# Independent System Calibration of Sentinel-1B
## 8 Polarimetric Characterization
### Channel Imbalance  
The amplitude imbalance is defined as the ratio of the signal power of the cross- and co-polarized channels. Figure 25 depicts the channel imbalance versus look angle for the different modes; filled diamonds indicate H-polarization on transmit (HV-HH); open diamonds V-polarization on transmit (VH-VV). The detected amplitude imbalance varies between \(-1.1\) dB and \(0.7\) dB; the highest deviations are indicated for steep look angles below \(23\)\(deg\). While the amplitude imbalance for H on transmit (full diamonds) is about \(0\) dB on average with a standard deviation of \(0.33\) dB, the channel imbalance for V on transmit (open diamonds) is mainly negative with a mean value of \(-0.4\) dB and a standard deviation of \(0.27\) dB. Furthermore, an opposite behavior can be observed for both channel differences (HV-HH and VH-VV) as shown in Figure 25, particularly for steep look angles below \(25\)\(deg\), i.e., there is an opposite trend between H and V polarization on transmit. However, changing the sign for one pair of differences (e.g., VH-VV then becomes VV-VH), more or less the same behaviour is reflected, independent of the polarization on transmit. This results in a flipped channel difference for this pair (VV-VH) while the other one remains (HV-HH). After applying this procedure, a specific curve as a function of elevation angle remains (so-called smile curve) with higher channel differences for steep and shallow elevation angles. The same characteristic for both channel combinations indicates that this smiling effect is mainly determined by the receiving channels and not by the transmit channels. The channel imbalance was investigated in order to reduce the number of potential error sources having an impact on the radiometric performance of the SAR data. By relating both receiving polarization channels to each other, all potential error contributions from the SAR processing are cancelled out which are applied to both polarization channels for a given acquisition in the same way. In particular, all factors arising from several normalization procedures (e.g., transmit pulse length, used bandwidth and filter parameters) vanish as the same factor is used for both polarizations. Furthermore, the contribution of all geometric related parameters (e.g., used DEM, elevation and squint angle) can be ignored as the same acquisition geometry is used for both polarizations at the same target. Thus, it is assumed that the remaining impact results from the reference antenna patterns derived from the antenna model and their application during SAR data processing. Another indication of an improper correction of the antenna patterns during SAR data processing results from the channel imbalance derived from rainforest acquisitions. Therefore, the EW mode with the widest elevation coverage is used to analyze the elevation angle dependency and compare near-, mid- and far-range beams. As the backscattering of co- and cross-channels is supposed to be almost flat over the elevation angle, the channel imbalance should also show this behavior. In Figure 26, two cases are depicted showing the channel imbalance derived from SLC products for both transmit polarizations:  
* the delivered standard product which includes a common correction by the antenna pattern (solid lines), and
* rainforest data without the correction of the antenna pattern (dotted lines). The results show that the data before antenna pattern correction are more homogeneous. The pattern correction adds both an additional bias which is different for both transmit polarizations, aswell as an additional shape as a function of the elevation angle. Therefore, we recommend investigating further the effective shape and offsets which are introduced by the antenna elevation pattern for all polarization channels. Hence, by applying the pattern correction during SAR data processing, the data seem to be overcompensated by the reference antenna patterns derived from the antenna model.