Research On On-board Radiation Calibration Of Large Aperture Remote Sensor Oriented To Geosynchronous Orbit

On-board calibration has become the main method of calibration due to its short transmission link and high calibration accuracy.According to the characteristics of large aperture remote sensors in geosynchronous orbits,this study proposes a partial aperture calibration method based on Solar Diffuser(SD).Solar diffuser calibration is currently the mainstream on-board calibration method for the reflective solar bands.The calibration method uses the sun that is relatively stable all the year round as the light source,and the solar diffuser plate reflects the sun light as the standard radiance source.The change of instrument responsivity with time can be calculated by the response of the instrument observation solar diffuser.The solar diffuser's surface characteristics are prone to change in the space environment,so a solar diffuser reflectance degradation monitor(SDRDM)is carried.The study used a two-port symmetrical monitoring method.This method observes the sun through the sun view port to monitor the response of the SDRDM detector,and observes the diffuser through the diffuser port to monitor the product of the response of the SDRDM detector and the reflectence of the diffuser.The ratio of the two types of monitoring can realize the monitoring of the reflectence of the diffuser plate.In order to reduce the error introduced by the measurement of partial aperture factors,the study also proposed the transfer of detector response standard.In this method,SDRDM detectors are used as transmission links.By performing on-board radiometric calibration of SDRDM,cross-calibration between detectors is realized within the platform.In order to solve the difficulty of full aperture and full optical path calibration for large aperture remote sensors,this study has designed a partial aperture solar diffuser calibrator.The calibrator is mainly composed of three important components:a calibration box(CB),a solar diffuser,a solar diffuser reflectance degradation monitor.The calibration box is the main body of the calibrator,which carries the SD and the SDRDM,and makes the calibrator an independent module externally attached to the remote sensor.The interface between the calibration box and the remote sensor is provided with a partial aperture stop to allow the optical path to enter the entrance pupil at the specified aperture.The sun entrance surface is provided with a calibration door to control whether light enters the calibration box.The SD is installed in the calibration box at a specified angle and can reflect sunlight to the entrance pupil of the remote sensor.SDRDM is designed to have dual observation ports for observing SD and the sun,respectively.The dual ports are symmetrical about the normal of a quartz light-diffusing glass inside SDRDM.Both ports are equipped with shutters,which can be opened alternately to realize SDRDM alternate observation.The two-port ratio is called the radiation ratio,and the radiation ratio can be characterized by the angle factor correction to reflect the change in SD reflectivity.The control of the calibration gate keeps the SD in a protected state most of the time,greatly improving the SD life.SDRDM is designed as a two-port symmetrical observation structure,a quartz light-diffusing glass is placed on the integrating sphere parting surface,so that the conditions of the incident light of the two ports being detected in the integrating sphere are nearly consistent.Therefore,when calculating the radiation ratio,many system variables can be eliminated,and the uncertainty introduced by the difference of the incident light angles of the two ports is reduced.In this paper,the orbit data is processed,and the calculation method of the same angle ratio method is proposed according to the characteristics of the geosynchronous orbit.According to the SDRDM monitoring model,the SD on-board state is analyzed.The monitoring uncertainty of this method is less than 0.4%.The monitoring results showed that the quartz soalr diffuser hardly degraded within two years.In order to verify the reliability of the SDRDM monitoring model,this paper proposes the same angle ratio method to verify its monitoring accuracy.This method is based on the same angle monitoring,so it can exclude the influence of the angle factor.The results show that the monitoring deviation of the two methods is less than 0.2%.The key factor that affects the accuracy of SDRDM monitoring is the angle factor of SDRDM observation.This paper proposes a method for calculating the angle factor on orbit.The data shows that the deviation between the laboratory measurement results and the on-board calculation results is less than 0.5%.According to the absolute radiometric calibration model,the F factor of absolute radiometric calibration is calculated.The uncertainty analysis shows that the uncertainty of absolute radiometric calibration is less than 3.6%.