Pre-flight calibration of a multi-angle polarimetric satellite sensor directional polarimetric camera

Abstract

The directional polarimetric camera (DPC), developed by Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Science, is a satellite sensor used to observe the polarization and directionality of the earth’s reflectance. It acquires the two-dimensional image of the earth with a large field of view (118.74°) and a high spatial resolution (3.3 km) in 8 spectral bands. The first DPC was successfully launched onboard the GaoFen-5 satellite in May 2018, subject to the Chinese high-resolution earth observation program. In this paper, a set of systematic and complete pre-flight calibrations of the DPC are proposed to ensure the effective characterization for in-flight calibration, so as to ensure the accuracy of DPC measured radiation polarization data and the reliability of inversion results. Since the geometric calibration method of the DPC has been presented in an early companion paper [Appl. Opt. 59 226 (2020)], this paper will not introduce it in detail. Instead, the geometric calibration results of each spectral band together with a discussion on the origin of differences between spectral bands are analyzed, and the error analysis of the method is conducted. The results of the DPC geometric calibration is that the residuals of all spectral bands are less than 0.1 pixel. For radiometric calibration, the radiometric models of non-polarized bands and polarized bands are derived in detail, respectively, and the specific calibration methods with error analysis, equipment, and main results with their related accuracies for each parameter of the radiometric models are described. To verify the accuracy of calibration parameters, a series of polarization detection accuracy verification experiments based on a non-polarized radiation source, a polarizing system, and a natural scene were carried out. The experimental results show that the maximum deviation of degree of polarization between the set values of the polarizing system and measured values of the DPC at the corresponding positions of four field of view angles of 0, 15, 30, and 45 degrees of each polarized spectral band is 0.009, 0.004, and 0.003, respectively. The average error in measuring the degree of polarization of a non-polarized light source by all pixels in the three polarized bands is 0.0043, 0.0046, and 0.0037, respectively. And the relative deviations of each field of view are within 0.020 when the DPC and CE318N simultaneously measure the DoLP of sky. All of these prove the effectiveness of the pre-flight calibration.

Analysis and correction of environmental differences in geometric performance of spaceborne optical instruments

“…The optical system of DPC is shown in Fig. 1 [16][17][18] , which consists of an objective lens, polarizer&filter wheel, and area array Charge Coupled Device (CCD) detector. The objective lens has the characteristics of wide FOV and telecentric in image space.…”

In-Flight Relative Radiometric Calibration of a Wide Field of View Directional Polarimetric Camera Based on the Rayleigh Scattering over Ocean

“…(3) The CCD sensor array is composed of 512 × 512 effective pixels from 544 × 512 useful pixels, realizing a high spatial resolution of 3.3 km at the nadir of the sun-synchronous orbit. Due to the design of the wide two-dimensional FOV with along-track migration at satellite velocity, multi-angular viewing of the same surface target in more than 9 angles is possible due to the over-riding of the successive images along the satellite track [19,20]. The intensity of light on the pixel is linearly correlated to the output digital number (DN) value.…”

“…Systematic and complete preflight radiometric calibration of DPC would be proposed before launch. A two-dimensional turntable and integrating sphere are taken to calibrate the relative radiometric variation in the laboratory with an accuracy of 0.3%, and the nominal absolute calibration accuracy is 5% pre-launch [19].…”

“…The relative radiometric calibration of DPC is defined as estimating the inconsistency sensitivity at different positions of the FOV to the same incident radiation, which are dimensionless functions related to the spectral band [19]. The relative response calibration usually consists in having the instrument look at a spatially uniform source, which can be an integrating sphere in the laboratory or uniform natural targets on the earth.…”

Research on automatic measurement of photoelectric performance for scientific-grade CCD detector

“…The cloud and aerosol polarization camera (CAPC) is a satellite sensor for the characterization of ultra-wide angle and low-distortion imaging [7][8][9] . The CCD detector is the key device of photo-electric conversion for CAPC, which is utilized for acquiring multi-angle and multi-spectral radiation information.…”