# Present and Future Marine Microwave Satellite Missions in China
## 2 Development of Marine Satellites in China
### Progress History of China's Marine Dynamic Environmental Satellites  
The HY-2A satellite was China's first polar orbit marine microwave (marine dynamic environment) satellite [14; 15]. It was successfully launched on 16 August 2011 and is still operating in orbit. The HY-2A satellite has a sun-synchronous orbit with an inclination of 99.34 degrees. The local time of the descending node is 6:00 am. In the early life of the satellite, a recursive frozen orbit with a repetition period of 14 days for multi-disciplinary ocean observations had been adopted, with an altitude of 971 km, a cycle of 104.46 min, and a daily operation of 13 + 11/14 cycles a day. However, during the later period of its life, a recursive orbit with a repetition period of 168 days for geodetic applications was adopted, with an altitude of 973 km, a cycle of 104.5 min, and a daily operation of 13 + 131/168 cycles a day. The HY-2A satellite integrates active and passive microwave remote sensors for high-precision orbital measurements, orbit determinations, and all-weather/all-day global detection processes. The main payloads of the satellite include a radar altimeter, microwave scatterometer, and a scanning radiometer, as well as a calibration microwave radiometer (CMR) to correct the wet tropospheric path delays of the radar altimeter. The main mission goals of the HY-2A satellite are to monitor and investigate marine environments; obtain a variety of marine dynamic environmental parameters (such as sea surface wind fields, significant wave heights, ocean currents, and sea surface temperatures); directly provide measured data for forecast global climate changes, air-sea interactions, and catastrophic sea states etc. [12; 14; 15; 16]. As a result, support services are offered for marine disaster prevention and mitigation, protection of marine rights and interests, development of marine resources, protection of marine environments, and marine scientific research [14; 15; 16]. The HY-2A satellite is one of the most complex earth remote sensing satellites in China. The active and passive microwave remote sensors observe the Earth at the same time. The electronic compatibility is complicated, and there are 16 antennas. Moreover, the satellite has the highest precision orbital determination ability among China's remote sensing satellites. By means of three high-precision orbital determination methods (for example, GPS, Doppler orbit determination system (DORIS), and laser ranging, the orbital determination precision of the HY-2A satellite can reach 2 cm. Although the HY-2A is essentially a scientific research satellite, its data products have been able to meet the needs of such marine applications as accurately identifying typhoons and tsunamis through monitoring, and significantly improving the timeliness of marine disaster forecasting. Therefore, the HY-2A satellite has been widely used in many fields [14; 15; 17; 18; 19]. The HY-2B satellite is China's second polar orbit marine microwave (marine dynamic environment) satellite, which was successfully launched by the Long March-4B rocket at the Taiyuan Satellite Launch Center on 25 October 2018 [20; 21; 22]. The HY-2B satellite is equipped with six payloads, including a radar altimeter, a microwave scatterometer, a scanning microwave radiometer, a calibration radiometer, a data collection system (DCS), and an automatic identification system (AIS). The radar altimeter is mainly used to measure sea surface heights, significant wave heights, and gravitational fields. The microwave scatterometer is used to observe the global sea surface wind fields. The scanning microwave radiometer is utilized to observe sea surface temperatures, water vapor content levels, liquid water, and rainfall intensities. The satellite's calibration radiometer is used to provide atmospheric wet tropospheric path delay corrections for the radar altimeter.