# Present and Future Marine Microwave Satellite Missions in China
## 3 Progress History of China's Marine Microwave Satellite Remote Sensing Technology
### Retrieval Technology for Marine Dynamic Environmental Elements  
Regarding the retrieval algorithms for significant wave heights, a new algorithm with skewness coefficient was derived based on a second-order theoretical echo model [37]. Then, singular value decomposition (SVD) filtering was introduced, and multiple re-tracking schemes were successfully obtained according to the different retrieval parameters of the maximum likelihood estimation algorithm. Comparative analysis was used to determine which four-parameter model was the most suitable for the HY-2A satellite's radar altimeter waveform retrievals, and the significant wave height retrieval accuracy reached 0.31 m [9]. [PERSON] et al. [38] re-evaluated the significant wave height products obtained by the HY-2A satellite's radar altimeter using the retrieval of the four-parameter re-tracking algorithm. It was found that the accuracy of the product was significantly improved when compared  
Figure 3: Observation diagram of the GF-3 satellite. The grey rectangle represents the schematic of different modes. with the previously used three-parameter re-tracking algorithm. It has been confirmed that sea state bias is a very important error source in the current sea level measurements obtained by radar altimeters [39]. Chinese and international researchers have proposed many correction algorithms for sea state bias [40; 41; 42; 43]. However, at the present time, all the effective correction algorithms are empirical models [44]. A neural network-based sea state correction algorithm has been developed based on those empirical models in recent years [45]. The sea surface wind fields obtained by satellite remote sensing include two key elements: speed and direction. Microwave scatterometers, microwave radiometers, radar altimeters, and synthetic aperture radar all can obtain sea surface wind speeds. However, microwave radiometers and radar altimeters cannot observe wind directions due to the limitations of their observational principles. Therefore, the observational accuracy of each payload in different wind speed ranges will tend to vary [46]. Regarding microwave scatterometer sea surface wind field retrieval algorithms, multi-solution retrieval algorithms and two-dimensional variation fuzzy solution removal methods are generally used to process the HY-2A satellite's scatterometer data [47]. The results have shown that the accuracy of the wind speeds and directions were greatly improved [48; 49]. In addition, the removal abilities of the wind direction fuzzy solutions were found to be obviously enhanced. In particular, for the wind field retrieval results under typhoon conditions, the accuracy of the wind direction retrievals has been significantly improved. [PERSON] [50] applied a multiple solution scheme (MSS) and the two-dimensional variation analysis method (2 DVAR) proposed by the Royal Netherlands Meteorological Institute for wind field retrievals from the HY-2A satellite's microwave scatterometer and achieved satisfactory results. In terms of radar altimeter sea surface wind speed retrievals, [PERSON] et al. [51] established the relationship between the Automatic Gain Control (AGC) from the HY-2A satellites radar altimeter and the sea surface wind speeds and solved the low wind speed problem of the HY-2A satellite's radar altimeter sea surface wind speed retrievals in their breakthrough research results. In the research of sea surface temperature retrieval algorithms, [PERSON] et al. [52] established a retrieval algorithm for sea surface temperatures, sea surface wind speeds, atmospheric water vapor content, and other marine environmental parameters using radiation transfer equation simulations. In addition, [PERSON] et al. [53] developed a grid-based method of sea surface temperature data fusion by applying optimal interpolation technology and expanded the products of the sea surface temperature data obtained via satellite remote sensing. In addition, in research involving sea ice concentration retrieval algorithms, [PERSON] et al.