Advanced Hyperspectral Imager (AHSI) on Chinese Gaofen-5 Satellite

The Advanced Hyperspectral Imager (AHSI) is the main payload of the Chinese Gaofen-5 (GF-5) satellite, which is a remote sensing satellite for scientific research on the Earth’s atmosphere and terrestrial observation launched on May 8, 2018. The satellite carries six payloads, a hyperspectral payload, and a multispectral payload for terrestrial Earth observation, along with four atmospheric observation payloads. These payloads will enable researches to study greenhouse gases, pollution, air quality, climate change, and map geological resources and crop production, among other tasks. The objectives of AHSI are to address many key science questions and operational needs using remote sensing technology, such as ecological and environmental monitoring, investigation of geology and mineral resources, land and resources, disaster monitoring, precision agriculture, forestry management, precision animal husbandry, and urban planning.

The AHSI was designed and built by the Shanghai Institute of Technical Physics, Chinese Academy of Science. It is China’s first spaceborne hyperspectral imager that uses a convex grating to disperse spectrum. AHSI has 330 spectral bands covering a wavelength range from 0.4 pm to

2.5 pm. The SSI is 5 nm in VNIR (0.4-1.0 pm) region and 10 nm in SWIR (1.0-2.5 pm) region. The ground sampling distance of AHSI is 30 m, which is the same as that of Hyperion, whereas the swath width of AHSI is 60 km, which is about 8 times wider than that of Hyperion. Figure 2.18 shows a photo of the AHSI payload before being launched (Liu 2018).

Table 2.27 lists the key parameters of the specification of the AHSI payload. The AHSI consists of a wide-field telescope, a field splitter, a slit, two Offner spectrometers with convex gratings, an ensemble of FPAs, a baffle, and an onboard calibration subsystem, as well as subsystems such as components, drivers, and signal acquisition and communication control and information processing electronics. The telescope is an off-axis TMA. The field splitter separates the input light from the telescope into VNIR and SWIR portions to fill the two corresponding spectrometers. The slit limits the radiation light to the spectrometers. The convex gratings of the spectrometers disperse the input light and image the spectrum onto the focal planes of the spectrometers. The 2D CCD detector array and the 2D HgCdTe detector array mounted on the focal planes of the VNIR and SWIR spectrometers sense the spectra and convert them to electronic signals.

FIGURE 2.18 Photo of the AHSI payload before launch. (Courtesy of Shanghai Institute of Technical Physics, Chinese Academy of Science.)

TABLE 2.27

Key Parameters of the Specification of the AHSI Payload

To achieve the requirements of the large FOV, a design of full reflection off-axis TMA was adopted for the telescope. On the basis of the traditional Offner configuration, a convex grating is added as a correction lens. The radiation lights pass through the grating twice prior entering the Offner structure and after leaving the Offner structure, respectively. The spectral curvature (smile) and spatial distortion (keystone) caused by the long slit are corrected by the different incident angles of the slit center and edge lights to the grating. The AHSI is also equipped with an onboard calibration subsystem to ensure the stability and quantification of acquired image data. This includes by imaging the onboard LED calibration components, combining the occultation to observe the atmospheric profile for spectrometer on-orbit spectral calibration, and by introducing sunlight to illuminate the diffuse panel to calibrate the spectrometer w'hile using a separate diffuser to monitor the attenuation of the main diffuse panel.

Table 2.28 reports the pre-launch characterization and test results of the ASH I payload witnessed by the representatives of the client and the mission management team. The test results demonstrate that the flight model of the AHSI payload met and exceeded all the required specifications. Compared to Hyperion hyperspectral sensor, the AHSI has a higher SNR (3-4 times), a wider sw'ath width (around 8 times), and more spectral bands (over 100 more; Liu 2018). This kind of progress of spaceborne hyperspectral sensors is encouraged and expected by the hyperspectral user community for almost two decades since the launch of Hyperion in 2000.

Italian Hyperspectral Satellite PRISMA

PRISMA (PRecursore IperSpettrale della Missione Applicativa) is a preoperative Italian hyperspectral satellite, aiming to qualify the technology, contribute to develop applications, and provide products to institutional and scientific users for environmental observation and risk management. It was launched on March 22, 2019, on a Vega launch vehicle from the European base of Kourou in French Guyana into a sun synchronous orbit. It focuses primarily on the European area of interest, enabling the download of the data on two ground stations located in Italy (Candela et al. 2016).

PRISMA instrument is composed of a hyperspectral imager and a PAN camera. The instrument is the core of the PRISMA mission, fully funded by the Agenzia Spaziale Italiana (ASI), and the prime

TABLE 2.28

Pre-Launch Test Results of the ASHI Payload

contractor is a consortium of Italian companies. SELEX ES is responsible to the development of the hyperspectral imager, including level 0-Level 1 (L0-L1) product algorithms (Meini et al. 2012).

The hyperspectral imager operates in pushbroom mode. It is made up of a VNIR spectrometer and a SWIR spectrometer to cover spectral bands ranging from 400 nm to 1010 nm and from 920 nm to 2505 nm. It provides hyperspectral images of the Earth with 30-m ground sample distance (GSD), 30-km swath width, and spectral bands at an SSI of 12 nm. The PAN camera provides black-and-white images at spatial resolution of 5 m within a spectral range of 400-700 nm, coregistered to the hyperspectral images, so as to allow images fusion to sharpen the spatial resolution of the hyperspectral images (Meini et al. 2016).

The Optical Head Unit houses a common telescope, a double-channel imaging spectrometers operating in VNIR and SWIR regions, and a PAN camera. It collects the input radiation from a scene on the ground by a telescope common to the hyperspectral imager and PAN camera, disperses the radiation by the prisms of the two spectrometers, converts photons to electrons by means of appropriate detector arrays, and amplifies the electronic signal and converts it into digital data stream. Figure 2.19 shows the optical layout of the PRISMA hyperspectral imager. The Main Electronics Unit controls the instrument and handles the bit stream representing the spectral images up to the interface with the spacecraft transmitter.

The telescope is a TMA design that assures excellent optical quality with a minimum number of optical elements. This solution is very compact and without obstruction. The TMA telescope optics layout is shown on the left in Figure 2.19. The shape of the three mirrors is aspherical with only conic constants. The secondary mirror is almost on-axis. The off-axis values of the primary mirror and of the tertiary mirror are designed in order to facilitate the mirror manufacturing. The position of the aperture stop lies on surface М2. The telescope optical system is telecentric with respect to the entrance pupil. The stray light effects have been extensively analyzed and addressed to define

FIGURE 2.19 Optical layout of the PRISMA hyperspectral imager. (Courtesy of SELEX ES.)

the particular requirements on the optics design in terms of element dimensions, mechanical profiling, finishing, scratch and digs, coatings, contamination, and baffling.

The spectrometers are of collimator-prism-imager configuration. A spectrometer consists of a collimator common to both VNIR and SWIR channels, a dispersing prism, and an objective (different for the two channels). The telescope images the spectral radiation of a cross-track line on Earth on the entrance slit of the VNIR and SWIR spectrometers. The dimension of the slit is 30 mm x 30 pm, with the 30 mm orientation toward cross-track direction and 30 pm to along-track direction. The overall input spectral radiation (400-2505 nm) is split into two channels (VNIR and SWIR) by a dichroic beam splitter (DBS). The collimator images the slit image at infinity, then the prism disperses radiation spectrum reaching on its surface, the objective focuses the chromatic images on the dedicated detector array placed on the corresponding VNIR and SWIR focal planes as detailed in Figure 2.19.

The main advantages of this kind of design are the same FOV for both VNIR and SWIR channels (i.e., same entrance slit) and the use of several common optical elements for both channels. The VNIR channel covers a wavelength range of 400-ЮЮ nm with 66 spectral bands, while the SWIR channel covers a wavelength range of 920-2505 nm with 171 spectral bands. The overlap between the VNIR and SWIR channels ranges from 920 nm to 1010 nm. This overlap allows a cross-calibration between the two channels, increasing the confidence in the calibration process.

The tw'o spectrometers use prisms as the spectral dispersive elements. This prism-based solution has advantage of obtaining higher efficiency and lower polarization sensitivity than those achievable by grating-based spectrometers. The high efficiency allows reducing the instrument dimension and mass with less demanding resources to the spacecraft and less criticalities for the optics design. The disadvantage is that the spectral dispersion is not constant with respect to the wavelength.

Both VNIR and SWIR channels have a magnification to match the detector array of 1000 x 256 pixels, with pitch size of 30 pm x 30 pm. The instrument design guarantees the spectral distortion (Smile) and the spatial distortion (Keystone) effects to be maintained within 10% of the pixel for both VNIR and SWIR detector plane arrays.

The PAN channel is obtained by separating the main beam coming from the TMA telescope by an in-field separator (FM2 in the telescope) that allows the use of a common fore-optics for

TABLE 2.29

Performance Parameters of PRISMA Instrument

both hyperspectral imager and PAN camera, greatly simplifying the overall instrument design and products co-registration. The in-field separation effect is a constant offset in terms of geo-location between hyperspectral and PAN images, which will be taken into account by image processing algorithms, when со-registering the hyperspectral and PAN images.

PRISMA instrument is also equipped with in-flight calibration unit to allow operations of absolute and relative radiometric calibration as well as geometric and spectral calibrations. Table 2.29 tabulates the performance parameters of PRISMA instrument.