The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
138 
Fig. 9 - Time-series of displacements of a sub-set of high- 
coherence points (p>0.99), according to Fig. 8. On the upper 
graphic, five points belonging to the dam top crest; on the lower, 
three points that are vertically widespread 
4.1 Data validation 
At the same time of GBInSAR measurements, the dam was 
under monitoring by a coordinatometer, whose results were 
compared to IBIS-L results and exhibited a good agreement, i.e. 
differences between measurements achieved by different 
instrumentations during 24 hours have been compared, resulting 
in discrepancies with a RMSE of ±0.2 mm (see Fig. 10). 
Fig. 10 - IBIS-L single pixel displacement (green circles) on 
the middle of the dam crest, compared to coordinatometer 
measurements along the entire period of observation 
5. CONCLUSIONS 
The application of an innovative, radar-based measuring system 
(IBIS-L by IDS, Italy) to static deformation of a large arch 
gravity dam has been described in the paper together with the 
system functioning principles. 
The test results highlight very good performance of this sensor 
coupled with the SAR technique in the measurement of slow 
movements, if compared with standard monitoring instruments. 
Achievements of this experimentation are very promising and 
open new perspectives for static analysis of large dam 
behaviour, and of other kinds of large constructions as well. 
Indeed, instruments and sensors which have been applied so far 
for measuring dam deformations feature an high accuracy and 
reliability, even though only a limited number of points could 
be observed. On the other hand, during this experimentation a 
very huge number of monitored points has been measured 
(some thousands) during a period of 37 hours. Also in case of 
the restricted sub-set S0.99 of only points featuring very high 
coherence, this account for more than 30 points which are 
widespread on the whole dam downstream face. In addition, no 
artificial reflectors are needed. The global behaviour of 
measured displacements and a quality check based on a 
comparison to readings from a coordinatometer have enhanced 
an accuracy of this GBInSAR system under ±1 mm. At the 
current state of the art, this cannot be achieved by other 
instruments, considering that mountain environments where 
dams are usually located might requirethe sensor positioning 
very far from the structure to be monitored (also some hundreds 
meters); obviously, this fact is a strong drawbacks for remote 
displacement sensors (e.g. for robotic total stations or automatic 
collimators), while IBIS-L can operate also from some 
thouthands meters. 
Moreover, the temporal data acquisition rate (9 minutes) and 
the degree of automation are excellent, if compared to other 
monitoring sensors. These characteristics make possible to use 
GBInSAR systems for continuos monitoring purpose, and to 
integrate them with other monitoring techniques 
A second important achievement of this test concerns the 
possibility by the end-users to adopt a commercial system, like 
IBIS-L is, for the current practise of dam monitoring activities. 
The presence of a simple GUI, the low number of parameters to 
set up and the standardization of SAR processing, allow its use 
also by non-skilled people in radar techniques. 
However, the experimental and theoretical research in this field 
needs further improvements. In particular, the precize 
localization of control points is to be developed. Furthermore, 
the integration between TLS and GBInSAR data is expected to 
open to further interesting applications, where the former 
system is able to detect lower frequency deformations with a 
higher point density, while the latter is capable to monitor 
higher frequency at a lower spatial resolution. 
AKNOWLEDGEMENTS 
Thanks go to A2A ATO-SIE (Grosio, Italy) for the availability 
of the dam of Cancano Lake and for the cooperation during 
experiments with the GBInSAR system. 
REFERENCES 
Alba, M., Fregonese, L., Prandi, F., Scaioni, M., and P. Valgoi, 
2006. Structural Monitoring of a Large Dam by Terrestrial 
Laser Scanning. IAPRSSIS, Vol. 36/5, pp. 6, on CDROM. 
Bemardini, G., De Pasquale, G., Bicci, A., Marra, M., Coppi, F., 
Ricci, P. and M. Pieraccini, 2007a. Microwave interferometer 
for ambient vibration measurements on civil engineering 
structures: 1. Principles of the radar technique and laboratory