# On-Orbit Calibration Method for Correction Microwave Radiometer of the HY-2 Satellite Constellation
###### Abstract  
The HY-2D satellite was successfully launched in 2022, which marks the first phase of the HY-2 satellite constellation. In order to reduce the deviation of wet path delay (WPD) between different satellites in the HY-2 satellite constellation and increase precision in the correction microwave radiometer (CMR) products, on-orbit calibration must be performed to the brightness temperature (BT) of the CMR in this constellation. This study describes the principle and process of on-orbit calibration for CMR in detail. For the three satellites of the HY-2 satellite constellation, after cross-matching with each other within a limited spatio-temporal range, the HY-2B satellite with sounding on the global ocean is selected to the calibration source, calibrating BT from the CMR of the HY-2C and HY-2D satellites to the BT dimension of the HY-2B satellite CMR. To check on-orbit calibration, a retrieval algorithm is built using atmospheric profile data from ECMWF and BT data, obtained from the CMR of the HY-2B satellite; this is used to calculate the atmospheric water vapor (AWV) and WPD from the HY-2 satellite constellation. After on-orbit calibration to the CMRs of the HY-2 satellite constellation, the deviation between the CMR products of different satellites is significantly reduced by over 20%, and the RMS of WPD for the same type of products from the Jason-3 satellite is less than 1 cm. It may be concluded that on-orbit calibration improves the accuracy of AWV and WPD by normalizing the BT dimension for CMRs of the HY-2 satellite constellation, so this calibration method is effective and credible for enhancing the quality of altimeter products in the HY-2 satellite constellation. on-orbit calibration; correction microwave radiometer; the HY-2 satellite constellation; brightness temperature; crossovers; reference satellite 28 September 202333 November 20231 December 20236 December 2  
tropospheric correction. For example, on 10 August 1992, the United States and France jointly launched the TOPEX/Poseidon (TP) satellite for sea-surface topography; it carries a special microwave radiometer (TMR, 18 GHz, 21 GHz, 37 GHz) for WPD, with an accuracy of 1.2 cm [5; 6]. As a follow-up satellite, the Jason-1 satellite was launched and changed the observed frequency of the microwave radiometer (JMR, 18.7 GHz, 23.8 GHz, 34 GHz) on board [7]. Subsequently, the Jason-2 and Jason-3 satellites were successfully launched, carrying advanced microwave radiometers (AMR, 18.7 GHz, 23.8 GHz, 34 GHz) with the same mechanism [8; 9]. On 21 November 2020, the United States and Europe jointly launched the Sentinel-6 MF (Jason-CS) satellite with the advanced microwave radiometer for climate (AMR-C, 18.7 GHz, 23.8 GHz, 34 GHz); it carried a new measurement system (Poseidon-4) combining dual-frequency synthetic aperture radar (SAR) and traditional altimeter radar [10]. The microwave radiometers of these satellites all have three frequencies for wet tropospheric correction. The HY-2 satellite constellation consists of three satellites for the marine dynamic environment; they all carry CMRs to measure WPD and include the HY-2B, HY-2C, and HY-2D satellites. In this constellation, every CMR has three channels for ocean observation: the 23.8 GHz channel is for the AWV absorption line, the 37 GHz channel is for cloud liquid water (CLW), and the 18.7 GHz channel is for observing sea-surface wind contributions [11]. Table 1 details their main specifications. The HY-2B, HY-2C and HY-2D satellites were successfully launched on 25 October 2018, 21 September 2020 and 19 May 2021, respectively. HY-2B runs on a sun-synchronous orbit (inclination of 99.34\({}^{\circ}\)), and HY-2C and HY-2D are designed with non-sun-synchronous inclined orbits (inclination of 66\({}^{\circ}\)) [12].