## 1 Introduction  
The Ku-band scatterometer onboard the Chinese HY-2D satellite (HSCAT-D) was launched on 19 May 2021. Together with the operating scatterometers onboard the HY-2B (HSCAT-B) and HY-2C (HSCAT-C) satellites, the HY-2 scatterometer constellation was built. This can greatly promote the applications of global sea surface winds by satellite scatterometers. Figure 1 shows the orbital positions of HY-2B, HY-2C, and HY-2D at one moment. This manuscript elaborates some methodologies that can be used to achieve intercalibration of radar backscatter measurements by the HY-2 series of scatterometers.  
Scatterometer backscatter measurements, i.e., normalized radar cross section (NRCS, or \(\sigma^{0}\)), can be used to monitor sea ice in polar areas [1, 2, 3], to estimate the accumulated land precipitation [4, 5], but are mainly designed for remote sensing of global sea surface wind speeds and directions (vector winds) [6, 7, 8, 9]. Previously, the HSCAT-B and HSCAT-C sea surface wind products were validated by comparing in situ buoy winds, numerical weather prediction (NWP) model winds, and winds from the advanced scatterometer (ASCAT) onboard the MetOp-series satellites [10]. It is reported that both HSCAT-B and HSCAT-C winds show quite good quality and comparable validating statistics as ASCAT winds, despite some well-known common error characteristics related to sea surface temperature (SST) and the rotating observing geometry of Ku-band scatterometers [10; 11; 12; 13].  
The HSCAT-B, C, and D instruments are very similar, but fly in different orbits. HY-2B is in a sun-synchronous orbit with 99.34\({}^{\circ}\) inclination and it crosses the equator at nearly the same local solar time (LST) every day (descending at 6 a.m., ascending at 6 p.m.), whereas the HY-2C and HY-2D are in non-sun-synchronous orbits with 66.0\({}^{\circ}\) inclination and their equator crossing times shift each orbit. The period of orbital repeat is 14 days for HY-2B, and 10 days for both HY-2C and HY-2D. The intercalibration of HSCAT backscatter measurements is a prime requirement to guarantee and improve the consistency of the HY-2 series scatterometer geophysical products. However, no absolute calibration approach is available for HSCAT instruments so far. Hence, we here consider relative calibration techniques.  
Studies of the intercalibration of scatterometer backscatter measurements have been performed for C-band scatterometers onboard the ERS-series and MetOp-series satellites [14; 15; 16], for Ku-band scatterometers onboard the QuikSCAT, OceanSAT-2, and international space station (ISS) [17; 18; 19]. In these studies, the presented methodologies include (1) direct comparison using collocated measurements; (2) intercalibration over the Amazon rainforest; (3) intercalibration using simulated backscatter over the global oceans; for each  
Figure 1: The orbital positions of HY-2B, HY-2C, and HY-2D satellites at one moment.  
observed \(\sigma^{0}\) data, a corresponding simulated \(\sigma^{0}\) can be obtained via a geophysical model function (GMF) with numerical weather prediction (NWP) winds and observing geometry as inputs [20]; (4) and comparison of the multi-dimensional measurement space NRCS distributions over the ocean from different scatterometers and periods, called \"cone metrics\" [16]. This latter method was not elaborated for Ku-band pencil-beam scatterers and will not be further addressed here. The above-mentioned methods must deal with differences in observing incidence angles (radar beam pattern gain). However, this part is not needed in this study, because all the HY-2 series pencil-beam scatterometers operate at the same incidence angles, i.e., 41.5\({}^{\circ}\) and 48.5\({}^{\circ}\) for horizontal (HH) and vertical (VV) beam polarizations respectively. Section 2 briefly introduces the datasets used in this study, and describes the intercalibration methods. The results and discussions are provided in Section 3. Finally, conclusions are presented in Section 4.