Research On The Instrument Uncertainty And The Calibration Method Of Visibility Measurement

Visibility is one of the key elements of meteorological observation.It plays an important role in the field of weather forecasting,transportation and environmental protection.Early studies of atmospheric visibility had been carried out in the early 20th century.But early visibility depended on manual observation mainly.Later,researchers tried to quantify observations with dedicated visibility meter.World Meteorological Organization provides suggests that the visibility meter relative error less 20%in the entire detection range of the visibility meter is allowed.Currently,Finnish company Vaisala scattering FD12P,PWD20,and American Novalynx's transmissive 8330 have been implemented.These instruments have high visibility measurement accuracy in the normal region.But in high and low visibility regions,there are some issues,such as the large degree of uncertainty,error randomness and inadequante accuracy.To take the above challenges,the article is based on the theory of visibility measurement uncertainty caused by the light propagation in the atmosphere.For transmissive and scattering measurement instruments,a design method of high precision measuring device and a visibility calibration method are proposed.Through the theoretical analysis and experimental verification,the uncertainty correction relationship between the atmospheric extinction coefficient,probe beam divergence angle and the atmospheric transmittance is established.The extinction coefficient calibration model is proposed and the model is verified by experiments.The main contents of this thesis are as follows:(1)To solve the uncertainty problems,solutions of visibility meter transceiver and receiver are proposed.With precision devices and high-performance sensors,high precision transmission and receiver module including two closed-loops,temperature controlling and steady light power controlling are designed.A noise model is established for each unit.After calculation,noise values are on the order of microvolts.In a 200Hz bandwidth,uncertainty theoretical maximum value caused by the circuit is 0.18‰.(2)The effect of noise to the instrument volatility indicators is analyzed.Visibility measurement uncertainty is consistent with electronic noise,and it changes with the temperature fluctuation within 24 hours.From night to early morning,the ambient temperature is continued slowdown.All types of radiation and noise gradually approach the lowest,the overall measurement uncertainty reduces.From morning to noon,ambient temperature gradually rises to its highest level.As a variety of external radiation occurs,thermal noise of the devices is increasing,and the uncertainty of measurement is also increased.At the 95%confidence interval,when 3 times the standard deviation for the uncertainty are taken,the percentage is 4.3‰.(3)The effect of the scattering effect on the emission and reception of the probe beam is analyzed.Based on geometrical optics and image processing,a divergent angle measuring platform is established.During the installation,the divergence angle of the light source is likely to be changing.To achieve the real divergence angle of the probe beam and optical power,the measurement platform of probe beam divergence angle is designed.The platform can detect beam parallelism without changing the original structure of the instrument.It is applicable for LEDs and LDs.The ivergence angle measurement error is 0.6%to 0.7%.(4)The relation between divergence angle error and visibility uncertainty is proposed.The test samples with different concentrations using the aerosol generator are produced.The relation between the probe beam divergence angle and the samples has been established.The relation between different divergence angle and detected value under various atmospheric samples has been obtained.The correction function of the detected value has also been obtained.Through the experiment,the results show that probe beam divergence angle has an impact on visibility measurement.The larger the divergence angle is,the greater the uncertainty is.Hence,divergence angle must be corrected.The method of divergent angle correction makes the measurement relative error reduced.Under the measurement tolerance of 5%,the divergence angle may allow 12mrad.Under the measurement tolerance of 10%,the divergence angle can be allowed to reach 28mrad.(5)A compensation model to calibrate the extinction coefficient is improved.Based on the nonlinear relationship between atmospheric extinction coefficient and transmittance,the new measurement model is proposed,by increasing the compensation in the measurement channel.The optical structure is designed with the model.Tunable filter is the compensator.It makes reverse compensation in the high and low value regions of test chamber.The two regions are mapped to the median region.To achieve fast track,the compensation transmittance measurement algorithms with boundary constraints is presented.The experimental results show that the optimized method overcomes the disadvantage of traditional measurement methods,in which the relative error is considerable in the all regions.With new model and method,the extinction coefficient relative error is reduced by 18%.