###### Abstract  
Acquiring high-quality images from space at low-latitude areas is challenging due to the orbital requirements of the satellites and the frequent cloud coverage. To address this issue, a low-latitude remote sensing satellite constellation--the Hainan Satellite Constellation (HSC)--was conceived with a spatial coverage-priority concept. This constellation integrates sensors with multispectral, hyperspectral, radar, and Automatic Identification System (AIS) capabilities for marine vessels with an onboard image processing technology. The design is tailored to the tropical/subtropical region. Once HSC becomes fully operational, it will provide high-frequency coverage in low-latitude regions, with a primary focus on ocean observations. The first four optical satellites (HN-1 01/02 and WC-1 01/02) were successfully launched in February 2022. They boast unique application characteristics, including satellite networking for ocean observations over large areas, onboard image processing and modeling for ship detection, as well as the synergy of onboard sensors with optical and ship AIS capabilities. This study focuses on the technical design and proposes implementation strategies for HSC, encompassing its technical characteristics, composition, and capacity. Additionally, it explores the construction of this satellite constellation and its uses while providing insights into potential follow-up satellites. low-latitude areas; Hainan Province; remote sensing satellite; constellation; ocean observations +
Footnote †: journal: Sensors  
0000-0002-4880-3333-3333-3333-3333-3333-3333-3333-3333-3333-3333-3333-3333-3333-3333-33333-3333-3333-33333-3333-3333-33333-3333-33333-33333-3333-3333-33333-3333-33333-3333-333 TROPICS [15], TRMM [16; 17; 18], and LAPAN-A2 [19; 20] were designed to handle rain prediction, soil moisture estimation, tropical rainfall measuring, and natural disaster monitoring in tropical countries. Jason-1 and Jason-3 were deployed with high-precision radar altimeters and a microwave radiometer, which is useful for measuring the sea level and modeling the sea surface [21; 22; 23]. The COSMIC-2 constellation integrates the radio occultation and space weather payload to observe the atmosphere and ionosphere and monitor space weather, such as tropical cyclones [24; 25; 26]. FengYun-3G, which was equipped with dual-frequency radar, a microwave imaging sensor, and a mid-resolution spectral imaging sensor, can recognize the change of weather in good time and precisely, especially precipitation [27; 28]. The payloads of these satellites mainly focus on the radar, microwave imaging sensor, multispectral imaging sensor, and wireless, which are good at capturing the weather conditions, such as rain, atmosphere, storms, and space weather. But the imaging characteristics and resolutions of these sensors make them inappropriate for the typical application scenarios in the Hainan region, which include ship detection, fishery management, vortex monitoring, crop recognition, etc. To handle these tasks effectively, sensors with higher spectral and spatial resolutions and efficient imaging are required. This means that different payloads, including optical, radar, hyperspectral, and AIS, should be considered to collect more complicated information. Advancements in remote sensing platforms and sensor technologies were reported [29; 30], and proposals for systems integrating remote sensing and ship Automatic Identification System (AIS) satellites for enhancing ship detection revisit times have been put forth [31]. Comprehensive accounts of China's remote sensing satellite constellations have also been documented [32; 33; 34; 35; 36; 37; 38; 39]. China has made significant strides in developing Earth observation satellites with a focus on ocean observations. Three series of satellites, targeting oceanic color, marine dynamic environments, and marine surveillance were successfully launched [40].