The versatility of self-developed advance space-borne imager
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Most of us have heard about the capabilities of images captured by airborne cameras. For instance, on May 10, with the aid of photos taken by a drone, a Shanghai driver was found using his mobile phone while behind the wheel of his car on Yan'an Road. The offense earned the culprit three demerit points on his driving record, and a fine of 200 yuan (US$28).
You might be less sure, however, of the uses of pictures taken by satellites orbiting 705 km above the earth, at an altitude that is roughly the distance between Wuhan and Shanghai, according to Liu Yinnian, a senior researcher at Shanghai Institute of Technical Physics, Chinese Academy of Sciences.
In spite of the altitude and speed of the satellite's orbit – it completes one orbit of the Earth about every 99 minutes – the Advanced Hyperspectral Imager (AHSI) is second to none among state-of-the-art detectors today, Liu explained recently at the institute in Jiading.
The imager, mounted on the GaoFen-5 satellite launched on May 9, 2008, could help detect target objects, safeguard the environment (for instance, by detecting methane emissions), or detect minerals in areas where geological prospecting might be difficult, and is always expensive.
It can also be used in precision agriculture, forestry surveying, or even urban planning.
These feats are enabled by the imager's amazing sensitivity. While our naked eye can only perceive visible light in several color bands, hyperspectral imaging technology can collect more information about objects on Earth, because it could sense visible and near-infrared, and short-wave infrared in hundreds of color bands.
Breakthroughs in theory do not automatically translate into progress in practical implementation.
As a consequence, a more formidable challenge lay ahead of the team, not least of all how to overcome the many engineering obstacles.
Just getting right the technique of tightening screws for a optical device cost some team members several months, Liu explained.
When Liu talked of the challenges of hyperspectral imaging work that started in 2000, the soft-spoken scientist waxed philosophical.
He pointed to the importance of the "unity of the opposites," successfully addressing the seeming impossibility of reconciling contradictions by thinking out of the box.
In 2008, under state support, and after experiencing many setbacks and tribulations, the hyperspectral imaging team led by Liu came up with a uniquely creative solution whose performance metrics turned out to be comparable to other spaceborne HSIs launched recently, and those scheduled for launch in the next few years.
The AHSI has a wide spectral range, large swath width, more spectral channels, and higher SNR (signal noise ratio) and spectral calibration accuracy.
The endeavor involved in these achievements is evidenced in a pile of documents consisting of 5,000 technological reports and over 40,000 drawings on a long desk in a display room.
The successful launch of China's GaoFen-5 satellite on May 9, 2008, was a landmark event. The AHSI on board the satellite, the first wide spectrum and swath spaceborne hyperspectral sensor in China, had 330 spectral bands, a 60-km swath width, and a 30-m spatial resolution.
In addition to the GaoFen-5, space-borne hyperspectral imagers were also on board four other satellites, which provide a steady supply of hyperspectral data to the Ministry of Natural Resources, and the Ministry of Ecology and Environment, to a tune of thousands of frames of data on a daily basis.
In terms of ecological monitoring, the advanced imager, given its swath width of 60km, could cover the whole body of Taihu Lake in one shot.
The imager also provides the only means of detecting weak emissions of methane from the satellite, thus significantly facilitating related supervision.
In prospecting of minerals, in 2021, the first ever extraction and plotting of over 10 minerals in space led to their identification with precision in excess of 90 percent.
Liu firmly believed that remote sensing would be an important development of commercial space technology.
With remote sensing data infiltrating every aspect of our life, its future industrial and commercial use can only be imagined at this stage.
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