# On-Orbit Calibration Method for Correction Microwave Radiometer of the HY-2 Satellite Constellation
## 6 Summary and Conclusions  
This calibration selects the CMRs of the HY-2B satellite as a reference and calibrates the CMRs of the HY-2C and HY-2D satellites so that the BT data of all CMRs are normalized to the same level. It greatly reduces differences between the three satellite observation products in the HY-2 satellite constellation. By cross-checking to AMR products from the Jason-3 satellite, it can be shown that the accuracy of CMRs products from the HY-2 satellite constellation is almost the same as that of similar satellites, and this on-orbit calibration also decreases the differences in similar radiometer products between the HY-2 satellite constellation and Jason-3 satellite. In this on-orbit calibration, it can be shown that normalization to the BT of CMRs can effectively improve the accuracy of products for the HY-2 satellite constellation. This also indicates, for a fixed retrieval algorithm, that there must be a unique set of BT values from the calibration equation that can make the AWV and CLW from CMR as close as possible to the true values in the atmosphere. For the ground segment, it is better method to use a weather radar to measure the atmospheric element and a satellite to synchronously observe atmospheric BT, and to construct the retrieval algorithm from these two datasets as the core algorithm; other methods for revision are based on this. For the satellite segment, it is an important task for satellite platforms to minimize the differences of working environments for CMR between the ground and space after determining the coefficients of the calibration equation from on-ground calibration, in order to ensure the stability of the observed BT. Figure 10: The distribution of crossovers between the HY-2 satellite constellation and Jason-3 satellite. For two-month datasets for all satellites, crossovers distributed on mid–high latitudes for HY-2B are concentrated at high latitudes for HY-2D and near the equator for HY-2C. Although good quality can be obtained for CMR products compared with AMR products, CMR products may still be further improved. Firstly, there is still room for improvement in the quality of BT data for the 18.7 GHz and 37 GHz channels. From the above comparison, it can be seen that the correlation between the BT data of the 23.8 GHz channels from different satellites is excellent, but the correlation between BT data of the other two channels is relatively low. This indicates that wind speed and cloud liquid water accuracy are not optimal, and the calibration equations of these two channels can still be optimized to improve WPD. Secondly, the retrieval algorithm can be improved based on the different geographical locations. The HY-2B satellite can observe polar areas, but the accuracy of CMR products in high-latitude regions is lower than that in low-latitude regions; for the three satellites, the accuracy of products in coastal waters is lower than that in distant sea waters; all of these can be solved by using different retrieval algorithms for different sea areas. Finally, standard BT data for on-orbit calibration can be further improved. WPD estimates are from the retrieval algorithm, which is built from atmosphere data and BT data obtained from the forward model. Therefore, acquiring more accurate BT data is the most critical step to improving the accuracy of WPD; the superior method is on-orbit calibration using BT obtained from field observation data and the forward model. However, due to the high cost and long period taken to acquire and accumulate effective field data, the main method of on-orbit calibration is still cross-calibration between two satellites or between satellite and model. The on-orbit calibration described in this study is one method to improve the accuracy of BT data for CMRs in the HY-2 satellite constellation.