Research On Water Vapor Radiometer And Its Application In Atmospheric Delay Measuremen

At present,VLBI(Very Long Baseline Interferometry)technology is widely used to accurately determine the orbit of deep space targets such as spacecraft.When the target electromagnetic wave signal received by the VLBI measuring station passes through the troposphere,the propagation speed of the signal is affected by the atmosphere,which will cause the signal propagation delay.This atmospheric propagation delay is affected by atmospheric parameters such as atmospheric temperature,air pressure,and water vapor,which seriously affects the measurement accuracy of the VLBI.The water vapor radiometer is a high-sensitivity microwave receiving device.By detecting the microwave radiation of water vapor and liquid water in the atmosphere,it can measure the water vapor in the tropospheric atmosphere in real time,thereby obtaining the delay of the electromagnetic wave signal.At present,VLBI measurement usually adopts empirical model,GPS/met measurement and other means to calibrate the delay error.The existing methods have the problem of poor real-time performance and inability to measure in the same direction as the radio telescope.With the increasing real-time requirements of VLBI measurements,the water vapor radiometer is considered to be an ideal means for atmospheric delay calibration in future VLBI measurements.However,because the water vapor radiometer adopts the passive remote sensing measurement method,the atmospheric radiation signal measured by it is very weak and has characteristics similar to noise,and its accurate and stable measurement is a difficult problem.Therefore,it is very meaningful to analyze its system error,calibration technology and delay calculation method.Based on the significance and research value of atmospheric delay measurement by water vapor radiometer,this paper presents the system design,error analysis,calibration technology and wet delay measurement method of water vapor radiometer.A large number of simulation calculations and comparative experiments are carried out.The results are as follows:(1)To meet the demand of water vapor measurement,a new method for selecting the optimal frequency of water vapor radiometer measurement is proposed.The historical radiosonde data set is used to calculate and simulate the optimal water vapor measurement frequency at different stations through the transformation rate of the weight function method,so that the uncertainty caused by frequency selection is reduced from 5% to 20% to less than5%.(2)The calibration error,the antenna feeder error,the receiver error and oblique path error are analyzed emphatically;The calibration methods,such as temperature compensation,receiver gain compensation using reference source,and oblique path compensation using atmospheric total attenuation model,were studied,which reduced the root mean square error of daily measurement of water vapor radiometer from 1.8K to 0.3K.(3)Through simulation analysis and experimental data,the influence of calibration scanning angle,atmospheric average temperature calculation error and system nonlinearity on the calibration success rate during the calibration of the sky tipping curve is studied.The nonlinear calibration model improves the calibration method of the sky tipping curve.The process of the improved calibration method is given and the correctness of the method is verified by experiments.Experiments show that: in areas with high average atmospheric brightness temperature on sunny days,the zenith angle for calibration should be appropriately reduced;after using the improved sky tipping curve calibration method,the calibration success rate is increased from 60% to 98%.(4)The main methods of measuring atmospheric delay are given,and the accuracy and consistency of delay measurement by tropospheric delay empirical model,radiosonde,GPS/met,and water vapor radiometer are systematically compared and analyzed.Combined with the analysis of the weather process,a method of jointly inverting the wet delay by the water vapor radiometer and GPS is proposed,and the mathematical model and inversion method are given.It has been proved by experiments that after the joint inversion method is adopted for the HZD-X water vapor radiometer,the wet delay in measuring cloudy and rainy days is reduced from 27.14 mm to 2.83 mm.Applying the error calibration,calibration improvement algorithm,joint inversion and other research results in this paper,the measurement accuracy of the domestic HZD-X water vapor radiometer has been significantly improved.