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Environmental Thermal Radiation Interference on Atmospheric Brightness Temperature Measurement with Ground-based K-band Microwave Radiometer
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摘要: 该文研究地基微波辐射计天线性能及其工作环境对K波段20~30 GHz亮温观测数据的影响，根据辐射传输理论和天线性能参数分析建立模型，通过模拟计算给出辐射计20~30 GHz波段亮温观测对天线性能及其工作环境的响应，提出针对工作环境温度变化影响的订正方案，并结合观测资料进行分析验证。结果表明：如果辐射计天线增益和3 dB波束宽度决定的等效主波束效率ηe较低，则即使在能够经常进行辐射计系统液氮定标的情况下也必须考虑天线工作环境 (环境温度与辐射计定标时的情景差异) 对K波段亮温观测的影响。对某一辐射计液氮定标后1年多观测资料的订正验证表明：订正效果明显，尤其是在28.0 GHz和30.0 GHz两通道。Abstract: Effects of operating environment thermal radiation interference on atmospheric brightness temperature measurement with ground-based K-band microwave radiometer especially for channels near 28.0 GHz and 30.0 GHz are studied. A model for simulating antenna temperature which expresses the energy received by the radiometer based on radiative transfer is derived and used to calculate the response of the brightness temperature measurements to parameters such as antenna specifications, radome, surrounding temperature and emissivity. Results show that the equivalent main beam efficiency (ηe) defined by 3 dB points is only 73.17% for a typical antenna, of which the half-beam half width α=3.1° and the gain G=30 dB. The value of ηe would be even smaller if factors like aperture radiation effect, shape-error, and occlusion and so on are taken into account. The brightness temperature would fluctuate by 4.0 K in case that ηe=70%, the surrounding temperature and emissivity would change by ΔTg=10 K and Δε=0.05 around Tg=280 K and ε=0.85 if the radome can be neglected. The fluctuation would increase up to 9.6 K if the size of the opening in the radome is just for the main beam. Therefore, if the equivalent main beam efficiency determined by the antenna gain and 3-dB beam width for the current radiometer system is not large enough, variation of the operating environment must be taken into account during the correction of K-band brightness temperature measurement even though LN calibration of the radiometer system can be performed as manual-required. For this, a brightness temperature correction method for operating environment variation is suggested according to the theoretical relationship and the result from application to observations. Over one year application after LN calibration shows that the fitness and correlation between the observed brightness temperature after correction and the calculated brightness temperature with radiative transfer equation is obviously better than before, especially for channels of 28 GHz and 30 GHz.
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图 2 2010年12月22日—2011年12月31日每日08:00和20:00晴空时辐射计环境温度变化ΔTg及K波段4个代表性通道亮温观测值TBM、订正值TBO和模拟值TBC的时间序列
Fig. 2 Time series of clear sky environment temperature change Tg and the observed brightness temperature TBM, the corrected brightness temperature TBO and the simulated TBC for 4 typical channels in K-band at 0800 BT and 2000 BT from 22 Dec 2010 to 31 Dec 2011
表 1 两类典型辐射计的K波段天线性能与分析
Table 1 Antenna performance and analysis for two typical types of K-band radiometers
行号 条件 参数 A类辐射计 B类辐射计 22~31 GHz 22 GHz 30 GHz 1 厂家提供 G/dB 33.2 30 32 2 X/dB < -30 < -23 < -24 3 WHPB/(°) 3.3~3.5 6.3 4.9 4 取α= 0.5WHPB α/(°) 1.7 3.1 2.5 5 γ=Xmax γ/dB -30.0 -23.0 -24.0 6 ηe /% 18.04 13.13 10.64 7 γ由G和α决定 γ/dB -35.9 -35.7 -38.1 8 ηe /% 45.98 73.17 75.42 9 由需求ηe=90%决定 γ/dB -46.2 -40.8 -42.8 10 α/(°) 2.38 3.44 2.73 
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表 2 B类辐射计K波段下行亮温和天线温度在天空不同状况下对比以及环境亮温变化影响 (TB计算中，假设TS=238 K，ΔTS=22.5 K，其余条件同文献[31])
Table 2 Comparison of the K-band downward brightness temperature and antenna temperature for type-B radiometer and analysis on the influence of environment under different sky conditions (let TS=238 K, ΔTS=22.5 K and other parameters for TB calculation are the same as Reference [31])
行号 条件 计算参数 22.2 GHz (ηe=73.17%) 25.0 GHz (ηe=73.98%) 28.0 GHz (ηe=74.84%) 30.0 GHz (ηe=75.42%) 1 晴空TB/K 31 28 17 16 2 云天TB/K 42 33 29 30 3 雨天TB /K 61 53 51 54 4 β=1 晴空TA/K 59 55 45 43 5 云天TA /K 68 60 55 56 6 雨天TA /K 85 77 75 77 7 δTB/K 3.5 3.4 3.2 3.2 8 晴空TB/δTB 8.9 8.3 5.3 5.1 9 云天TB/δTB 12.0 9.8 9.0 9.5 10 雨天TB/δTB 17.5 15.8 15.8 17.1 11 β=0 晴空TA/K 87 83 73 71 12 云天TA /K 95 86 82 81 13 雨天TA /K 108 101 98 99 14 δTB/K 8.3 7.9 7.6 7.3 15 晴空TB/δTB 3.8 3.5 2.2 2.2 16 云天TB/δTB 5.1 4.2 3.8 4.1 17 雨天TB/δTB 7.4 6.7 6.7 7.4
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表 3 B类辐射计K波段亮温的环境温度变化订正系数及订正效果统计量
Table 3 Coefficient c for calibrating the environment temperature influence on K-band brightness temperature measured by type-B radiometer and the statistics to show the efficiency of calibration
通道 频率/GHz 订正系数c 订正前拟合直线
TBM=aTBC+b拟合度
RMC2订正后拟合直线
TBO=aTBC+b拟合度
ROC21 22.23 0.215088 y=1.1112x-2.4141 0.9537 y=0.9862x-0.2928 0.9763 2 22.50 0.224503 y=1.1119x-1.1412 0.9571 y=0.9792x+0.9970 0.9763 3 23.03 0.352293 y=1.1879x-0.9221 0.9194 y=0.9700x+2.1907 0.9680 4 23.83 0.354430 y=1.2161x-0.7508 0.8861 y=0.9634x+2.4390 0.9563 5 25.00 0.329222 y=1.2402x-0.9890 0.8174 y=0.9577x+2.0860 0.9395 6 26.23 0.265974 y=1.2885x-2.6649 0.8019 y=0.9890x+0.6199 0.9431 7 28.00 0.350330 y=1.0714x+0.3436 0.3846 y=1.0091x+0.0591 0.9138 8 30.00 0.457644 y=0.1129x+12.659 0.0049 y=0.9314x+2.2196 0.7931
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