Spectral modulation polarization technology is a snapshot spectral polarization measurement technique without moving parts, offering high accuracy. Anhui Institute of Optics and Fine Mechanics has developed a prototype Ultraviolet-visible Imaging Spectropolarimeter. Prototype consists of a spectral modulation module, a telescope, and an Offner spectrometer, enabling dual-channel polarization imaging. The spectral modulation module uses Wollaston prisms for beam splitting, which has the disadvantage of asymmetric splitting angles, reducing the matching accuracy of the dual-channel polarized images. Therefore, it is necessary to analyze the imaging asymmetry caused by the prism wedge angle and determine the range for the wedge angle. To converge the dual-channel polarized beams after Wollaston prisms at the spectrometer slit, a telescope with F-number of 5 was designed. To verify the dual-channel imaging performance of the system, an overall analysis was conducted with the spectrometer, ensuring a modulation transfer function greater than 0.4 for each spectral and the specifications met the design requirements.
A high-resolution simultaneous polarization imager based on off-axis three-mirror telescopic objective detects polarization information to enhance contrast and identify target. Stray light can increase system noise, reduce image contrast, and affect imaging quality. It is necessary to analyze and suppress stray light in order to ensure the polarization measurement accuracy of the instrument. The three-dimensional solid model and optical properties of the instrument were established by stray light analysis software Tracepro. The stray light of channel 1 was simulated and analyzed. The important surface of the system and the first-order stray light paths were found through forward trace and backward trace. The main baffle and vane, secondary mirror baffle and third mirror light barrier were designed by using CAD drawing software. Then the PST (Point Source Transmittance) curve of stray light is given under off-axis angle is within ± 50 °. When the off-axis angle is 30 °, PST is less than 10-10. The VGI (Veiling Glare Index) is 1.47%, which was obtained by fitting the PST curve and integrating. The influence of the VGI on the polarization accuracy was 0.020@p=0.3. The results show that the stray light suppression system have obvious effects, and the influence on the polarization accuracy is acceptable.
The reasonable design of the dynamic range of spaceborne remote sensor is the basic guarantee for its effective observation in-orbit, but the complex radiation characteristics of the earth-atmosphere system and other factors have brought some1 challenges to its design. Aiming at the Polarized Scanning Atmospheric Corrector (PSAC) on board Huanjing-2A/B(HJ-2A/B) satellite, the dynamic range of apparent radiance observed by PSAC is analyzed and obtained according to its detection target and band setting, based on the method of statistics of historical data of remote sensors such as Moderate-resolution Imaging Spectroradiometer (MODIS), considering the complex radiation characteristics of earth atmosphere system and scale effect. In order to meet the requirements of dynamic range design and ensure better radiation resolution, PSAC designed four gear adjustable electronic gain (relative gain is 1,2,3,4), and carried out the system radiation response gain adjustment and dynamic range test under laboratory conditions. The statistical results of on orbit observation data of PSAC show that the design results of its dynamic range meet the needs of on-orbit observation application, and lay a foundation for the retrieval of atmospheric and aerosol parameters and the realization of atmospheric correction objectives.
Data preprocessing of the Polarized Scanning Atmospheric Corrector (PSAC) onboard HuanjingJianzai-2(HJ-2)A/B satellites is a key step for further applications. Based on the principles and characteristics of PSAC sensor, this paper elaborates on the methods and procedures of its data preprocessing including parameters quality supervision, data precorrection, calibration implementation and geolocation, etc. The results show that the data preprocessing from the original data to the L1 product is accurate and effective after preliminary analysis and evaluation, which can be used for subsequent atmospheric parameters retrieval and atmospheric correction applications.
The Off-axis Three-mirror Simultaneous Imaging Polarimeter (OTSIP) is a kind of polarimetric remote sensor with high spatial resolution. In OTSIP, simultaneous measurements were performed by means of prism dividing amplitude. Due to various equipped polarizers and complex polarimetric characteristics of OTSIP, its instrument matrix will deviate from the ideal value. In order to ensure the polarimetric accuracy of OTSIP, the development of an efficient polarimetric calibration is indispensable. In this paper, a calibration method using a standard linear polarization light source and circular polarization light source was proposed. The first three columns of the instrument matrix were firstly calibrated by a linear polarimetric calibration source to obtain the calibration coefficients via the least-squares fitting algorithm, and then the fourth column of the instrument matrix was calibrated by a circular polarimetric calibration source. Moreover, the nonideality of circular polarization state light was significantly improved by averaging measured results at 0 and 90° azimuths. As for the full field of view polarization calibration, a linear fitting method to each element of the instrument matrixes at multiple field of view angles was used. The resulting polarimetric measurement accuracy showed that the linear and circular polarization measurement accuracy was better than 1% (DOP<=0.3), validating the effectiveness and feasibility of this polarimetric calibration method. This method greatly improves the calibration efficiency of the OTSIP, making it possible to calibrate the polarimeter in flight.
Particulate Observing Scanning Polarimeter (POSP) is a spaceborne spectropolarimetric sensor to map radiometric and polarimetric information of global atmospheric particulate matter. POSP has three SWIR bands, respectively are 1.38μm, 1.61μm and 2.25μm. So the SWIR polarimetric system is designed, which has the characteristic of division of aperture and division of amplitude to realize the simultaneous polarization measurement. The system has two identical optical paths, their orientation is 45 degree, and it can measure first three Stokes parameters. In order to realize to cool the InGaAs detectors below -60°C, combined with active cooler and passive cooler, the InGaAs detectors assembly and cooling system are integrated compact. The thermal vacuum test shows that cooling system could meet the cooling requirements of InGaAs detectors.
In order to meet the application requirements of a space borne polarizing radiometer infrared band, a high-precision on-orbit temperature control scheme for the infrared detector combining active temperature control and passive temperature control is proposed. The infrared detector is installed on the heat sink copper block, and the temperature of heat sink copper block is controlled at -20°C~-30°C through the method of auxiliary cold plate + heat pipe thermal conduction. Combined with the infrared detector built-in three-level thermoelectric cooler, the photosensitive surface temperature of the infrared detector is cooled to below -60°C by a method of constant current driving. In order to ensure the measurement accuracy of infrared radiation polarization, the short-term temperature fluctuation of the photosensitive surface of the infrared detector is required to be less than 0.03°C/s. This article has designed the infrared detector temperature control scheme verification test, and actually measured the stability of infrared detector temperature and dark current. The results of the simulation and tests show that the range of infrared detector heat sink temperature is - 25±5°C, the range of infrared detector photosensitive surface temperature is -65°C ~ -75°C,the rate of short-term temperature change of the infrared detector photo-sensitive surface is better than 0.01°C/s, and the dark current fluctuation is less than 1.3pA. Satisfying the on-orbit high-precision polarization measurement requirements.
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