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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
temporary stand-points, because the replacement of the 
instrument hasn’t been foreseen for this test. As shown in figure 
6, the radar system has been placed just in front of the dam’s 
downstream face. This positioning has allowed to capture the 
entire dam displacement along in the range direction during the 
whole observation time. The time needed to scan the whole 
downstream has been 9 minutes, and each scanning cycle has 
been repeated without intermediate breaks. It is relevant to 
notice that the configuration of parameters needed to correctly 
perform the data acquisition is very easy, because this task can 
be performed by also non-skilled operators. During the data 
acquisition, the control software allows one to check the 
focalisation map, which is an information of the reflectivity 
properties of the object illuminated by the SAR sensor. In 
addition, a check on intermediate displacements of points 
tracked on the structure can be seen. This option is very 
important for continuous monitoring applications. 
* 
Fig. 6 - IBIS-L positioned in front of the downstream face of 
the Cancano dam (Alta Valtellina, Italy) 
The results of GBInSAR measurements are 2-D deformation 
maps of displacements, which reached a maximum value of 
about 4 mm in the central section. Consider that, in years with 
the biggest variation of water level in the basin, the full 
displacement of this point might reach about 80 mm. In figure 7, 
maps corresponding to three different epochs (after 10, 22 and 
37 h) during the test at Cancano dam are shown. 
According to an integer ambiguity of 4.5 mm (Acp= 1 rad), such 
a displacement can be automatically tracked if IBIS-L is 
continuosly acquiring data, and not more that 1 cycle is 
increased from an observation epoch and another (in this 
application these are separated by a 9 minute rate). In case of 
repositioning of IBIS-L, the processing tool is not able to detect 
displacements larger than the integer ambiguity. A solution, to 
this problem could be provided by integrating the GBInSAR 
system to other ranging sensors featuring lower precision and 
measurement rate, but that would be able to reconstruct low 
frequency displacements (e.g. TLS or robotic total stations). 
By thresholding points on the basis of their coherence (p) and 
their Signal-to-Noise Ratio (SNR), it is possible to select those 
which are high accurate control points. The higher are the 
selected thresholds, the higher is the accuracy in the 
measurement of the displacement of the selected point. In 
figure 8, a sub-set S0.99 of only points with p>0.99 is shown. For 
each of these the time-series of displacements is fully available. 
For 5 points on the dam crest, the displacements are shown in 
the upper part of figure 9, while in the lower part displacements 
of points which are vertically widespread are shown. As you 
can see, the trend of the observed dam displacements agrees to 
the foreseen static behavior in both cases. Note the magnitude 
of maxima displacements along the observation period is under 
2 mm. Some outliers are present in time-series corresponding to 
points P34 and P30, which probably are placed on positions 
disturbed by local turbulence or they might be located on a 
surface featuring bad reflectivity, despite of the high coherence. 
On the other hand, these outliers does not result in trend errors, 
and can be removed by low-pass filtering in the time domain, or 
by averaging displacements evaluated on close points in the 
space domain. Indeed, by comparing displacements of points 
P30 and P29, whose position differ for only 15 m, it is possible 
to see that only the first of them is affected by anomalous noise 
(see Fig. 9). 
Fig. 7 - Complete displacement maps after 10, 22 and 37 
hours of observation 
Fig. 8 - Location of tracked points on the dam downstream face 
with a coherence p>0.99