地基微波辐射计工作环境对K波段亮温观测影响

Environmental Thermal Radiation Interference on Atmospheric Brightness Temperature Measurement with Ground-based K-band Microwave Radiometer

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。

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 widthα= 3.1°and the gain G= 30 dB.The val-ue ofηe 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 thatηe= 70%,the surrounding temperature and emissivity would change byΔTg= 10 K andΔε= 0.05 around Tg= 280 K andε= 0.85 if the radome can be neglected.The fluctuation would increase up to 9.6 K if the size of the open-ing 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 tem-perature measurement even though LN calibration of the radiometer system can be performed as manual-re-quired.For this,a brightness temperature correction method for operating environment variation is sug-gested 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.