## 2 Principle and Method  
Specifications of GeoFWHIS are shown in Table 1. Its wavelength range covers from near ultraviolet to long wavelength infrared (0.3-12.5 \(\upmu\)m). The full spectrum is divided into five sub-bands, including the near-ultraviolet and visible band (B1-UVIS), the visible and near-infrared band (B2-VNIR), the short-wave infrared band (B3-SWIR), the medium wave infrared band (B4-MWIR), and the long wave infrared band (B5-LWIR). They have different specifications for spatial resolution, spectral resolution, modulation transfer function (MTF), and signal-to-noise ratio (SNR). To meet the different requirements of each band, a reasonable layout of the optical system is particularly important. The layout block diagram of GeoFWHIS is shown in Figure 1. It is composed of an afocal telescope, three imaging objectives, and spectrometers for five bands. The GeoFWHIS keeps relatively stationary with the ground, so its integral time will not be limited by the movement of the platform. Sufficient SNR can be achieved by actively increasing the integral time, to reduce the requirement of a large relative aperture. Meanwhile, GeoFWHIS cannot work in a push-broom manner such as LEO (Low Earth Orbit) imaging spectrometers because of the relatively stationary state. Imaging of a single scene covering 400 \(\times\) 400 km is completed through internal scanning. The afocal telescopic compresses the incident light into a small aperture. At the telescope's exit pupil, the scanning mirror scans the compressed beam into the imaging objectives, and the full spectrum is divided into five sub-bands by dichroic mirrors. It can be seen from Table 1 that the GeoFWHIS has a wide swath and high spatial resolution. There are 16,000 spatial sampling points in B1 and B2 bands within 400 km, and the instantaneous field of view is up to 0.7 \(\upmu\)rad. Wide swath and high spatial resolution determine that the system has long slits; the longest is 241.3 mm. Such a long slit will result in a very large spectrometer, which poses great challenges to the manufacture of both the spectrometer and detector. Therefore, we use multiple spectrometers for each band to cover the super-long slit. According to the specifications of the selected detector, four spectrometers are spliced, and there is an overlap between every two adjacent spectrometers.  
We compare the specifications between GeoFWHIS and some current satellite sensors in orbit in Table 2. The GEO meteorological satellite sensor GOES-R ABI (Advanced Baseline Imager), FY-4 AGRI (Advanced Geostationary Radiation Imager) and the LEO hyperspectral satellite sensor EO-1 Hyperion, GF-5 AHSI (Advanced Hyperspectral Imager), and EnMAP HSI (Hyperspectral Imager) are compared. Compared with the GEO meteorological satellite sensors, GeoFWHIS has significantly higher spatial and spectral resolution in the full spectrum. Its spatial and spectral resolution is more than one order of magnitude higher than ABI and AGRI. Compared with the LEO hyperspectral satellite sensors, GeoFWHIS has a wider swath width, a wider spectral range, and the same order of spatial and spectral resolution. Therefore, GeoFWHIS not only has the advantage of wide coverage of GEO satellite sensors, but also the advantage of high resolution of LEO satellite sensors.