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Sun L.,Beijing Satellite Navigation Center | Chen J.,Beijing Satellite Navigation Center | Tan S.,Beijing Satellite Navigation Center | Chai Z.,Beijing Institute of Environmental Features
GPS Solutions | Year: 2015

As the Compass satellite system is under construction, various applications are emerging. Multipath mitigation is still a critical problem that must be resolved, especially short delay multipath. Research on multipath mitigation is reported and a multipath limiting antenna array including the algorithm is discussed. The carrier phase data of the receiver are subjected to forward–backward spatial smoothing to decorrelate the signals. The multiple signal classification (MUSIC) algorithm estimates the direction of the satellite signal and the direction of the multipath. Based on the direction of arrival (DOA), the beamforming maximizes the gain in the satellite signal direction and places nulls in the multipath directions. Compared to other beamforming algorithms, the method presented places nulls in the multipath directions and obtains high gain in the satellite direction. At the same time, the phase is not changed during the vector weighting method. The multipath limiting antenna array discussed resolves the problem that the short delay multipath cannot be reduced in signal processing multipath mitigation technology and that in the process, the phase in the direction of the satellite signal is not changed. An extra carrier phase measurement error of the receiver is not introduced, and a phase correction after beamforming is not necessary. The multipath limiting antenna array and the algorithm can provide support for the high-performance satellite navigation system. © 2014, Springer-Verlag Berlin Heidelberg.

Huang J.,Beijing Institute of Environmental Features | Fan X.,Science and Technology on Opticl Radiation Laboratory | Wang J.,Science and Technology on Opticl Radiation Laboratory
Hongwai yu Jiguang Gongcheng/Infrared and Laser Engineering | Year: 2013

Based on the principles of infrared imaging system, a 3D infrared simulation framework was implemented with OSG rendering engine in this paper. It includes the geometric characteristics of the target and background model, the infrared radiation characteristics of the model, atmospheric radioactive transfer model, the detector photoelectric conversion model and the detector noise model. This system can generate the IR scene image varying with the target speed, weather and the location of observer.

Cui Y.,Tsinghua University | Yang J.,Tsinghua University | Zhang X.,Beijing Institute of Environmental Features
Journal of Systems Engineering and Electronics | Year: 2012

A new constant false alarm rate (CFAR) target detector for synthetic aperture radar (SAR) images is developed. For each pixel under test, both the local probability density function (PDF) of the pixel and the clutter PDF in the reference window are estimated by the non-parametric density estimation. The target detector is defined as the mean square error (MSE) distance between the two PDFs. The CFAR detection in SAR images having multiplicative noise is achieved by adaptive kernel bandwidth proportional to the clutter level. In addition, for obtaining a threshold with respect to a given probability of false alarm (PFA), an unsupervised null distribution fitting method with outlier rejection is proposed. The effectiveness of the proposed target detector is demonstrated by the experiment result using the RADATSAT-2 SAR image.

Jinji S.,Beihang University | Dong C.,Beijing Institute of Environmental Features
Acta Astronautica | Year: 2013

This paper analyses the eddy current loss in Homopolar magnetic bearings with laminated rotor cores produced by the high speed rotation in order to reduce the power loss for the aerospace applications. The analytical model of rotational power loss is proposed in Homopolar magnetic bearings with laminated rotor cores considering the magnetic circuit difference between Homopolar and Heteropolar magnetic bearings. Therefore, the eddy current power loss can be calculated accurately using the analytical model by magnetic field solutions according to the distribution of magnetic fields around the pole surface and boundary conditions at the surface of the rotor cores. The measurement method of rotational power loss in Homopolar magnetic bearing is proposed, and the results of the theoretical analysis are verified by experiments in the prototype MSCMG. The experimental results show the correctness of calculation results.

Sun J.,Beihang University | Bai G.,Zhengzhou University | Li L.,Beijing Institute of Environmental Features
Transactions of the Institute of Measurement and Control | Year: 2015

To measure the current stiffness and displacement stiffness of a radial hybrid magnetic bearing, a new stiffness measurement method is proposed for a magnetically suspended flywheel (MSFW). Firstly, the suspension force and stiffness characteristics of the radial hybrid magnetic bearing are studied using magnetic circuit method and finite element method. Secondly, a detailed stiffness measurement method for a radial hybrid magnetic bearing is proposed. It has two procedures: one is the determination of the relationship between the rotor displacement and the corresponding current when the gravity of the rotor is in the +X (or +Y) direction and X (or Y) direction respectively, and the other is the calculation of the current stiffness and the displacement stiffness of the radial hybrid magnetic bearing in an MSFW according to the relationship between the rotor displacement and the current. Three prototype MSFWs with angular moments of 15 Nms are manufactured, and they mainly consist of two axial hybrid magnetic bearings and one radial hybrid magnetic bearing. The proposed stiffness measurement method of the radial hybrid magnetic bearing is verified by all prototype experiments. © SAGE Publications.

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