The performance of attitude determination and control system using
sun-sensor/magnetometer/gyro as attitude sensors would be impacted by the remanence of
micro-satellite. To reduce the influences of remanence, a two-step Extended Kalman Filter (EKF)
method is developed to estimate the steady magnetometer bias and residual magnetic moment.
Firstly, the magnetometer bias is estimated. Then the magnetometer is calibrated with the
estimation of magnetometer bias, and the corrected measurement is employed to estimate the
attitude as well as the residual magnetic moment. Simulation results indicate that the accuracy of
attitude determination is about 1° and the accuracy of magnetometer bias and magnetic moment
estimation is about 10nT and 0.03A•m2. The impact of remanence can be observably eliminated
via the two-step EKF method.
In this paper, an improved second generation digital image watermarking scheme is proposed. This scheme exploits the region feature instead of point or line feature. The region features are retrieved by watershed transform, which allows watermark recovery after common attacks. The experiments have shown that this proposed scheme is robust against compression, noise intrinsically and more robust against geometrical attacks and JPEG compression compared with
Kutter’s method. The watermark capacity is improved because the robustness of region feature is more than point or line feature.
Multiple digital watermarking is attracting more and more researchers because it is more valuable in the practical applications than single watermarking. In this paper, a multiple watermarking algorithm based on 1-D and 2-D chaotic sequences is proposed. The chaotic sequences have the advantages of massive, high security, and weakest correlation. The massive and independent digital watermark signals are generated through 1-D chaotic maps, which are determined by different initial conditions and parameters. The chaotic digital watermark signals effectively resolve the construction of massive watermarks with good performance. The embedding of multiple watermakrs is more complex than the single watermarking scheme. In this paper, each watermark is added to the middle frequency coefficients of wavelet domain randomly by exploiting 2-D chaotic system, so the embedding and extracting of each watermark would not disturb each other. Considering the parameters of 2-D chaotic systsem as the key to embedding procedure can prevent the watermarks to be removed maliciously, therefore the performance of security is better. The capacity of the multiple watermarking is also analyzed in this paper. The experimental results demonstrate that this proposed watermarking algorithm is robust to many common attacks and it is a reliable copyright protection for multiple legal owners.