Jichun Tan

Autonomous on-orbit calibration of a star tracker camera

We have developed a calibration approach for a star tracker camera. A modified version of the least-squares iteration algorithm com- bining Kalman filter is put forward, which allows for autonomous on-orbit calibration of the star tracker camera even with nonlinear camera distor- tions. In the calibration approach, the optimal principal point and focal length are achieved at first via the modified algorithm, and then the high- order focal-plane distortions are estimated using the solution of the first step. To validate this proposed calibration approach, the real star catalog and synthetic attitude data are adopted to test its performance. The test results have demonstrated the proposed approach performs well in terms of accuracy, robustness, and performance. It can satisfy the autonomous on-orbit calibration of the star tracker camera. C 2011 Society of Photo-Optical

Novel approach for laboratory calibration of star tracker

An autonomous star tracker is an opto-electronic instrument used to provide the absolute three-axis attitude of a spacecraft utilizing star observations. The precise calibration of the measurement model is crucial, as the performance of the star tracker is highly dependent on the star camera parameters. We focus on proposing a simple and available calibration approach for a star tracker with wide field of view. The star tracker measurement model is described, and a novel approach for laboratory calibration is put forward. This approach is based on a collimator, a two-dimensional adjustable plane mirror, and other ordinary instruments. The calibration procedure consists of two steps: (1) the principal point is estimated using autocollimation adjustment; and (2) the other camera parameters, mainly the principal distance and distortions, are estimated via least-squares iteration, taking into account the extrinsic parameters. To validate this proposed calibration method, simulations with synthetic data are used to quantify its performance considering the errors of the distortion model and calibration data. The theoretical analysis and simulation results indicate that the uncertainties of the measured star direction vectors are less than 4.0×10−5 rad after calibration, and this can be further improved.

Star spot location estimation using Kalman filter for star tracker.

Star pattern recognition and attitude determination accuracy is highly dependent on star spot location accuracy for the star tracker. A star spot location estimation approach with the Kalman filter for a star tracker has been proposed, which consists of three steps. In the proposed approach, the approximate locations of the star spots in successive frames are predicted first; then the measurement star spot locations are achieved by defining a series of small windows around each predictive star spot location. Finally, the star spot locations are updated by the designed Kalman filter. To confirm the proposed star spot location estimation approach, the simulations based on the orbit data of the CHAMP satellite and the real guide star catalog are performed. The simulation results indicate that the proposed approach can filter out noises from the measurements remarkably if the sampling frequency is sufficient.

Angular velocity estimation from measurement vectors of star tracker

In most spacecraft, there is a need to know the crafts angular rate. Approaches with least squares and an adaptive Kalman filter are proposed for estimating the angular rate directly from the star tracker measurements. In these approaches, only knowledge of the vector measurements and sampling interval is required. The designed adaptive Kalman filter can filter out noise without information of the dynamic model and inertia dyadic. To verify the proposed estimation approaches, simulations based on the orbit data of the challenging minisatellite payload (CHAMP) satellite and experimental tests with night-sky observation are performed. Both the simulations and experimental testing results have demonstrated that the proposed approach performs well in terms of accuracy, robustness, and performance.

Detection of TNT in acetone using Raman spectroscopic signature

The detection of explosive agents is becoming more important and receiving much greater emphasis for homeland defense. Raman spectroscopy is a well established tool for vibration spectroscopic analysis and can be applied to the field of explosives identification and detection. The major bands of the Raman spectroscopy of industrial TNT (Trinitrotoluene, CH3C6H2(NO2)3) are analyzed and seven prominent peaks, that is 1616.9cm-1 (C=C aromatic stretching vibration), 1533.9cm-1 (NO2 asymmetric stretching vibration), 1360.1cm-1 (NO2 symmetric stretching vibration ), 1210.5cm-1 (C6H2-C vibration), 822.9cm-1 (nitro-group scissoring mode), 792.3cm-1 (C-H out-of-plane bend), and 326.7cm-1 (framework distortion mode) are used to identify the TNT. The Raman spectroscopes of TNT solved in acetone at different mass ratios are studied, and the TNT in the solution can be detected correctly according the relative distance, intensity, and peak area of the seven peaks. The TNT prominent peaks appear clearly in high level solution (the mass ration of TNT and acetone is more than 1:10). With the decrease of TNT concentration in solution, the signature of TNT becomes more and more weak. The low detection limit of TNT is limited by the noise of the instrument (NXR FT-Raman accessory module with Nicolet 5700 FT-IR spectrometer is used for our experiments. The low detection limit in our experiments is mass ratio 1:200, which is about 4mg/mL). The prominent peak heights are discussed in consideration of the TNT concentration. Taking one of the acetones peaks (1716.9cm-1) as the internal standard line, the relative height of the prominent TNT peaks is almost proportional to the concentration of the TNT in the solution. A fitting curve for the relations of prominent peak height according to the concentration is proposed with multinomial fitting method, which can be used to analyze the concentration of TNT more accurately.

Generation and analysis of both in-phase and out-phase radially polarized femtosecond-pulse beam

Abstract. Both in-phase and out-phase radially polarized femtosecond-pulse (RPFP) beams have been generated with one phase-only liquid crystal spatial light modulator, which effectively modulates the phase retardation distributions of a pulse beam wavefront by two reflections. The intensity distributions and polarizing properties of both in-phase and out-phase RPFP beams are detected, and the temporal properties of in-phase RPFP beams are investigated in detail. Experimental results indicate that we effectively produce an RPFP beam. And the temporal duration of the output in-phase RPFP beam is 183 fs about 14 fs shorter than the input Gaussian femtosecond-pulse beam. The temporal durations of arbitrary polarized components of an in-phase RPFP beam vary less than 3.5%.

Fluorescence reabsorption calculation and influence on solid-state optical cooling

The calculation model of fluorescence reabsorption and reemission with consideration of reflection on the boundary and material size using Monte Carlo method is proposed. To validate this stochastic model, experiments were conducted, and the calculated steady state spectra showed a good agreement with measurements. Using the absorption and fluorescence spectra of Yb-doped phosphate glass by careful measurements and corrections, we calculated the redshift in the observed fluorescence spectra and external quantum efficiency caused by fluorescence reabsorption and re-emission for the samples with the geometries of cylinder and cuboid. The calculation results show that the fluorescence reabsorption and re-emission have significant influence on the cooling efficiency. The calculation results also show that the cylinder with small waist beam incident (the incident light beam diameter is much less than the size of the sample, and goes through the center of the sample) is suitable for optical cooling.

Reconstruction of target image from inhomogeneous degradations through backscattering medium images using self-calibration

Target images recorded with range-gated laser imaging systems and conventional passive imaging systems through rapidly changing turbid mediums inevitably suffer from inhomogeneous degradations. Consequently, this makes the images partly or entirely different from their true targets and eventually has adverse effects on target identification. To date, the inhomogeneous degradations are still not finely eliminable despite utilizing adaptive optical methods and pure mathematical signal improvement techniques. Herein, we demonstrate an image restoration method involving intrinsic physical evolution of light beams based on the backscattering images of a turbid medium. The corresponding mathematical signal processing algorithms are applied for restoring the true target images in the presence of rapidly changing inhomogeneous degradations. This technique would benefit target imaging through moving cloud/mist in air and flowing muddy masses under water.

Analysis of femtosecond optical vortex beam generated by direct wave-front modulation

Abstract. The generation of femtosecond optical vortex beam based on direct wave-front modulation with phase-only liquid crystal spatial light modulator is demonstrated. The spatial and temporal properties of the generated femtosecond vortices are investigated in detail. The experimental results show remarkable agreement with the results of the theoretical analysis and simulations, and indicate that the method we utilized can efficiently generate femtosecond optical vortex beam of arbitrary topological charge. The temporal and spectral properties of the femtosecond pulsed beam are hardly affected by the phase dislocation imposed on the wave-front.

Restoration of longitudinal laser tomography target image from inhomogeneous medium degradation under common conditions

In order to overcome the shortages of the target image restoration method for longitudinal laser tomography using self-calibration, a more general restoration method through backscattering medium images associated with prior parameters is developed for common conditions. The system parameters are extracted from pre-calibration, and the LIDAR ratio is estimated according to the medium types. Assisted by these prior parameters, the degradation caused by inhomogeneous turbid media can be established with the backscattering medium images, which can further be used for removal of the interferences of turbid media. The results of simulations and experiments demonstrate that the proposed image restoration method can effectively eliminate the inhomogeneous interferences of turbid media and achieve exactly the reflectivity distribution of targets behind inhomogeneous turbid media. Furthermore, the restoration method can work beyond the limitation of the previous method that only works well under the conditions of localized turbid attenuations and some types of targets with fairly uniform reflectivity distributions.