The Experts below are selected from a list of 61971 Experts worldwide ranked by ideXlab platform
Lawrence W. Townsend - One of the best experts on this subject based on the ideXlab platform.
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Radiation protection guidance for activities in low-Earth Orbit
Advances in Space Research, 2002Co-Authors: Lawrence W. Townsend, R. J.m. FryAbstract:Scientific Committee 75 (SC 75) of the National Council on Radiation Protection and Measurements (NCRP) was assembled for the purpose of providing guidance to NASA concerning radiation protection in low-Earth Orbit. The report of SC 75 was published in December 2000 as NCRP Report No. 132. In this presentation an overview of the findings and recommendations of the committee report will be presented. © 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
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radiation protection guidance for activities in low Earth Orbit
Advances in Space Research, 2002Co-Authors: Lawrence W. TownsendAbstract:Abstract Scientific Committee 75 (SC 75) of the National Council on Radiation Protection and Measurements (NCRP) was assembled for the purpose of providing guidance to NASA concerning radiation protection in low-Earth Orbit. The report of SC 75 was published in December 2000 as NCRP Report No. 132. In this presentation an overview of the findings and recommendations of the committee report will be presented.
Chibiao Ding - One of the best experts on this subject based on the ideXlab platform.
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Medium-Earth-Orbit SAR Focusing Using Range Doppler Algorithm With Integrated Two-Step Azimuth Perturbation
IEEE Geoscience and Remote Sensing Letters, 2015Co-Authors: Lijia Huang, Donghui Hu, Chibiao DingAbstract:Existing low-Earth-Orbit synthetic aperture radar (SAR) algorithms generally assume that the data are azimuth invariant. However, this assumption does not hold for the medium-Earth-Orbit (MEO) SAR systems due to the significantly longer azimuth integration time and complex imaging geometries. As a result, the MEO SAR data cannot be processed accurately and efficiently using the existing algorithms. To solve this problem, this letter proposes a two-step azimuth perturbation (AP) method that uses the first-step AP to remove the bulk azimuth variance at the range processing stage and the second-step AP to remove the residual variance at the azimuth processing stage. As an example, an improved range Doppler algorithm with the integrated two-step AP is discussed in this letter. Simulations of an L-band MEO SAR with 5-m resolution at 10 000-km Orbit height are used to demonstrate the validity and accuracy of this algorithm.
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focusing of medium Earth Orbit sar with advanced nonlinear chirp scaling algorithm
IEEE Transactions on Geoscience and Remote Sensing, 2011Co-Authors: Lijia Huang, Donghui Hu, Chibiao DingAbstract:The signal processing of the medium-Earth-Orbit synthetic aperture radar (SAR) is more challenging than that of the current low-Earth-Orbit SAR because the imaging geometry is more complicated, and the range and azimuth variances are more severe. This paper deals with these imaging problems in three aspects. First, an advanced hyperbolic range equation (AHRE) is proposed for the first time, which is more precise for a spaceborne SAR than the conventional hyperbolic range equation (CHRE). Second, the point target spectrum based on the AHRE is analytically derived, which is useful for developing efficient SAR processing algorithms. Third, the well-known nonlinear chirp scaling (NLCS) algorithm is modified according to this new spectrum, and the so-called AHRE-based advanced NLCS (A-NLCS) algorithm is established. The simulation results validate the correctness of our method for L-band SAR systems at altitudes from 1000 to 10000 km with an azimuth resolution around 3 m. It is also shown that the A-NLCS algorithm has better performance than the CHRE-based algorithms in longer integration time cases. Therefore, we recommend the A-NLCS algorithm for a spaceborne SAR with a lower frequency, finer resolution, and higher satellite altitude.
Lijia Huang - One of the best experts on this subject based on the ideXlab platform.
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Medium-Earth-Orbit SAR Focusing Using Range Doppler Algorithm With Integrated Two-Step Azimuth Perturbation
IEEE Geoscience and Remote Sensing Letters, 2015Co-Authors: Lijia Huang, Donghui Hu, Chibiao DingAbstract:Existing low-Earth-Orbit synthetic aperture radar (SAR) algorithms generally assume that the data are azimuth invariant. However, this assumption does not hold for the medium-Earth-Orbit (MEO) SAR systems due to the significantly longer azimuth integration time and complex imaging geometries. As a result, the MEO SAR data cannot be processed accurately and efficiently using the existing algorithms. To solve this problem, this letter proposes a two-step azimuth perturbation (AP) method that uses the first-step AP to remove the bulk azimuth variance at the range processing stage and the second-step AP to remove the residual variance at the azimuth processing stage. As an example, an improved range Doppler algorithm with the integrated two-step AP is discussed in this letter. Simulations of an L-band MEO SAR with 5-m resolution at 10 000-km Orbit height are used to demonstrate the validity and accuracy of this algorithm.
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focusing of medium Earth Orbit sar with advanced nonlinear chirp scaling algorithm
IEEE Transactions on Geoscience and Remote Sensing, 2011Co-Authors: Lijia Huang, Donghui Hu, Chibiao DingAbstract:The signal processing of the medium-Earth-Orbit synthetic aperture radar (SAR) is more challenging than that of the current low-Earth-Orbit SAR because the imaging geometry is more complicated, and the range and azimuth variances are more severe. This paper deals with these imaging problems in three aspects. First, an advanced hyperbolic range equation (AHRE) is proposed for the first time, which is more precise for a spaceborne SAR than the conventional hyperbolic range equation (CHRE). Second, the point target spectrum based on the AHRE is analytically derived, which is useful for developing efficient SAR processing algorithms. Third, the well-known nonlinear chirp scaling (NLCS) algorithm is modified according to this new spectrum, and the so-called AHRE-based advanced NLCS (A-NLCS) algorithm is established. The simulation results validate the correctness of our method for L-band SAR systems at altitudes from 1000 to 10000 km with an azimuth resolution around 3 m. It is also shown that the A-NLCS algorithm has better performance than the CHRE-based algorithms in longer integration time cases. Therefore, we recommend the A-NLCS algorithm for a spaceborne SAR with a lower frequency, finer resolution, and higher satellite altitude.
R. J.m. Fry - One of the best experts on this subject based on the ideXlab platform.
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Radiation protection guidance for activities in low-Earth Orbit
Advances in Space Research, 2002Co-Authors: Lawrence W. Townsend, R. J.m. FryAbstract:Scientific Committee 75 (SC 75) of the National Council on Radiation Protection and Measurements (NCRP) was assembled for the purpose of providing guidance to NASA concerning radiation protection in low-Earth Orbit. The report of SC 75 was published in December 2000 as NCRP Report No. 132. In this presentation an overview of the findings and recommendations of the committee report will be presented. © 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
Yebin Gong - One of the best experts on this subject based on the ideXlab platform.
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Performance study of low Earth-Orbit satellite systems
IEEE Transactions on Communications, 1994Co-Authors: Aura Ganz, Yebin Gong, Bo LiAbstract:Considerable interest has been focusing on the possibility of employing a large number of low Earth Orbit (LEG) satellites to provide an overall global coverage. However, there has been no performance study of such systems. The authors investigate the performance of low Earth Orbit-satellite systems in terms of the system capacity, the average number of beam-to-beam handoffs and satellite-to-satellite handoffs the channel occupancy distribution and the average call drop probability. These performance measures are obtained as a function of: (1) the network constellation, (2) the satellite speed, (3) the cell size, and (4) the average transaction duration. These metrics are exemplified on the Iridium system. The results derived provide useful and simple tools for system design as well as for system analysis. >
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Performance study of low Earth-Orbit satellite systems
Proceedings of ICC '93 - IEEE International Conference on Communications, 1993Co-Authors: Aura Ganz, Bo Li, Yebin GongAbstract:The performance of low-Earth-Orbit satellite systems is investigated in terms of the average number of beam-to-beam handoffs, the average number of satellite-to-satellite handoffs, the channel occupancy distribution, and the average call drop probability. These performance measures are obtained as a function of: 1) the network constellation; 2) the satellite speed; 3) the cell size; and 4) the average transaction duration. These metrics are illustrated on the Iridium system. The results derived provide useful and simple tools for system design, as well as for system analysis.
