can be easily found by making some geometric considerations 
(see the example reported in figure 5). 
Monitored Area 
Nd t 
J LOS 
Fig. 4: Example of a spatial resolution grid 
3.3 Interferometric technique 
Once the 2-D map of a structure has been determined at 
uniform sampling intervals, the displacement response of each 
pixel is evaluated by using the Interferometic technique. 
Interferometry is a powerful technique that allows the 
displacement of a scattering object to be evaluated by 
comparing the phase information of the electromagnetic waves 
Fig. 5. Line-of-Sight displacement vs projected displacement 
4. APPLICATION TO A FULL SCALE DAM: 
SUMMARY OF RESULTS 
An arch-gravity dam was investigated by using IBIS-L system 
to measure deformations due to the filling of the water basin as 
well as to variation of the structure temperature. The Cancano 
dam (Alta Valtellina, Italy) features about 90 m height from 
bottom to top, and 350 m length on the crest, and it is managed 
by A2A company (see Fig. 6). The dam is currently controlled 
by topographic techniques, that consist in two high precision 
leveling lines on the top crest and on the middle height corridor 
of the structure, in optical collimators to detect horizontal 
displacements, and in a column of coordinatometers positioned 
in the middle vertical section. In addition, a precise DSM of the 
structure itself was achieved by TLS. The availability of all 
these measurements allows to perform a GBInSAR data 
validation. 
Data acquisition by IBIS-L has been performed at a mean 
distance of 400 m far from the dam mid-point, during a period 
lasting 37 hours. The IBIS-L system has been setup on a 
136 
mmsm