# The Customized Automatic Processing Framework for HY-2A Satellite Marine Advanced Products
## 2 Products and Related Work
### The Onboard Sensors of HY-2A Satellite and Their Products  
As mentioned in the previous section, there are four kinds of sensors onboard the HY-2A satellite. On the one hand, the two active microwave sensors are a radar altimeter and a microwave scatterometer. On the other hand, the two passive microwave sensors are a scanning microwave radiometer and a calibration microwave radiometer [10]. The radar altimeter on HY-2A is mainly used to measure the sea surface height and the significant wave height. The radar altimeter is an active microwave sensor that emits short electromagnetic bursts vertically downward to the ocean surface and receives the reflected signal. By calculating the time difference between the transmitted and received signals, the distance between the satellite and the sea surface can be measured. With the precise orbit determination by the laser ranging, the Global Positioning System and the geoid, these observations can determine the changes in sea surface height to an accuracy on the order of a few centimeters, and the altimetry data can provide critical information on ocean circulation and dynamics [11]. In addition, the shape of the return signals yields the significant wave height and the magnitude yields the scalar wind speed [12]. The products derived from the radar altimeter divide into three levels. The level-1 products have geometric corrections applied. The level-2 products are the geophysical records derived by different processing methods. The level-3 products include gridded products, seasonal and annual products. Additional details of the radar altimeter carried on board HY-2A are given below in Table 1.  
As the other microwave sensor, the scatterometer mounted on HY-2A is used to provide observations of vector wind at the sea surface. With a rotating dish antenna that can generate a pair of conically scanned pencil beams at two incidence angles, it takes multiple looks at the same sea surface area by transmitting pulses and receiving the returns either from different directions or at different polarizations. From the backscatter returns, the value of surface scattering cross section \(\sigma 0\) can be derived. With the geophysical model function (GMF), a functional relation between \(\sigma 0\) and the near-surface winds can be determined. In other words, the wave properties and the \(\sigma 0\) are functions of wind speed and the azimuthal difference between the wind direction and the scatterometer look angle. Therefore, such multiple measurements of \(\sigma 0\) can provide the wind speed and direction [12]. The scatterometer-derived wind products are also divided into three levels. The level-1 products are corrected by geometric correction and instrument noise removal, and calculated to the data of Kp coefficient, backscatter coefficient, etc. The level-2 products have two sub-levels: 2A and 2B. The 2A products are derived from correcting the return for atmospheric interference and mask processing, and the 2B products are wind velocity products. The level-3 products are the gridded products that are separated into ascending and descending passes. The technical specifications of HY-2A microwave scatterometer can be briefly summarized in Table 2.  
The scanning microwave radiometer is a multi-channel imager with center frequencies at 6.6, 10.7, 18.7, 23.8 and 37 GHz. Both vertical and horizontal polarizations are used for all channels except the 23.8 GHz, which only has the vertical. With the calibrated brightness temperatures and retrieval algorithms, the radiometer can retrieve surface and atmospheric variables, such as water vapor, cloud liquid water, wind speed, and sea surface temperature. Because the emissivity or transmissivity associated with each atmosphere or oceanic constituent has a different frequency