| Microwave radiometer is a high sensitivity noise receiver, which can obtainphysical characteristics of the target by the radiation information received from thetarget and the retrieving methods. So, microwave radiometer has been deeply studiedand successfully applied in many fields, such as land remote sensing, ocean remotesensing, atmospheric remote sensing, military, medical treatment, security detectionand many other fields in recent decades. The cloud liquid water content(LWC) is avery important cloud physical parameters, and the cloud supercooled watercontent(SCWC) has received particular attention as a physical quantity in the weathermodification field. Because of the limitations of measurement methods andinstruments, quantitative data of LWC and SCWC is still quite lack. In all the existingdetection methods, Ground-based(surface to air) microwave radiometer (GBMR) hasbeen widely used in meso-scale synoptic system and weather modification field as anew tool for measuring cloud liquid water due to its features of high temporalresolution, high sensitivity, unattended continuous operation and ability of detectingsupercooled liquid water in mixed phase states cloud under certain condition. However,due to the restrictions of moving speed and detection range, GBMR is difficult tocompletely meet the requirements of meso-scale researches. Airborne radiometer,which makes up for the weaknesses of the GBMR and can fly directly into the targetcloud system for detection, becomes an ideal tool for microwave remote sensing.The first challenge we faced in developing the airborne radiometer is the weaknoise signal detection problem: the brightness temperature received by the radiometerat an altitude of several kilometers is lower than on the ground, sometimes just a fewkelvin. Therefore, airborne air detection microwave radiometer has very high demandson the weak noise signal detection ability and developing such measuring instrument isdifficult at home and abroad.Supported by the project "Aviation Multi-frequency Passive Microwave RemoteSensor Integration and Upgrades" from Main Direction Program of KnowledgeInnovation of Chinese Academy of Sciences (KZCX2-YW-JS30), Chinese Academy ofSciences Institute of Atmospheric Physics and Jilin Weather Modification Office, thispaper complete the following research work: 1. Analyze the principle of all the classic type microwave radiometers and adoptthe auto gain compensation technology based on software algorithm to eliminate theeffects of the system gain fluctuation. In addition, in order to detect small change ofthe supercooled water content from ultralow brightness temperature cosmicbackground, the noise coupled technique is proposed to enhance the input signal. Thethermal noise signal of interest is received by the antennas and coupled into an extranoise signal generated from the solid-state noise source, so that the signal can beuplifted to the optimum detecting range of the square-law detector.2. For more conducive to the practical application of the radiometer and gainingmore valid observing data to retrieve high spatial resolution2D and3D distributions ofcloud liquid content (LWC), this paper develops a double-antenna structure whichconsists of two fixed angle Ka-band dielectric lens antennas. According to results ofnumerical simulations, the best combination of the double-antenna elevation angles are30°and90°(elevation angle).3. During the airborne observing process, the temperature of the receiver boxchanges from high temperature on the surface to the low temperature at high altitude.With the deep analysis on the generation mechanism of the airborne double-antennaradiometer’s brightness temperature measurement error due to the changes inenvironmental temperature, a one-point and a multi-point temperature correctionmethods based on the two-point calibration equations are proposed in this paper.Thenew multipoint temperature correction method can correct the two-point calibrationequations through building the relationship between the physical temperature of thetemperature-sensitive units and measurement error using the least square regressionmethod.4. On the basis of above work, an airborne double-antenna millimeter waveradiometer for detecting atmospheric SCWC is developed. The radiometer operates at31.65GHz with a sensitivity of smaller than0.2K,a linearity of better than0.9995, abrightness measuring range of10～350K, a system stability of better than1.5K, abeam angle of4.2°and a working temperature of-20～+50℃.The research in this paper will provide new methods and instruments for thequantitative measurement of LWC and SCWC, improve the efficiency andeffectiveness of the artificial precipitation operation and service for the atmosphericscience research and agricultural production. |
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