# On-Orbit Polarization Calibration for Multichannel Polarimetric Camera
## 2 The Multichannel Polarimetric Camera and Main Polarization Elements
### Main Polarization Parameters of the Polarimetric Camera
From the figures above, we can see that the maximum polarization degree \(\varepsilon_{1}\) for this system is up to 2%, and the biggest phase retardance for this system is less than 0.005\({}^{\circ}\). In this case, the influence of the diattenuation \(\varepsilon_{1}\) for this system cannot be ignored, and the phase retardance for this system can be ignored, which can be indicated by Figure 3.  
The performance of a linear analyzer is described by the extinction ratio E, which is defined by \(E=T_{y}/T_{x}\), where \(T_{x}\) and \(T_{y}\) represent the maximum and minimum transmittance of the two orthogonal directions, respectively [19]. Here, the extinction ratio E can represent the diattenuation of the linear analyzer, because they can be converted into each other [13]. For an ideal analyzer, the extinction ratio E is quite close to zero. But the extinction ratio of this analyzer (Wire Grid Polarizers on Glass Substrates, WP25M-UB, THORLABS) is not high enough, because it is usually made up of a wire grid for a large acceptance angle. For a common wire grid analyzer, it isn't an ideal analyzer as the extinction ratio at visual band is E = 1/200. When we are only concerned about the analyzer, the relative deviation \(\delta(E)\) of polarization detection with reference to an ideal analyzer is calculated as [20]. \[\delta(E)=\frac{2E}{1+E}\times 100\%. \tag{2}\]  
When the extinction ratio is \(E=1/200\), the relative deviation is \(\delta\)(E) = 1%, which also should be calibrated for high accuracy. Figure 3: (**a**) Design model of the pre-optical system; (**b**) analysis result of polarization state at 870 nm; (**c**) analysis result of diattenuation; (**d**) analysis result of phase retardance. The field of view of the optical system is \(2\omega=12^{\circ}\). The polarizer axis angles of the linear analyzers can also influence the accuracy of the polarimetric camera. The accuracy of polarization axis inclination of linear analyzer affects the precision of the polarization camera [21]. It can be calibrated by a computer-aided method. We can see from the above analysis that the main polarization parameters of the camera, which need to be calibrated, are the diattenuation of the pre-optical system (\(\epsilon_{1}\)) and the wire grid analyzer (E). As there is only an on-orbit solar calibration assembly on this satellite for calibration, the polarization calibration method should be newly designed. A new method of on-orbit polarization calibration should be presented to improve the accuracy of the polarimetric camera, which needs no extra device except for the solar calibration assembly. On-Orbit Rotating Analyzer Polarization Calibration Method for the Multichannel Polarimetric Camera  
In this section, we present a novel polarization calibration method. The main purpose is to complete the calibration using the solar calibration assembly. At a 670 nm channel, the instrument measurement accuracy can reach 0.8% after calibration by this method. The on-orbit polarization calibration assembly for the EOSP and the APS is shown in Figure 4.  
We can see from Figure 4 that the main idea of the calibration for the EOSP and the APS is to create polarized light at the entrance pupil of the camera. On the satellite, it's hard to have sufficient space and power for the on-board polarized light source, when we imitate the calibration method of EOSP. As the diameter of the polarizer should cover all the entrance pupil of the camera, it's hard to produce a polarizer with such a large diameter (at least 150 mm), if imitating the calibration method of the APS. As the on-board solar diffuser (see Figure 2) is used for on-orbit radiometric calibration in the satellite, it's quite suitable for polarization calibration. Unlike the method of EOSP and APS, the calibration light of this method is unpolarized light rather than linear polarized light. The polarizing effects of the camera are analyzed by a rotatable analyzer, which is adapted from the built-in fixed linear analyzer.