Sihua Fu

An Adaptive Contoured Window Filter for Interferometric Synthetic Aperture Radar

An adaptive contoured window filter is proposed to filter off the noise from phase images of interferometric synthetic aperture radar (InSAR) in this letter. The contoured windows can best satisfy the requirement that constrains the phase signal constant inside windows on which low-pass filtering can remove the noise well while the fringe phases are well preserved. The contoured windows are determined by tracing along the local fringe orientation. An algorithm for determining window sizes adaptive to the fringe density is also proposed. The theoretical analysis and experiments prove that the proposed filter can greatly remove decorrelation noise while preserving the fringe phase well, even for those fringes with strong curvatures for InSAR processing

Fold-ray videometrics method for the deformation measurement of nonintervisible large structures.

An optical measurement method, the fold-ray videometrics method, that is applicable to the deformation measurement of large structures is proposed. Through an illustration of ship deformation, the principle of fold-ray videometrics and the composition of the deformation measurement system are introduced. The fold-ray videometrics method is able to transfer or relay three-dimensional geometric information with a fold-ray optical path and thus is capable of real-time measurement of three-dimensional positions, attitudes, and deformations between nonintervisible objects and those of intervisible objects with a very large angle of view. The proposed method therefore has the potential to be applied in deformation measurement of large structures.

Two improved algorithms with which to obtain contoured windows for fringe patterns generated by electronic speckle-pattern interferometry

Fringe patterns generated by electronic speckle-pattern interferometry are full of high-spatial-frequency and high-contrast speckle noise. Filtering with contoured windows has proved to be an efficient approach to filtering out speckle noise while retaining the fringe patterns. Furthermore, with contoured windows the contoured correlation fringe pattern method can be used to derive smooth, normalized, consistent fringes. Contoured windows previously were determined by fringe orientation only, and this process generated accumulated errors. We propose two new algorithms with which to obtain the contoured windows according to the fringe intensity slope and the distance ratio to neighboring skeletons. These new techniques can determine contoured windows more precisely.

Attitude-correlated frames approach for a star sensor to improve attitude accuracy under highly dynamic conditions

The attitude accuracy of a star sensor decreases rapidly when star images become motion-blurred under dynamic conditions. Existing techniques concentrate on a single frame of star images to solve this problem and improvements are obtained to a certain extent. An attitude-correlated frames (ACF) approach, which concentrates on the features of the attitude transforms of the adjacent star image frames, is proposed to improve upon the existing techniques. The attitude transforms between different star image frames are measured by the strap-down gyro unit precisely. With the ACF method, a much larger star image frame is obtained through the combination of adjacent frames. As a result, the degradation of attitude accuracy caused by motion-blurring are compensated for. The improvement of the attitude accuracy is approximately proportional to the square root of the number of correlated star image frames. Simulations and experimental results indicate that the ACF approach is effective in removing random noises and improving the attitude determination accuracy of the star sensor under highly dynamic conditions.

Study of a pose-relay videometric method using a parallel camera series

This study proposes a pose-relay videometric method that uses a parallel camera series and is applicable to measuring deformation between nonintervisible and intervisible objects with a very wide angle of view. The measuring system is constructed by adding symmetrical cameras to the pose-relay stations of a single camera measuring system to improve its robustness and precision. An adjustment data fusion method is suggested to take full advantage of the data redundancy among neighboring relay stations in the proposed system. Simulated results show that the adjusted method enhances the measuring precision achieved with the classic weighted average data fusion method owing to its use of the restraint condition inherent in the system.

Generation of speckle-reduced phase images from three complex parts for synthetic aperture radar interferometry

Interferometric synthetic aperture radar (InSAR) has applications in many fields. However, decorrelation speckle noise hinders the wider application of InSAR. In this letter, we propose an innovative method termed contoured correlation interferometry to generate speckle-noise-reduced phase images. This method only needs three part images of the two complex images instead of four for the conventional method, which is a significant advantage if processing synthetic aperture radar images on satellites. It proves an efficient tool that reduces speckle noise while preserving the phase derived, which is one of most difficult problems in InSAR data processing. The proposed method can serve as an alternative to generate better InSAR interferograms, whose physical meaning is clearer.

Coregistration Based on Three Parts of Two Complex Images and Contoured Windows for Synthetic Aperture Radar Interferometry

The coregistration of complex image pairs is a very important step in synthetic aperture radar interferometry (InSAR) data processing. This letter proposes a novel coregistration method that only needs three arbitrary parts of the two complex images instead of four parts in the existing coregistration methods. This method constitutes an integrated three-part method for InSAR data processing with our contoured-correlation-interferometry method for phase-image generation. Saving one part transmission makes a significant advantage when processing SAR images on satellites. Furthermore, we demonstrate that, by means of using fringe contoured windows instead of squared windows, the accuracy of the coregistration for both the three-part coregistration method and the existing methods can be improved considerably

Two-step camera calibration method based on the SPGD algorithm.

Given the rapid convergence characteristic of the stochastic parallel gradient descent (SPGD) algorithm, this study proposes a method that applies the algorithm to a two-step camera calibration method to resolve the frequent iteration and long calibration time deficiencies that exist under the traditional two-step camera calibration method, thereby achieving rapid calibration. The method first uses image coordinates obtained with subpixel positioning technology as initial values of control variables, in addition to positive disturbances produced on a two-dimensional plane, then uses two-step theory to calculate the average value of aberrations. Based on the same rationale, negative disturbances are then produced and the average value of the aberrations is calculated. Finally if, after assessing whether to continue with further iterations based on the difference in these values, continued iterations confirm new control variables based on the SPGD algorithm iteration formula, a new cycle is started until the results satisfy requirements. Theoretical analysis and experimental results show that the proposed rapid calibration method using the SPGD algorithm in the two-step camera calibration method is 3-4 times faster than the traditional two-step calibration method, and that it has significant potential value for use in certain time-constrained projects.

The compensation effects of temperature variation in long-term videometric measurement

Temperature variations will affect the geometric and optical characteristics of the camera and lens in the videometric system, especially when the optical systems are working in a long period. The paper presents an analysis of the image drift caused by temperature variations and the corresponding temperature compensation method. Firstly, image drift model is established based on the pinhole camera model; then two simplified models are introduced by analyzing the coupling relations between the variations of camera parameters and temperature; furthermore, influence of temperature changes on camera parameters were modeled and calibrated; pose estimation experiments as well as temperature compensations are also conducted in deformation measurements, and the results prove the feasibility and efficiency of the proposed models.

Determination of speckle fringe orientation map by image sequence and removing the speckle noise from speckle fringe patterns

Removing speckle noise in electronic speckle pattern interferometry (ESPI) from a single speckle fringe pattern while keeping the fringe features is a difficult problem. The spin filtering with curved surface windows proposed by the authors is successful to filter out speckle noise nearly completely from a single speckle fringe pattern. However the new filtering has a difficulty to be overcome that the speckle fringe orientation map (SFOM) depends on the processing window size which is tryout and is difficult to be derived correctly when the speckle fringe density changes considerably. In this paper we utilize the original speckle pattern sequence with one-beam setup to determine the speckle movement direction field by digital correlation methods so that the SFOM is determined from the direction field. In this way the SFOM can be derived regardless of fringe density.