# On-Orbit Polarization Calibration for Multichannel Polarimetric Camera
## 1 Introduction  
The polarimetric remote sensing camera is an optical detection system, which is designed for the polarimetric imaging of the target. It can obtain the degree and the angle of linear polarization, not just the radiance. The polarization information has a multitude of applications in many fields, including agriculture, military reconnaissance, meteorological environment monitoring and other fields [1; 2; 3]. At present, most of the detection instruments used for polarization remote sensing are the first-generation polarization detection systems. The types of these systems include the division-of-time (DoT), the division-of-amplitude (DoAM), the division-of-aperture (DoAP) and the division-of-focal plane (DoFP) [4; 5]. However, each type has its own weakness, such as the calibration problem of the types of DoAM and DoAP [6], the imaging field of view (IFOV) errors of the type of DoFP [7; 8]. However, the polarizing effects caused by the polarimetric camera itself will affect the accuracy of polarization detection. In addition, the polarizing effects are changed by the optical structure,material and so on. For the above reasons, the polarization characteristics of polarized cameras operating in space will be different from those in laboratory calibration. Therefore, the instrument should make the on-orbit polarization calibration at regular intervals to ensure that the detection accuracy is not reduced. This paper intends to provide an on-orbit polarization calibration method for a multichannel polarimetric camera with DoAM. This method has many technological advantages, such as a high maturity and stability, wide spectral range, and large field of view. At present, the commonly used on-orbit calibration methods include the scene calibration, the on-board polarized light source method and the spaceborne linear polarization method. The Polarization and Directionality of the Earth's Reflectance (POLDER) and the Directional Polarized Camera (DPC) has adopted the scene calibration [9; 10]. When the polarimetric camera observes the atmosphere in a specific angle, the light reflected by the atmosphere is close to the unpolarized light. The camera uses this source as a calibration source for polarization calibration. However, it is hard to meet the requirement of obtaining unpolarized light when observing the atmosphere in reality. Even if the scene calibration method is a simple and easy to operate method, the calibration accuracy is low. The on-board polarized light source method (see Figure 3) has the highest precision, which is used by the Earth Observing Scanning Polarimeter (EOSP) [11]. The advantage of this method is that the polarization calibration can be taken in real time. However, one of the problems of this method is that the system lifetime is restricted by the polarized light source. The other problem is that the volume and power cannot meet the satellite constrains. The spaceborne linear polarization method which has been adopted in Aerosol Polarimetry Sensor (APS) can solve the above problems [12]. When the polarimetric camera observes the atmosphere in a specific angle, the on-board polarizer will create polarized light which can be used for the calibration. The accuracy is high enough, and the lifetime of the on-board polarizer is as long as the satellite. The main problem of this method is that the polarizer should cover all fields of view for the optical system, which means that the polarizing plate will be very large. All of the methods above have some inherent problems. Therefore, it is necessary to study a new on-orbit polarization calibration method with high accuracy, small volume and reliable performance. In this paper, we propose a new on-orbit polarization calibration method, which is called the on-orbit rotating analyzer polarization calibration method. The calibration source is the unpolarized light created by an on-board solar diffuser which can be shared with the on-orbit radiometric calibration.