The performance of the Advanced Geosynchronous Radiation Imager (AGRI) onboard the Fengyun-4 (FY-4) satellite was evaluated. The results showed that its performance has approached the international advanced level of similar instruments. A wavelet filter was designed to filter the striping noises of the AGRI, which come from the inconsistency of the spectrum and thus varies with the observed target. The non-uniformity of the whole image was evaluated by weighting the probability variance of its count values. After filtering, the infrared image of the AGRI had a uniform visual effect on a single channel, and the pattern noise was reduced to below 0.2 K, which ensures the uniformity of products such as cloud detection and sea temperature that use the infrared splitting window channels. Furthermore, the AGRI carries the surface source blackbody as the on-orbit radiation calibration benchmark and rotates the north-south scanning mirror to point to the blackbody to obtain the blackbody observation data in each infrared band. In the current mode, the infrared calibration is performed every 15 minutes. The long-term evaluation results showed that the radiation calibration accuracy of the infrared channels of the AGRI was better than 0.5 K, and some of the channels reached 0.2 K. In addition, the long-term monitoring of the sensitivity and radiative cooling of the AGRI has shown the on-orbit variation of the instrument, which provides a reference for the development of the subsequent instrument.
Image navigation is a primary process for on-orbit optical payloads involving environmental disaster monitoring, meteorological observation, and the positioning and tracking of space-aeronautics targets. However, because they are affected by solar illumination and orbital heat flux, as well as shock and vibration during launch, the installation structures between the instruments and satellite platform, especially for geostationary satellites, will inevitably generate a displacement resulting in the reduction of positioning accuracy. During the application of FengYun-4A (FY-4A), it is found that the further away from the subsatellite point, the greater the positioning error of Advanced Geostationary Radiation Imager (AGRI) on FY-4A will be. The positioning error can reach 14 pixels at the Arabian Peninsula in operational images. In addition, compared with orbital and attitude measurement errors, long-term observations show that the installation matrix is likely to be the most significant factor determining the navigation accuracy of AGRI. Therefore, an on-orbit installation matrix calibration approach as well as a high-precision navigation algorithm is proposed to modify the positioning error of AGRI. Experimental results show that the navigation error of the processed images corrected by the proposed method can be reduced to 1.3 pixels, which greatly improves the navigation procession of AGRI. In general, this method could be a supplement to the correction of positioning error for geostationary payloads.
FengYun-4A (FY-4A) is the first three-axis stabilized geostationary meteorological satellite in China, which was launched in the early morning of 11 Dec., 2016. Advanced Geosynchronous Radiation Imager (AGRI) is one of the four payloads onboard FY-4A, and acquired the first image on 20 Feb., 2017. FY-4A AGRI contains 14 spectral bands, in which 6 bands are in reflective solar region, with the nominal wavelengths at 0.47, 0.65, 0.825, 1.375, 1.61 and 2.25μm. The spatial resolution is 0.5 km for 0.65 μm band, 2 km for the shortwave infrared bands and 1 km for others. AGRI is designed with a solar diffuser, however it revealed the insufficient capacity for in-flight calibration mainly due to the partial aperture effect. The first vicarious calibration field campaign was conducted at the Dunhuang site of China Radiometric Calibration Site (CRCS) in Apr. 2017. It revealed the large bias of the AGRI data calibrated using the prelaunch calibration parameters, mostly underestimated. Using multiple land sites in Asia and Oceania, the calibration correction factors were derived combined with the CRCS data. The sensor's on-orbit radiometric response variation and observation bias against the simulated radiation were also monitored. It revealed that the bands at 0.65 μm was most stable, while 0.47 μm band showed the large degradation with an annual rate nearly 17%. In this paper, the calibration status of the FY-4A AGRI solar bands was presented.
In the domain of modern space-borne remote sensing in optical band, the operation mode of multiple-element or linear sensor with plane mirror's two-dimensional (2-D) scanning is popular and has been applied in some real applications successfully. Three basic image rotation models of different 2-D plane mirror scanning modes are analyzed in mathematic detail, and their performances are also compared in the aspects of scanning coverage, observing efficiency and degree of image rotation respectively. It is shown that two of them are feasible for real applications and have some advantages with each other. On the other hand, some raw observed sub-image series of Indian National Satellite (INSAT) Charge Coupled Device (CCD) imager, whose plane mirror uses one of the modes to complete the full-disc view of the Earth, are used to validate the authenticity of the image rotation models above. It is shown that, the accuracy of image positioning (about 2~3 pixels) is some better than that (about 5 pixels) achieved by the investigators who cared the launch and the first accuracy assessment of this satellite. Moreover, both the image rotation model based on the plane mirror's 2-D scanning and the corresponding image derotation method are useful for end-user who wants to utilize the images quantitatively on ground.
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