as master) are shown in Figure 18(c) and (d). The panels convey that a regular component characterized by periodic fluctuations can be identified in the residual's behavior. By simply looking at both the residual and humidity curves, a close relationship between these two data sets appears evident. As shown in Figure 19, the calibration effects on humidity were a magnification of its fluctuations. As expected, such a model refinement brought visible improvements [see the residuals in Figure 16(b)] on middle-range targets but was nonetheless capable of improving near- and far-range targets as well, suggesting that the atmosphere 14 7 0 -7 -14 7 0 -7 -14 8 4 0 -4 -8 2 7 8 ) 3 4 5 6 Day Number (b) (m ) (m 1 e e 800 600 400 200 ng ng 7 8 4 5 6 1 2 3 Day Number (a) IBIS-L Data Ra Ra 800 600 400 200 14 Displacement (mm) Data Compensated Without Calibration Displacement (mm) Displacement (mm) IBIS-L Data 7 8 4 5 6 1 2 3 Day Number (c) 4 0 -4 800 600 400 200 7 8 ) 3 4 5 6 Day Number (d) (m 2 e 1 ng -8 8 8 4 0 -4 -8 Displacement (mm) Compensated Without Calibration Ra Displacement (mm) 8 Displacement (mm) Data Compensated With Calibration 4 0 -4 -8 7 8 4 5 6 3 2 1 Day Number (e) Compensated With Calibration 1 2 3 4 5 6 Day Number (f) 7 8 FIGURE 18. The Bolzano campaign. (a) The 3D surfaces and (b) the planar projections are shown for the interferometric phases measured by the IBIS-L and for the residual phases after the compensation process performed (c) and (d) without and (e) and (f) with the calibration procedure [78]. (β ) 10 0.5 0.4 0 0.3 0.2 -10 0.1 -20 0.8 1 (α) (a) 1.2 Humidity Difference 2 Relative Humidity (%) Calibrated Results on a 570-m GCP 20 0.6 0 -2 -4 -6 15 June 16 June 17 June 18 June 19 June 21 June 22 June 15 June 2008 2008 2008 2008 2008 2008 2008 2008 (b) FIGURE 19. The results of the calibration procedure performed on a ground control point (GCP) at a 570-m range. (a) The figure of merit (target coherence) in the (a, b) domain and (b) the difference between the measured humidity and the one calibrated according to the a, b best candidates [78]. MARCH 2020 IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE 57