# On-Orbit Calibration Method for Correction Microwave Radiometer of the HY-2 Satellite Constellation
## 3 Cross-Checking in the HY-2 Satellite Constellation  
The constraints for selecting crossovers for on-orbit calibration are as follows: they are all more than 50 km from the coastline, the temporal range is less than 30 min, and the spatial range is a radius of 15 km across a circular area. The data for one month (see Table 2 from 27 December 2021 to 27 January 2022) are extracted from the datasets used for on-orbit calibration, and any two of three satellites in the HY-2 satellite constellation are cross-matched according to the above constraints. Among them, there are 1254 crossovers between HY-2B and HY-2C; 10,898 points are between HY-2B and HY-2D and have no points between HY-2C and HY-2D. We take data for another month (see Table 2 from 21 March to 21 April 2022) to cross with each other under the same constraints in the HY-2 satellite constellation; HY-2B crosses HY-2C and HY-2D, respectively, for 4424 and 3050 points, though we still have no crossovers between HY-2C and HY-2D. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Dataset** & **Function** & **Level** & **Volume** & **Satellite** & **Timeframe** \\ \hline
1 st group & Verification & L2B & 2.52 GB & HY-2B, HY-2C, & 27 December 2021–27 January 2022 \\  & & & & HY-2D & 21 March 2022–21 April 2022 \\ \hline
2 nd group & Retrieval & & & & 1 January 2021–1 March 2021 \\  & algorithm & & & HY-2B & 1 April 2021–1 June 2021 \\  & & & & & 1 July 2021–1 September 2021 \\  & & & & HY-2B, HY-2C, & \\  & & & HY-2D & 1 May 2022–1 May 2023 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The datasets for on-orbit calibration. The distribution of crossovers is shown in Figure 3a-d. It can be shown in two durations: HY-2B has the most crossing points with HY-2D, with the second-most being between HY-2B and HY-2C. However, the distribution and number of crossovers varies in different months between HY-2B and HY-2C (shown in Figure 3a,c) as well as HY-2B and HY-2D (shown in Figure 3b,d). Therefore, long-term data accumulation can ensure sufficient points for on-orbit calibration, but there are no points between HY-2C and HY-2D. These are mainly determined from the orbit characteristics of the three satellites in the constellation (see paragraph 4 in Section 1). Therefore, the HY-2B satellite can be used as the reference satellite for this on-orbit calibration. Figure 3: _Cont._  
We compare the BT data of the above crossovers between the satellites and take three channels from this constellation; as shown in Figure 4a, their deviations are quite obvious. It may be seen that their BT differences are all concentrated on a fixed threshold and the trend line of their BT differences is very straight, indicating that the relative stability of BT is excellent between them [28] from 2021 to 2022. Therefore, it may be deduced that if crossovers conform to the above constraints between HY-2C and HY-2D, these stable differences must also occur for them. For two satellites with crossovers, we removed the BT data contaminated by sea ice and rainfall from all points; the invalid data for HY-2B and HY-2C accounted for 1.92% of the total, and the invalid data for HY-2B and HY-2D accounted for 1.86%. In Figure 4b, the BT data of crossovers for the same frequency channels  
Figure 3: The distribution of crossovers with each other in the HY-2 satellite constellation; (**a,b**) are within the same time period and (**c,d**) are within another time period. (**a,c**) Crossovers between HY-2B and HY-2C and (**b,d**) crossovers between HY-2B and HY-2D. (**a,d**) Crossovers from 30\({}^{\circ}\) to 60\({}^{\circ}\) at south and north latitudes, respectively, and (**b,c**) crossovers where HY-2B crosses HY-2C and HY-2D, distributed on the globe from 60\({}^{\circ}\) of the north and south latitudes to the equator. from two satellites are linearly correlated, indicating that the CMRs of the three satellites have consistency and inheritance, which is also a prerequisite for on-orbit calibration in the HY-2 satellite constellation.