Environment-2 (HJ-2) A/B satellites will be launched in 2020, which are expected to work as the successors of Environment-1 (HJ-1) satellites in Chinese Environment and Disaster Monitoring and Prediction Satellite Constellation. A new space-borne instrument called Polarized Scanning Atmospheric Corrector (PSAC) also will be onboard HJ-2 satellites, aiming to provide the atmospheric properties for synchronous atmospheric correction of the main sensors, such as the charge-coupled device cameras onboard the same satellite. PSAC is a cross-track scanning polarimeter with polarized channels from near-ultraviolet to shortwave infrared, centered in 410, 443, 555, 670, 865, 910, 1380, 1610 and 2250 nm. In order to test the performance of inversion algorithms and software modules, synthetic data simulated by the vector radiative transfer is indispensable. In this paper, the regional simulation of PSAC multispectral measurements are preliminarily studied, and the Unified Linearized Vector Radiative Transfer Model (UNL-VRTM) has been used as the forward model. For the observation geometries, the viewing zenith angles are calculated by the linear interpolation over the cross-track scanning angle range from west to east, while the viewing azimuth angle are simulated by following the azimuth angle distribution of other corresponding satellite. By taking the vegetated surface type as an example, the multispectral Lambertian surface reflectance and wavelength-independent BPDF model are used in the forward simulation, and different aerosol optical depth with fine-dominated and coarse-dominated aerosols are considered. In this way, the multispectral measurements can be obtained by the forward simulations over a regional grid with the predefined latitude and longitude, and further analysis are carried out based on the synthetic data. Thus, this study can provide key support to the testbed of inversion algorithms and software modules before and after the satellite launch.
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.
To obtain high accuracy polarization observation information is highly expecting in aerosol parameter retrieval and atmosphere environment research. In this paper, we described a space-borne multispectral polarized scanning atmospheric corrector (PSAC), which can provide extremely high polarimetric accuracy. And we use wavelet-based denoising method to improve the stability of PSAC measurement results by reducing internal noise. The result shows that the STD of DOLP difference between PSAC measurement value and theoretical value is significantly reduced, reached 5%~10% in the requirement DOLP difference accuracy 0.5%, the proposed threshold function can increase the evaluation accuracy stability of instrument polarization and reduce the uncertainty, also, the measurement results of PSAC inflight improved as well.