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