Quaternion versus Rotation Matrix Feedback for Spacecraft Attitude Stabilization Using Magnetorquers

The purpose of this paper is to compare performances between stabilization algorithms of quaternion plus attitude rate feedback and rotation matrix plus attitude rate feedback for an Earth-pointing spacecraft with magnetorquers as the only torque actuators. From a mathematical point of view, an important difference between the two stabilizing laws is that only quaternion feedback can exhibit an undesired behavior known as the unwinding phenomenon. A numerical case study is considered, and two Monte Carlo campaigns are carried out: one in nominal conditions and one in perturbed conditions. It turns out that quaternion feedback compares more favorably in terms of the speed of convergence in both campaigns, and it requires less energy in perturbed conditions.

Efficient SAR Azimuth Ambiguity Reduction in Coastal Waters Using a Simple Rotation Matrix: The Case Study of the Northern Coast of Jeju Island

Azimuth ambiguities, or ghosts on SAR images, represent one of the main obstacles for SAR applications involving coastal monitoring activities such as ship detection. While most previous methods based on azimuth antenna pattern and direct filtering are effective for azimuth ambiguity suppression, they may not be effective for fast cruising small ships. This paper proposes a unique approach for the reduction of azimuth ambiguities or ghosts in SAR single-look complex (SLC) images using a simple rotation matrix. It exploits the fact that the signal powers of azimuth ambiguities are concentrated on narrow bands, while those of vessels or other true ground targets are dispersed over broad bands. Through sub-aperture processing and simple axis rotation, it is possible to concentrate the dispersed energy of vessels onto a single axis while the ghost signal powers are dispersed onto three different axes. Then, the azimuth ambiguities can be easily suppressed by a simple calculation of weighted sum and difference, while preserving vessels. Applied results achieved by processing TerrSAR-X SLC images are provided and discussed. An optimum weight of 0.5 was determined by Receiver Operating Characteristic (ROC) analysis. Capabilities of ship detection from the test image were significantly improved by removing 93% of false alarms. Application results demonstrate its high performance of ghost suppression. This method can be employed as a pre-processing tool of SAR images for ship detection in coastal waters.

A New Criterion for Bounded Component Analysis

<div>In this paper we present a new criterion for bounded component analysis, a quite new approach for the Blind Source Separation problem. For the determined case, we show that the `1-norm of the estimated sources can be used as a contrast for the problem. We present a blind algorithm for the source separation of independents sources or mixtures of correlated sources by only a rotation matrix. We also present a variety of simulations assessing the performance of the proposed approach.</div>

A New Criterion for Bounded Component Analysis

<div>In this paper we present a new criterion for bounded component analysis, a quite new approach for the Blind Source Separation problem. For the determined case, we show that the `1-norm of the estimated sources can be used as a contrast for the problem. We present a blind algorithm for the source separation of independents sources or mixtures of correlated sources by only a rotation matrix. We also present a variety of simulations assessing the performance of the proposed approach.</div>

Vector Strategy of Formation of the Compensating Current of Shunt Active Filter In Sliding Mode

A space-vector strategy for controlling of a three-phase shunt active power filter (SAPF), which is connected to a utility grid in which there is a nonlinear load, is proposed. The filter consists of a semiconductor voltage inverter with fully controlled switches, a capacitive storage and a single-link output RL filter. The DC voltage of the storage capacitor is stabilized by a second-order sliding mode, which is based on the implementation of a finite double-twisting algorithm. To form the compensation current, an indirect control strategy and a space-vector algorithm for tracking the instantaneous value of the current of the "parallelogram" type are used. Space-vector method avoids ambiguity in the choice of vectors of the output voltage of the inverter under the solution of the problem of current tracking. The proposed algorithm uses the methods of projective geometry to determine the relative position of the spatial vectors of voltages and currents of the power supply system, so it does not contain the difficult mathematical operations for calculating inverse trigonometric functions. The selection of the appropriate output voltage vector of the SAPF inverter on the location of the end of the current error vector relative to some switching lines is based. Switching lines from the projections of the two-dimensional sliding surface on the plane α-β are formed. These projections are subject to further linear transformation, the matrix of which is non-degenerate. Linear transformation of projections of these sliding surfaces includes sequential operations of rotation at a fixed angle, parallel movement and application of the rotation matrix. This rotation matrix has constant coefficients, which change abruply at some points in time. The classical method of equivalent control is applied. The transition from one group of coefficients to another occurs based on the spatial location of the equivalent control vector, the coordinates of which are calculated in real time. The choice of the rotation matrix provides unambiguous determination of the spatial position and the choice of the corresponding output voltage vector of the SAPF inverter. A simulation model of the power supply system with a two-dimensional sliding mode and taking into account the features that are inherent in the real-time sliding mode is built, and the simulation results are analyzed. The "switching freeze" method to eliminate chattering was used. The proposed strategy with the traditional three-channels hysteresis algorithm of the compensation current tracking according to the criteria of the number of switches is compared.

Separation of Split Shear Waves in Two Non-orthogonal Sets of Vertical Fractures

The phenomenon of S-wave splitting indicates the development of fractures in the shallow crust. Therefore, methods based on S-wave splitting have been established to predict the development of one set of parallel fractures. However, for rocks containing two non-orthogonal sets of vertical fractures, the mechanism of S-wave splitting is more complex, and the available methods cannot be applied. To resolve this inadequacy, we propose a two-way rotation method to separate split S-waves with the aim of restoring the split S-wave polarizations and predicting the fracture azimuths. First, we calculate the stiffness matrix of fractured media based on the linear slip theory and derive the phase velocities and polarizations of split S-waves induced by fractures using the Christoffel equation. Second, we clarify the S-wave splitting mechanism in this media by employing velocity analysis and deconstruct the S-wave polarizations on the horizontal components. Third, we deduce a two-way rotation matrix obtained by the S-wave splitting modes to separate the split S-waves. To solve for the angle parameters related to the fracture azimuths in the two-way rotation matrix, we superpose the subspace polarizations in two dimensions to determine the polarization azimuths of the split S-waves. Numerical model tests demonstrate that the proposed method is stable under noisy conditions. Finally, we apply the proposed method to real near-offset and walkaround VSP data, and the predicted fracture results are verified by imaging logs and prior knowledge.