# On-Orbit Calibration Method for Correction Microwave Radiometer of the HY-2 Satellite Constellation
## 2 Datasets  
On-orbit calibration must be supported with sufficient data; one portion of these are used to build models for calibration and inversion, and the other is used to verify them. The  
Figure 1: The HY-2 satellite constellation ground tracks. (**a**) A dual-satellite grid including HY-2C and HY-2D, with symmetrical tracks. (**b**) A constellation grid including HY-2B, HY-2C and HY-2D; it is an uneven grid due to the HY-2B orbit differing from that of HY-2C and HY-2D, but its tracks cover polar regions.  
Figure 2: The CMR feed sources of the HY-2 satellite constellation. The HY-2B satellite flies forward along the \(+x\) axis; (**a**) shows its CMR and has three feed sources at three frequencies; these are arranged in a straight line along the flight direction, and the centers of the three-channel footprint are collinear. Both the HY-2C and HY-2D satellites fly on the yaw, which no longer stays flying forward along the \(+x\) axis. (**b**) The resulting CMRs, which have only one feed source to the three frequencies.  
observed times for the three sets of data must be different, to prevent data for calibrating or modeling being used to verify them [20; 21]. Therefore, all experimental datasets used for this on-orbit calibration should be divided into three parts: verification, retrieval algorithm, and calibration equation. The datasets are given in Table 2.  
The first group is used to cross-check on-orbit calibration. In order to normalize the BT of the three CMRs in the HY-2 satellite constellation, it is necessary to select one of them as the reference. The reference satellite should have more crossovers than the other two satellites in the constellation, to provide as many samples as possible for on-orbit calibration [22; 23]. The task that checks the effect of on-orbit calibration includes two steps: one observes the change of differences for CMR products in the crossovers of two satellites before and after calibration [24], and the other checks the accuracy of products after calibration. In this experiment, the CMR products of the HY-2 satellite constellation for two discontinuous months are selected to cross-match and compare, and after on-orbit calibration, the AMR products of Jason-3 are compared with CMR products to verify.  
The second group is used to build a new retrieval algorithm. The current operational algorithm for the CMRs of the HY-2 satellite constellation is from the HY-2A satellite; it was fitted using WPD (AWV) data from the AMR and TB data of CMR, as obtained from the HY-2A satellite [25]. The WPD and AWV of the HY-2 satellite constellation were validated by comparing them with the synchronous data from in-situ observations of the ocean or the AMR of Jason-3, which all show minor deviations [26]. In this on-orbit calibration, new models and coefficients are tried experimentally.  
The third group is used to establish an on-orbit calibration equation. After selecting the reference satellite, the BT data of the other two satellite CMRs are calibrated to the BT data's dimension of the reference satellite CMR. In order to ensure the accuracy of BT data, this calibration must be performed when every satellite of the constellation is running stably [27]. The HY-2D was sufficiently stable after April 2022. This experiment accumulated datasets for about 1 year until 2023, which was enough to determine whether the on-orbit calibration method had an optimal effect.