# On-Orbit Calibration Method for Correction Microwave Radiometer of the HY-2 Satellite Constellation
###### Abstract
Although both HY-2C and HY-2D have the same orbit inclination, they are not on the same orbital plane and there is a fixed angle between the orbital planes of the two satellites, to maintain a fixed-phase difference between the ground tracks of the two satellites and produce a denser ground-track grid than a single satellite. Because HY-2C and HY-2D are different from HY-2B in terms of orbit character, it is impossible for HY-2C and HY-2D to produce a fixed-phase difference with HY-2B [13]. However, the dual-satellite system that consists of HY-2C and HY-2D (with the same orbit character) can achieve fixed-cycle (about 10 days) observation coverage, so their ground tracks are evenly spaced on the Earth, with a spacing of about 146.3 km for the grid formed by the tracks [14]. When HY-2B (fixed-cycle: about 14 days) is joined to this system, this grid can reduce the average spacing to about 100 km, and the density is increased significantly. These grids are shown in Figure 1.  
The radiometer must be calibrated before launch via on-ground calibration. The calibration equation of the receiver shows the relationship between the output voltage and antenna temperature, which extends from the mechanism to the receiver [15]. The calibration result of every CMR achieves satisfactory accuracy on the ground for the HY-2 satellite constellation; however, post-launch, the brightness temperature (BT) deviation still exists between them, for multiple reasons:  
* As time increases in satellite operation, the decline of CMR performance will cause the observed BT to drift [16]. * For the CMR antenna and feed, the ambient temperature will change after launch, meaning calculation from on-ground calibration formulas can deviate from real BT values [17]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Marker** & \begin{tabular}{c} **Frequency** \\ **(GHz)** \\ \end{tabular} & \begin{tabular}{c} **Bandwidth** \\ **(MHz)** \\ \end{tabular} & \begin{tabular}{c} **Footprint** \\ **(km)** \\ \end{tabular} & \
\begin{tabular}{c} **Beamwidth (3 dB)** \\ **(C)** \\ \end{tabular} &
\begin{tabular}{c} **Antenna Aperture** \\ **(mm)** \\ \end{tabular} \\ \hline HY-2B & 18.7, 23.8, 37 & 250, 250, 500 & 24, 19, 10 & \(1.4\pm 0.2\), \(0.6\pm 0.1\) & 920 \(\pm\) 10 \\ \hline HY-2C & 18.7, 23.8, 37 & 250, 250, 500 & 21, 19, 13 & \(1.25\pm 0.2\), \(0.75\pm 0.2\) & 920 \(\pm\) 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main parameters of correction microwave radiometers (CMRs) for the HY-2 satellite constellation. The CMR of HY-2B has three feed sources that correspond to three channels for observations; the CMRs of HY-2C and HY-2D satellites have only one feed source to three channels respectively. This is shown in Figure 2.  
Therefore, every CMR must be calibrated after launch; this is on-orbit calibration. It is customary to revise the calibration equation for stable satellites, which improves the accuracy of WPD. In 1992, Jason-1 was cross-calibrated by TOPEX in the \"verification tandem phase\" [18]. In wetter conditions, the bias of WPD between JMR and TMR before calibration is 5-7 mm; after on-orbit calibration, the bias is approximately zero; in dry atmospheric conditions, this bias before calibration is 15 mm, and it drops to 5 mm after calibration. In 1997, ERS-2 was cross-calibrated by TOPEX and ERS-1, which improved the WPD [19]. The objective of the present study is to normalize the BT values obtained from the on-ground calibration equations for the CMRs of the three satellites, and to minimize the bias among BT values of different CMRs, so as to further improve the accuracy and consistency of the WPD for the HY-2 satellite constellation.