Research On Polar Sampling And Calibration Of Rotating Synthetic Aperature Interferometric Radiometers

Passive microwave remote sensing technology,with its cloud and rain penetration characteristics,has all-weather and all-weather observation capabilities,and is an important technique in the field of temperature and humidity sounding.Compared with the polar-orbiting satellites,geostationary satellites have the advantages in terms of large field of view and high temporal resolution.They are ideal platforms for monitoring and forecasting fast-changing disastrous weather phenomenon,such as typhoons and rainstorms.At present,microwave atmospheric sounding in geostationary orbit is still a technological gap in the world.It is one of the most cutting-edge,most urgent,and most challenging topics in the field of earth observation,and it is of great significance to effectively monitor the medium and small-scale disastrous weather system and improve the accuracy of weather forecast.Limited by the spatial resolution and the related technical barriers of large-aperture antennas,it is difficult to implement traditional real-aperture microwave microwave radiometers on geostationary orbit for achieving high-resolution atmospheric sounding.Synthetic aperture radiometer synthesizes the sparse small antennas into an equivalent virtual aperture,which can achieve the instantaneous imaging of the brightness temperatures in the field of view,avoiding the manufacturing and scanning of the large-aperture antenna.Therefore,it is promising to realize microwave atmospheric sounding on geostationary orbit by synthetic aperture radiometers.To meet the urgent needs of Chinese national economic construction and disaster prevention,the National High-tech Research and Development(863)Program and the National Natural Science Foundation of China(NSFC)have supported the cutting-edge research of geostationary microwave atmospheric sounding.The National Space Science Center of Chinese Academy of Sciences has undertaken the related projects of synthetic-aperture technical scheme.Aiming at the technical difficulties of high complexity of high-resolution synthetic aperture radiometer system,an array-rotating time-sharing sampling mode is proposed.This paper focuses on the research of rotational sampling synthetic aperture radiometer,aiming at the two key problems of rotational sampling mechanism and calibration technique.The main research contents and innovative achievements are summarized as follows:1.Aiming at the polar coordinate sampling grid formed by the rotating synthetic aperture radiometers,Fourier spectrum analysis was carried out on the visibility function from radial direction and circular direction,respectively.The bandlimited theory of visibility function in radial direction and circular direction is established for the first time.And the effective bandwidth estimation method of the polar coordinate sampling visibility function is proposed according to the characteristics of the observation scene and the system parameters.In order to avoid any information loss of the sampled visibility,a sampling criterion of the visibility function in polar coordinates is proposed based on the Nyquist sampling theorem,which provides a theoretical basis for the system design and operation scheme of rotating synthetic aperture radiometers.2.As the time-sharing sampling mode is accomplished by the array rotation and integral sampling simultaneously,the blurring effect of sampled visibility caused by dynamic integral sampling is studied for the first time.The blurring of sampled visibility by dynamic integral sampling is established,based on the derivation of the analytical expression of the visibility blurring of point-source observation in small-angle approximation.The dual relationship between the dynamic integral sampling trajectory of visibility function and the corresponding reconstructed brightness temperature artifacts is verified by numerical simulations.3.As it is difficult to implement traditional noise-injection calibration scheme on the highresolution geostationary synthetic aperture radiometers,a novel calibration strategy based on redundant space calibration is proposed,which utilizes the unique structure of equispaced circular array and rotational sampling strategy.Without dedicated calibration hardware or external calibration references,the proposed strategy calibrates the phase and amplitude simultaneously.Although the measured visibility phase is naturally wrapped,the phase retrieval still works correctly without any additional phase unwrapping technique,which truly realizes self-calibration of phase.The calibration performance is guaranteed by the strong self-consistency achieved by the array rotation,and it leads a brand-new technical route for the calibration scheme design of geostationary synthetic aperture radiometers.4.Based on the error model of synthetic aperture radiometer,a full-link data processing scheme for the second-generation full-scale engineering prototype of geostationary interferometric microwave sounder is built.The visibility function characteristics of the centrosymmetric observation target in the time-sharing sampling mode are studied,and a novel phase calibration and correlation bias correction method based on the external point source is proposed.It effectively accomplishes the phase calibration and correlation bias correction during the ground-based experiment,where the ideal far-field observation target is difficult to obtain.The data processing scheme is verified by the experimental results and it serves as a technical foundation of the further application of geostationary synthetic aperture radiometers.