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The Analysis and Correction of Lunar Intrusion to Space View of FY-3A/MWHS
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摘要: 为解决月球对风云三号卫星微波湿度计在轨实时辐射定标的影响问题，该文根据风云三号卫星微波湿度计在轨实时观测数据分析，通过多项式拟合分析技术，建立修正模型，消除月球影响，得到合理的辐射定标结果。风云三号卫星微波湿度计在轨实测数据分析表明：月球对风云三号卫星微波湿度计在轨辐射定标结果影响最严重时一天内可干扰4~5条轨道，每条轨道有近100个扫描周期受到污染。月球影响致使冷空定标观测数据跳升1000个计数值，如果不进行污染滤除会造成对地观测亮温下降约20 K，导致辐射资料无法同化进入数值天气预报模式。通过与同类载荷的交叉比对分析，修正月球影响后，风云三号卫星微波湿度计在轨实时辐射定标结果精度与在轨测试结果相当。Abstract: The microwave humidity sounder (MWHS) is an important payload of FY-3A launched on 27 May 2008, for global all-weather atmosphere sounding. FY-3A/MWHS has been in operational application on orbit for more than 5 years. FY-3A/MWHS has five channels in the range of 150-191 GHz. In the normal working mode, it performs a cross-track scanning and cycle two points calibration by using of the blackbody and space views. However, the moon may occasionally appear within the cold space calibration field of view. Due to the polar orbit of the platform, it will always appear to be near the-90 phase and then have a brightness temperature of approximately 170-200 K. Its angular extent is about 0.5. Lunar radiation could therefore be significant against a cold sky background, especially for the narrow-beamed MWHS. Lunar intrusion to space view of MWHS may make a moon-glint, which typically affects about 100 scan lines per orbit for 4-5 consecutive orbits. Lunar intrusion cases are detectable as the degradation in NWP model statistics.Based on analysis of lunar intrusion cases and effects, a mitigation algorithm is developed to improve calibration results of FY-3A/MWHS in orbit.In the algorithm, the position of the moon is calculated using standard astronomical formula, and the intruding time is flagged. After that an approximation by polynomial model is devised to remove the moon-glint. On 4 July 2010, the moon-glint makes more than 1000 digital number abnormal jumping, and 20 degree degradation in brightness at sub-points line for FY-3A/MWHS 183.31±1 GHz channel.By using of the cross calibration technique between corresponding channels of FY-3/MWHS and MetOp-A/MHS, an extensive analysis is carried out to make the best fitted global SNO samples.The lunar intrusion mitigation algorithm is tested, and results show that after removing the moon-glint effects, corrected calibration results are almost the same with what got during check in orbit. The lunar intrusion mitigation algorithm works well and has been used in operation, providing the foundation of quantitative application for data of FY-3A/MWHS in NWP. Lunar intrusion also gives a clear view of the moon when it appears in the field of space view. Since there is no intervening atmosphere to attenuate the signal, which makes it possible to perform a cross-channel calibration.
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图 2 月球影响过程中扫描线间冷空观测数据变化情况
Fig. 2 The change of space view data between scan-lines during a moon-glint
图 4 2008年7月22日微波湿度计183±1 GHz通道冷空观测数据连续多轨分布
Fig. 4 The distribution of space view data for FY-3A/MWHS 183.31±1 GHz channel over orbits on 22 Jul 2008
图 5 风云三号卫星微波湿度计受月球影响示意图
(a)月球进入微波湿度计冷空观域示意图，(b)月球对卫星视张角
Fig. 5 Sketch map of FY-3/MWHS affected by the moon
(a) Scanning sketch map of FY-3/MWHS when the moon in space area，(b) moon solid angle to FY-3 satellite center
图 6 月球影响微波湿度计定标示意图
Fig. 6 Sketch map of how the moon affecting on the FY-3A/MWHS space view
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