CRESDA planned to have the entire CHEOS series of satellites in orbit by 2020, but the window has been extended to allow additional satellites to be deployed. Gaofen Missions in the CHEOS Program The satellite is configured with two 2 m barrel-mounted panchromatic/8 m MS (Multispectral) cameras, and a set of 4 WFIs (Wide Field Imagers) with 16 m MS medium-resolution and a combined swath of 830 km. The revisit frequency of the spacecraft is ≤4 days and the observation range of the mission covers a region between 80ºN to ~80ºS. The data of the GF-1 mission are being used in the following application areas: land resource investigation, mineral resource management, atmospheric and water environment quality monitoring, and natural disaster emergency response and monitoring. Gaofen-1 (gao fen = high resolution) is the first of a series of high-resolution optical Earth observation satellites of CNSA (China National Space Administration), Beijing, China. The civilian HDEOS (High-Definition Earth Observation Satellite) program was proposed in 2006, it received government approval and was initiated in 2010. China plans to launch six HDEOS spacecraft between 2013 and 2016. 1) RF communications: the spacecraft features an S-band TT&C subsystem and an X-band data transmission subsystem. The Pan imagery is compressed by a JPEG 3:1 algorithm, while the 8 m and 16 m MS data are compressed with a lossless algorithm to ensure image quality.

Gaofen-4, employs GEO remote sensing bus, configured with a 50m staring camera, operating on the geo-synchronous orbit. GF-4 can provide an imaging area of 7000km×7000km with individual scene covering an area of 400km×400km, and with capacity for high temporal resolution remote sensing monitor at minute-level. GF-4 is designed for 8 years life. Launched in late 2015, GF-4 was China's first geosynchronous orbit high-definition optical imaging satellite and claimed to be "the world's most sophisticated". The satellite can cover more than 26 million sq km including 8.5 million sq km in China and 17.7 million sq km overseas, with an optical spatial resolution of better than 50 metres as well as infrared sensing capabilities. The 4,600kg satellite is part of the China High-resolution Earth Observation System (CHEOS) which, according to the China National Space Administration (CNSA), aims to provide China with all-weather, all-day and global Earth observation coverage by 2020. The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates ( [email protected]). Orbit: Sun-synchronous orbit, altitude 645 km, inclination = 98º, LTDN (Local Time on Descending Node) = 10:30 hours, revisit frequency of ≤4 days. H. Qi, "China High-resolution Earth Observation System (CHEOS) and its Latest Development," Proceedings of the 51 st Session of Scientific & Technical Subcommittee of UNCOPUOS, Vienna, Austria, Feb. 11-22, 2014, URL: http://www.unoosa.org/pdf/pres/stsc2014/tech-47E.pdf Figure 8: Fused image of the Beijing Forbidden City with 2 m and 8 m resolution data of GF-1 (image credit: DFH)

Exceptions To The Rules

Figure 16: Side-slither raw image (left) on May 1, 2013 and shift of each detector image (right), image credit: DFH China also stated earlier in the year that it planned to launch a 20-meter-resolution synthetic aperture radar satellite to geostationary orbit in 2023. Designed and developed by CAST, Gaofen-2 employs the long life CAST-CS-L3000A bus with breakthrough technologies including fast roll and high stable attitude control, long focal length and lightweight camera design to enhance the satellite’s performance and reliability.

The main goal of the HDEOS series is to provide NRT (Near-Real-Time) observations for disaster prevention and relief, climate change monitoring, geographical mapping, environment and resource surveying, as well as for precision agriculture support. The major users of the observation data will be the Ministry of Land and Resources, Ministry of Environmental Protection, and the Ministry of Agriculture.On May 1, 2013, the GF-1 spacecraft yawed 90° and performed side-slither calibrations three times on orbit. This maneuver produced a radiometric flat-field input to the sensor so that the relative response of each detector was determined for the same exposure level ( Figure 16). The side-slither derived detector correction parameters were then used to improve the quality of the GF-1 imagery. A significant improvement in image correction was achieved (Ref. 14). The Gaofen-5, with a designed lifespan of eight years, is equipped with six state-of-the-art observation payloads that would provide the most comprehensive observation data compared to other Chinese remote sensing satellites, Li Zhengqiang, an expert at the Chinese Academy of Sciences' Institute of Remote Sensing and Digital Earth, told the Global Times. The Gaofen-1 mission has been extended and, as of June 2023, the GF-1 spacecraft and its payload are still operational. 23) CHEOS comprise optical, multispectral, hyperspectral and synthetic aperture radar satellites. A CHEOS near space segment comprising airships, air flight platforms, and airborne Earth observation instruments and data processing system was however also included in the plan to augment its capabilities and data collection, though little information has been published on this aspect. Gaofen-1 is based on the CAST small satellite bus designed and developed by the China SpaceSat Co. Ltd. of Beijing (also referred to as DFH Satellite Co. Ltd.), the commercial subsidiary of CAST (Chinese Academy of Space Technology). The satellite is equipped with a pair of solar panel wings, each with three solar panels. The CAST-2000 bus is a small platform with high performance, expandability and flexibility. The CAST-2000 bus is of HJ (Huan Jing)-1A and -1B mission heritage, both spacecraft were launched in Sept. 2008.