Mingyu Cong

Scattering near specular direction for horizontally oriented ice discs

Scattering phase function on horizontally oriented ice particles near the specular reflective direction is analytically modeled using a mixed method combining direct reflection and Fraunhofer diffraction components, where particles are simply treated as circular facets and the effect of fluttering is introduced under the assumption of Gauss distribution. The obtained model expression reveals that the essence of far-field scattering around specular direction is the diffraction pattern modulated by fluttered geometric reflection. Four groups of experiments are designed to validate this model at different wavelengths and incidence angles, and the calculated phase functions present good agreement both in distributions and peak values with that of T-matrix method in conjunction with a Monte Carlo stochastic process.

Modeling and simulation for optical sensor imaging in space rendezvous and docking

In order to realize the dynamic imaging simulation for optical sensor in space rendezvous and docking, in this paper, a dynamic imaging modeling and simulation method for optical sensor is presented based on the general idea of surface mesh-creating. The simulation models include geometry and optical characteristics models of space object and star background, imaging model of space based optical sensor, and the noise model during the process of imaging are given out. Taking advantage of these models, dynamic imaging simulation of optical sensor is finally accomplished. For two satellite objects, the imaging simulations are realized with this method in different conditions. Results show that the method could accurately and efficiently simulate the images of optical sensor, so this method could be applied in the development of space rendezvous and docking system.

On-Orbit Calibration Data-Processing Technologies for the Space-Based Infrared Camera

During the period of processing and analysis of on-orbit radiometric calibration data received from a space-based infrared camera launched recently, three practical issues were extracted and resolved at the data level, specifically how to exclude the invalid calibration data; how to determine the appropriate on-orbit decontamination time in the presence of increasing contaminants inside the camera system; and how to calibrate images without suitable calibration coefficients. Three major types of invalid data were summarized according to their appearances and possible causes after analyzing data from many on-orbit calibration tests, and the targeted filtering strategies were proposed with proven excellent performance in practice. A two-term exponential model was established to characterize the observed camera degradation by dividing the corresponding digital number into the blackbody and non-blackbody terms. Based on the model, degradation trends of the camera response, radiance resolution, and signal-to-noise ratio were estimated, respectively, to help determine the contamination tolerance from different aspects. Attempts were made to predict calibration coefficients by the pixelwise degradation models, and then they were applied to image calibration. Results show that the predicted coefficients can effectively compensate the calibration errors due to degradation and can be treated as an alternative if time-matched coefficients are unavailable.

Bad pixel replacement based on spatial statistics for IR sensor

IR focal plane arrays typically contain bad pixels. Bad pixels have to be corrected because those can significantly impair the performance of target-detection algorithms. On the other hand, particularly as an aid to visual interpretation, it is desirable to replace the bad pixels. IR image contains spatial information and is correlative in spatial domain. In spatial statistics the semivariogram is an important function that relates semivariance to sampling lag. This function can characterize the spatial dependence of each point on its neighbor and provide a concise and unbiased description of the scale and pattern of spatial variability. One of the main reasons for deriving semivariogram is to use it in the process of estimation. Kriging is an interpolation and estimation technique that considers both the distance and the degree of variation between known data points when estimating values in unknown areas. In this paper a new technique based on spatial statistics is developed for bad pixel replacement. The main objective of the technique is to replace bad pixels through Kriging estimation. Theory analysis and experiments show that the method is reasonable and efficient.

Modeling and analysis for infrared clutter radiance of atmospheric absorption band sensor

An extended analytic model for atmospheric clutter radiance in absorption bands is developed. In this model, clutter radiance is mainly due to temperature and reflectance fluctuations of atmospheric, cloud and earth. A simplified line-of-sight (LOS) radiance model for short-wave infrared (SWIR) and mid-wave infrared (MWIR) absorption bands is introduced, based on the one-dimensional radiation transfer equation (RTE) and Youngs semi-empirical model for diffuse reflectance of clouds. Under the assumption that atmospheric temperature fluctuations are isotropic horizontally, the relations between clutter radiance and temperature fluctuations as well as other factors are obtained. The clutter radiance characteristics are analyzed in 2.7μm and 4.3μm absorption bands, the long wavelength wing of 2.85μm and 4.35μm shows a much larger clutter contribution from earth and cloud. The models present here are efficient and reasonable by comparing the results of MODTRAN and data from SPIRIT-III radiometer.

Pixel-level analysis of calibration precision for the space-based infrared camera

A space-based infrared camera was launched to collect atmospheric radiation data. In order to investigate its performance quantitatively both under pre-launch and post-launch conditions, a practical estimation model of radiometric calibration precision was proposed that only depended on the measured image data from ground and on-orbit blackbody-based calibration tests. The model treated the calibration error as a consequence of two independent factors. One was introduced by using the calibration equation to represent the relationship between the object apparent radiance and the camera digital response, and the other was the measurement uncertainty when imaging a target with known constant emission. Distribution maps of the errors for the focal plane array were constructed by means of estimating the calibration error pixel-wisely. Results show that the camera’s performance after launch is slightly worse than that before launch. The pixels with calibration errors more than 10% only account for about 5% for this camera, and they generally locate in the edge of the focal plane. The maps will be helpful in weighing the validity of sampled data at the pixel level.

A simulation method of 3D cirrus radiance images for space-based missile warning detectors

High-altitude cirrus clouds are main clutter sources for space-based missile warning in 2.7μm band, their radiance images can serve as an important guide in designing and evaluating early warning systems. First of all, the 3D finite element model of atmosphere (include clouds) was constructed, and its corresponding discrete radiative transfer calculation method was derived as well. Then, the geometry and scattering properties of cirrus were given for the special application in 2.7μm band. With the auxiliary calculation of MODTRAN, the primary and secondary factors in the radiative transfer process were distinguished leading to a simplified radiance calculation means. Finally, a new method was proposed to generate radiance images by integrating source functions in the chief ray pathway of each camera pixel. The simulated results were given and compared with the results of SHDOMPP (Spherical Harmonic Discrete Ordinate Method for Plane-Parallel Atmospheric Radiative Transfer) program, which prove the correct projecting relationship between the 3D cirrus sense and its image on detector focal plane, and validate the rationality and credibility of the proposed method in quantitative radiance calculation.

3D Stochastic cloud generation for performance evaluation of space-based optical system

A three-dimensional (3D) stochastic cloud generation architecture is developed to simulate radiative scenes for performance evaluation of space-based optical system. The rescale-and-add fractal algorithm is employed to generate internal and external structure data of clouds. The Spherical Harmonic Discrete Ordinate (SHDOM) code is selected to solve the 3D radiative transfer equation numerically. The flowchart of the simulation system is shown. The methods for generating 3D grid data of cloud spatial structure and liquid water content (LWC), as well as the radiative quantities, are presented. Two types of clouds are generated as illustrations. Radiative cloud scenes under different imaging conditions could be conveniently available using this framework. The simulation results demonstrate the effectiveness of the proposed methods and architecture.

Tracking algorithm design and precision analysis of space object measure for orbiting optical camera

The evolution of technology made it possible to place elements of the space surveillance network in space, include electro-optical systems, which will effectively make up the limitation of ground-based space surveillance system in existence. The orbiting optical camera will play more and more important roles in the activities of surveillance and measurement of space object. There are two operation modes in the track & measurement of interested space object using orbiting optical camera, which are autonomous tracking mode and program tracking mode. If using program tracking mode, it is necessary to analyze the precision of the tracking program when designing the program tracking algorithm, in order to keep the object in the field of view during the tracking process. The productions of this paper include putting forward the design flow of orientation tracking algorithm and calculating method of orientation tracking angle for the orbiting optical camera, analyzing the dominant random errors influencing the precision of tracking program, and deducing the error transfer function through the calculating formula of program tracking angle. The conclusion is obtained based on analyzing the calculating result of a simulating instance using the error transfer model deduced, which is the relative distance, attitude control error of the measure satellite and orbit prediction error of the space object tracked are the dominant factors influencing the precision of tracking program.

Application of Spatial Statistics for IR Background Suppression

Background suppression is an effective method for extracting the signal of target in infrared remote sensing image. Background clutter contains spatial information and is correlative in spatial domain. In spatial statistics the semivariogram is an important function that relates semivariance to sampling lag. This function can characterize the spatial dependence of each point on its neighbor and provide a concise and unbiased description of the scale and pattern of spatial variability. One of the main reasons for deriving semivariogram is to use it in the process of estimation. Kriging is an interpolation and estimation technique that considers both the distance and the degree of variation between known data points when estimating values in unknown areas. A kriged estimate is a weighted linear combination of the known sample values around the point to be estimated. In this paper a new algorithm based on spatial statistics is developed for IR background suppression. The main objective of the algorithms is to suppress background clutter through Kriging estimation. Theory analysis and experiments show that the method is reasonable and efficient.