Study On L,C-band Microwave Radiometer Calibration And Active Cold Noise Source

The sea surface salinity remote sensing is a hot issue in the field of microwave remote sensing in recent years. The salinity has important reference significance for the study of the ocean thermal-salt circulation and the global sea level change. It is not only an essential parameter to reveal the complex phenomena of the ocean, but also an important basis for the study of global climate change and sea water circulation. Microwave radiometer is an effective method to detect the sea surface salinity, and the L band is the best frequency band for sea surface salinity remote sensing. Working together with the C-band radiometer witch measures sea surface temperature, L-band radiometer can effectively extract the information of sea surface salinity.Firstly, to observe the sea surface temperature and salinity, this paper studies the L/C dual band airborne microwave radiometer system. The performance of antenna, receiver, calibration unit and so on are analyzed. The linearity and sensitivity of the receiver, and the accuracy of the temperature measurement unit are tested. The calibration experiment was carried out on the shore of the Qingdao Bay. The aviation remote sensing experiment was carried out in Bohai Bay. The performance indexes of the system were verified, and the radiation brightness temperature data of the sea surface L/C wave band was obtained.Secondly, the generation and development of active cold noise source(ACNS) technology are overall expounded. The 2-port microwave network noise theory and transistor small signal equivalent circuit model are analyzed. Based on the two theories, the mechanism of microwave cold noise generation is described, and the minimum equivalent noise temperature generated by the transistor gate is given.Thirdly, the principle and design of active microwave cold noise source are studied. Based on the cold noise theory, a complete design scheme of active microwave cold noise source is proposed. The L-band and C-band active microwave cold noise source device are developed, with breakthroughs in core technologies such as minimum load reflection and minimum gate noise temperature. The lowest output brightness temperature of L-band active microwave cold noise source at room temperature is 85.3K, and the C-band ACNS’s is 137.8K, witch reach the international level. The key problems affecting the output of the cold noise are analyzed. The integration of noise source and radiometer system is considered, and the effect of noise source is affected by environment temperature and power supply. The constant temperature control strategy is given, which lays the foundation for the practical engineering application in the future.Fourthly, the calibration test and flight test of airborne microwave radiometer system are analyzed in this paper. In shore test, a calibration method for real time cross four point calibration is proposed. The correction of calibration data by using atmospheric radiative transfer characteristics and its influence on the calibration performance are analyzed. The method of determining the noise temperature of internal calibration source is analyzed. The influence of front end loss and temperature is determined. In flight test, the calibration method and the error correction method are studied. The main factors affecting the calibration results are analyzed.In the end of this paper, the experimental method of testing the performance of active microwave cold noise source is studied. A detailed analysis of the cold active microwave noise source as the calibration test results of internal calibration source is given.The calibration accuracy is verified. On the basis of this, the brightness temperature output characteristic of the line grid calibration source is tested. The measured reflectivity is 98.4%, and the transmittance is better than 99.3%. The experimental results show that the radiation signal is separated and mixed according to the sine law in the rotating process. Controllable radiation signal can be generated in the brightness temperature range of 8K~248K. The feasibility of the grid set standard source as the external calibration temperature source is verified. Aiming at the problems in the experiment, the improvement direction is analyzed.