KEYWORDS: Instrumentation engineering, Charge-coupled devices, Imaging systems, Signal to noise ratio, Analog electronics, Sensors, Signal processing, Cameras, Polarization, Polarimetry
A CCD imaging system in the Directional Polarimetric Camera is developed, which is comprised of timing driving unit, pro-processing and analog-front-end unit, FPGA main control unit, internal communication and remote measurement unit, image transmission interface unit, and so on. A CCD driving timing method with twice frame transfer and once horizontal readout is proposed to effectively eliminate the residual charge of last frame image and improve the signal-to-noise ratio. The performance of this CCD imaging system is verified. The experimental results show that the 14bit image data can output steadily with a frame frequency of 2.02 frames per second. The imaging signal-to-noise ratio can reach 54.93dB when the CCD is lower than saturated. The dynamic range of the CCD detectable signals is 68.98 dB and the nonlinearity error under different wavelengths is less than 1%. The instrumentation can satisfy the operational requirements such as stable output, excellent linear performance, high signal-to-noise ratio and large dynamic range.
The directional polarimetric camera (DPC) is a satellite sensor with the aim to observe the polarization and directionality of the earth’s reflectance, developed by Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. The analog front-end signal processor is the key to realize the conversion from the analog input signal of the front-end CCD image to the digital processing signal of the back-end. Its performance directly determines the image quality of the instrument. In this paper, the single particle locking effect of the CCD analog front-end signal processor of the Directional Polarization camera after being irradiated by high-energy particles is studied, the test scheme and the test system composition of the single particle locking effect are introduced, and the dynamic working current of the device and the change rule of the dynamic output digital code signal and other parameters are analyzed. The experimental results show that when the LET threshold of high-energy particles is greater than 13.4 MeV•cm2•mg-1, the dynamic working current of the analog front-end signal processor increases significantly, the dynamic output digital code cannot change with the change of the input signal, and single particle locking occurs in the chip. These research results provide significant reference data for in-depth study of single particle locking protection of analog front-end signal processor in space application environment.
This paper studies the effect of the analog front-end signal processor in CCD imaging system on the total ionizing dose radiation effect after 60Co-gamma-ray irradiation, and focuses on the change of the device's static operating current, low-level reference voltage, high-level reference voltage and dynamic output digital code signal. The results show that after certain dose radiation, the static operating current increases significantly with the total dose increase, the high and low reference voltage decreases with the total dose increase, and the dynamic output signal does not correspond to the input signal with the increase of the total dose. The above work can provide reference for in-depth study of radiation reinforcement and radiation damage assessment of analog front-end signal processors in space application environment.
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.
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