Further, we find that the mean values of over SA1, SA2, and SA4 are of the same order as the corresponding NESZ. Therefore, it is reasonable to assume that measurement error induces an equivalent effect on SA1 and SA2, so if the observed phase difference is caused by the measurement error, PDFs over SA1 and SA2 in Fig. 4 are expected to have a similar shape. However, the experimental results demonstrate that the various phase differences cannot be attributed to measurement errors. In addition, the PDFs are estimated within an area of , which is large enough to neglect the effect of statistical fluctuations. Thus, the effects of speckle noise need to be taken into consideration. The real part of is shown in Fig. 5 and for every case, the scatter diagrams illustrate the behavior of , the multiplicative term and the additive term, respectively. The scatter diagrams are plotted by employing pixel nonoverlapping windows. It should be noted that the imaginary part is not displayed here because this term presents a similar behavior as . In combination with the conclusions reached in Sec. 2 (Fig. 2), the mean value versus standard deviation for multiplicative noise is approximately an equality relation. From Fig. 5, we can see that the dominant noise over SA1 is multiplicative noise, which only introduces noise in the amplitude. In this case, the speckle noise has no effect on the individual phases of HV and VH channels; therefore, the distributions remain centered at 0 deg, which is in agreement with the theoretical expectation as shown in Fig. 4. There is no clear relation between mean value of additive noise and its standard deviation as in Fig. 2, where the only feature is that the mean value is about 0 deg. According to this pattern, it is clear in Fig. 5 that the dominant noise type over areas SA2 and SA4 is additive noise which induces noise both in amplitude and phase. This introduced additive noise results in the distribution for SA2 and SA4 being significantly deviating from the 0 degree as shown in Fig. 4. From this experiment, we could conclude that the reciprocity theorem does not hold in the case of strong additive noise.