## 3 Results and Discussions  
The intercalibration results obtained using the presented three methods are provided in Table 2. \"Collocated\" stands for the direct comparison using collocated measurements, \"Rainforest\" stands for intercalibration over the Amazon rainforest and \"NOC\" stands for the double difference based on backscatter simulations over the global oceans. In performing all methods, the same period (three months) of HY-2B, HY-2C, and HY-2D data were used. As introduced above, the collocation method uses the CDF matching technique, and thus can generate nonlinear calibrations as a function of \(\sigma^{0}\) value. Figure 7a,b shows the nonlinear corrections for HH and VV polarizations separately. We can see that the differences among HSCAT \(\sigma^{0}\) data are small, particularly between HY-2C and HY-2D. However, the HY-2B \(\sigma^{0}\) data below \(-25\) dB shows noticeable separation against HY-2C or HY-2D. This can be related to the gain variation caused by the telemetric antenna temperature, noise processing, and different thermal conditions for the instrumentation in the different orbits (different solar illumination conditions). Note that backscatter values below \(-25\) dB are relatively infrequent (see Figure 3a) and will mainly affect low wind speed retrievals over the ocean. The HY-2B is in a sun-synchronous dawn-dusk orbit (6:00/18:00 LST), while HY-2C and HY-2D are in a similar asynchronous orbit with respect to the sun. This implies 2 groups of scatterometer with different sun irradiation conditions and hence different thermal space environments with possible effects on the calibration of noise and gain. The  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Case**} & \multicolumn{2}{c}{**Collocated**} & \multicolumn{2}{c}{**Rainforest**} & \multicolumn{2}{c}{**NOC**} \\ \cline{2-7}  & **HH (dB)** & **VV (dB)** & **HH (dB)** & **VV (dB)** & **HH (dB)** & **VV (dB)** \\ \hline HY-2B and HY-2C & +0.132 & \(-\)0.060 & +0.043 & \(-\)0.056 & +0.133 & +0.022 \\ HY-2B and HY-2D & +0.155 & +0.019 & +0.128 & +0.039 & +0.202 & +0.089 \\ HY-2C and HY-2D & +0.076 & +0.067 & +0.086 & +0.096 & +0.069 & +0.067 \\ \hlinefact that the calibrations are different, does not imply correctness of one or the other, since both systems can, in principle, be varying due to thermal effects, but the variation along the orbits is rather similar for HY-2C and HY-2D. All scatterometers hence may be affected. At moderate winds, 0.1 dB bias implies 0.1 m/s wind speed error. At low winds, the amount of dB's in 0.1 m/s increases. So, whereas the calibration bias below \(-25\) dB increases, its effect on the wind speed error remains modest and only a few 10 ths of a m/s. Integrating the backscatter PDF from low to high values corresponds to integrating the wind speed PDF from low to high values. So, below \(-25\) dB implies about 10% of the lowest winds. We furthermore considered that radar backscatter over land is relatively more stable than over the dynamic ocean in the collocation time window. However, we tested the direct comparison method for only the measurements over land or ocean, or for shorter time difference, where the results shown in Table 2 and Figure 7 only change negligibly (less than 0.05 dB; not shown). Figure 8 shows the hourly average \(\sigma^{0}\) over the Amazon as measured by HH or VV polarization of HY-2B, HY-2C, and HY-2D scatterometers in summer. In performing the rainforest method, considering the diurnal variations, the scatterometer measurements that are used are limited to 5:00 to 7:00 and 17:00 to 19:00 according to local solar time. The overall diurnal variations of HY-2C and HY-2D are clear, and the peaks are expected to occur around 6:00 near sunrise. However, this is not exactly the case for HY-2D. These results are assessed in the in-orbit test phase of scatterometers, while based on these results, further analyses of satellite gain and noise variations as a function of, inter alia, thermal conditions, will be useful.