PSI - Issue 64

Donato Fiore et al. / Procedia Structural Integrity 64 (2024) 740–748 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Figure 2 - Idealization of radar image profile (range profile)

The radar continuously scans the bandwidth at a rate ranging up to 100 Hz, so that the corresponding sweep time ∆t of 0.01 s is in principle well suitable to provide a good waveform definition of the displacement response for a civil engineering structure. This range profile gives a one-dimensional map of targets in the viewable space in function of their relative distance from the equipment. The concept of range profile is better illustrated in Fig. 2, showing an ideal range profile obtained when the radar transmitting beam illuminates a series of targets at different distances and different angles from the system. Thus, has to be observed that, as shown in Fig. 2, the lBlS-S system has only range resolution, i.e. different targets can be individually detected if they are placed at different distances from the radar. Hence, measurement errors may arise from the multiplicity of contributions to the same range bin, coming from different points placed at the same distance from the radar. Once the images of the scenario illuminated by the radar have been determined at sampling intervals ∆t, the displacement of each target is evaluated using the differential interferometry technique (see e.g. Pieraccini et al. (2004)), based on the comparison of the phase information of the back- scattered electromagnetic waves collected in different times. Since two images acquired at different times exhibit phase differences, depending on the motion of the scatterers along the direction of wave propagation, the radial displacement ∆ r (i.e. the displacement along the radar Line Of Sight - LOS) is simply computed from the phase shift ∆Ɵ: = − 4 ∆Ɵ (3) where λ is the wavelength of the elec tromagnetic signal.

Figure 3 - Radial displacement vs. actual (projected) displacement