Haishao-1 satellite: Low-inclination orbit spaceborne synthetic aperture radar

The spaceborne synthetic aperture radar (SAR) has revolutionized Earth observation by providing unparalleled all-weather, day-and-night imaging capabilities. Unlike optical systems, SAR utilizes microwave signals to penetrate clouds and darkness, making it indispensable for monitoring dynamic Earth processes such as land deformation, vegetation cover, oceanic dynamics, and polar changes.1 Since the pioneering SEASAT mission in 1978,2 spaceborne SAR technology has evolved dramatically, with advancements in resolution, coverage, and operational versatility. Today, a new era of SAR innovation is being driven by commercial small satellites like Capella and ICEYE constellations,3 which offer sub-meter resolution, hourly revisit frequencies, and rapid response capabilities. Against this backdrop, the Haishao-1 (HS-1) satellite (meaning “sea whistle” in English), launched on December 4, 2024, carrying an X-band (9.6 GHz) SAR payload and a nightlight camera, represents a groundbreaking achievement in China’s spaceborne SAR development. As the first Chinese SAR satellite to operate in a low-inclination orbit (43°) and a very low Earth orbit (VLEO) at 350 km (see Figure 1A and the table within the figure), HS-1 combines cutting-edge technologies to deliver unprecedented monitoring capabilities for low-latitude regions and beyond. This paper highlights the technological innovations and scientific potential of HS-1, showcasing its role in advancing global Earth observation.

First low-inclination-orbit spaceborne SAR in China

While most remote sensing satellites operate in sun-synchronous orbits (near-polar orbits) to achieve global coverage, their ability to monitor low-latitude regions, particularly the tropics and subtropics, remains limited. Especially for SAR satellites, sun-synchronous orbits are more capable of meeting radars’ stringent energy requirements. Low-inclination orbits (typically between 30° and 45°) offer a compelling alternative, enabling high-frequency observations and continuous spatiotemporal monitoring of these critical areas. For instance, the South China Sea (SCS), spanning over 1,000 km east-west and located between 3°N and 23°N, demands frequent monitoring to track dynamic oceanic processes and ensure maritime safety. However, the adoption of low-inclination orbits presents significant technical challenges, including reduced power generation due to shorter sunlight exposure and complex thermal management requirements caused by dramatic environmental shifts between sunlight and shadow. These challenges explain why, out of over 100 operational spaceborne SAR satellites, only seven operate in low-inclination orbits.