2019 Vol. 39, No. 4

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Research Articles
Numerical Simulation for Solar Wind Background by Entropy Conservation Scheme
WANG Tenglong, FENG Xueshang, LI Caixia, LIU Xiaojing
2019, 39(4): 417-431. doi: 10.11728/cjss2019.04.417
Background solar wind is a key factor for interplanetary disturbance propagation. Magnetohydrodynamic (MHD) simulation is an important tool for background solar wind study. In this paper an entropy conservation scheme is adopted. Ideal GLM-MHD is used to handle the magnetic divergence. The divergence of the magnetic field generated during the calculation is propagated outside the calculation domain with the maximum characteristic speed of MHD system. With the analytical divergence-free condition of magnetic field as additional constraint condition, the reconstruction of solution variables uses the constrained least squares method. The reconstructed magnetic field gradient is further modified by the way of iteration. The flux calculation adopts an entropy conservation scheme which satisfies the second law of thermodynamics. This formulation can ensure that the entropy does not increase in the calculation process, and the numerical stability can be guaranteed. The results show that numerical simulation for solar wind background by entropy conservation scheme can obtain more stable results.
Automatical Scaling of Es Layer Echoes on Ionograms ormalsize
WANG Sheng, ZHOU Chen, JIANG Chunhua, YANG Guobin, ZHAO Zhengyu
2019, 39(4): 432-441. doi: 10.11728/cjss2019.04.432
Sporadic E (Es) and its echoes have a big impact on the measurement and inversion of the F-layer echoes, and it also affect the short-wave communication. This paper proposed a method based on decision tree algorithm to automatically identify Es layer and its echoes on ionograms. First, the ionogram was preprocessed by adaptive binarization and median filtering algorithms, the effective Es echo region was extracted from ionograms. Moreover, with characteristics of Es layer echoes on ionograms, the features were selected by image projection method. The features of Es layer on the projection values of the virtual height and the occurrence time of the Es layer were used as input of the decision tree algorithm to construct the decision tree. And manually labeled tag on ionograms and compare it with the output of the decision tree algorithm. Then trained to get closer to the results of manual labeled. In the present study, ionograms recorded at Pu'er Station (22.7°N, 101.5°E) in Yunnan province were used to construct the decision tree.the ionograms of Yunnan Pu'er Station (22.7°N, 101.5°E) and Sichuan Leshan Station (29.5°N, 103.7°E) were used as test sets respectively. The method was tested and verified. That method has high accuracy for the identification of Es two-hop echoes in both Pu'er and Leshan stations, reaching 84.2% and 82.8% respectively.
Spherical Calibration Method on Atmospheric Density Model
REN Tingling, MIAO Juan, LIU Siqing, WANG Hong, CAO Yong
2019, 39(4): 442-448. doi: 10.11728/cjss2019.04.442
Making calibration on the atmospheric model density is an important way to improve the accuracy of atmospheric model. By using the atmospheric density data derived from the high-accuracy accelerometer onboard satellite CHAMP, a calibration on NRLMSISE-00 model is developed with the spherical method. Firstly, the atmospheric density is assimilated to the same altitude, then the calibration and prediction error are calculated based on the calibration results. Finally, the atmospheric density of the next 3 days are forcasted. The results show that the calibration and prediction errors are reduced significantly after the spherical calibration. The calibration error is reduced to about 10% during high solar activity year, and the prediction accuracy is improved by 31.34%, 21.39%, 13.75% for the next 3 days respectively. The calibration error is reduced to about 14% during low solar activity year, and the prediction accuracy is improved by 55.03%, 47.79%, 43.60% for the next 3 days respectively.
Simulation on the Mountain Wave and Its Propagation Generated by Terrain
WEI Jiarui, LIU Xiao, XU Jiyao
2019, 39(4): 449-459. doi: 10.11728/cjss2019.04.449
Mountain waves generated as airflow pass the terrains are one of the important classes of atmospheric gravity waves. Based on the equations of controlling atmospheric motion, a two-dimensional nonlinear numerical model is established to simulate the mountain waves generation and its propagation process. Using the relationship among the horizontal background wind, the terrain and the vertical velocity, a vertical velocity disturbance is introduced into the model to act as the source of the mountain waves. The entire processes of generation, propagation, and fully development of mountain waves are reproduced by use of our model. The aspects of the horizontal wavelength, the vertical wavelength, the potential temperature disturbance and the streamlines are analyzed to describe the characteristics of the mountain wave different stages. During the propagation of the mountain waves, the horizontal wavelength λx ranges from 2.5 to 5km, and the vertical wavelength λz is about 2.5km. The results are consistent with the calculations by linear theory. The analyses illustrate that the model can successfully simulate the mountain wave generated by the terrains. The study is helpful for understanding the generation and the propagation of mountain waves, as well as their effects on the middle and upper atmosphere.
Predictions of the Single Event Upset in Space Applicationormalsize
CHEN Ao, CHEN Hongfei, XIANG Hongwen, YU Xiangqian, SHI Weihong, SHAO Sipei, ZOU Hong, ZOU Jiqing, ZHONG Weiying
2019, 39(4): 460-468. doi: 10.11728/cjss2019.04.460
According to the heavy-ion LET spectra measured by 3 PREMs onboard 2 satellites of SSO and several devices' σ-LET curves which obey the Weibull distribution, the SEU-rate caused by heavy ions in space is estimated. The prediction of SEU-rate and the measured values are compared, and the influencing factors of SEU-rate are analyzed. The position and facing direction of the device are important factors for SEU-rate. Devices located at the end of the satellite and facing to anti-forward (-x) direction have higher SEU-rate than those located at the bottom of the satellite and facing to ground. Apart from that, the SEU-rate during the maximum of the solar activity is higher than that during solar ebb. Both above can attribute to the intensity and range of the differential LET spectra of heavy ions in space. Above the SEU threshold of the devices, the stronger the intensity of LET spectrum is, the higher the SEU-rate will be. And the harder the LET spectrum is, the higher the SEU-rate will be. The SEU-rate will differ according to different device.
Numerical Simulation of Bubble Dynamics and Heat Transfer during Pool Boiling in Microgravity
YI Tianhao, CHEN Chaoyue, LEI Zuosheng, ZHAO Jianfu
2019, 39(4): 469-477. doi: 10.11728/cjss2019.04.469
A two-dimensional model of the single bubble is performed. The surface tension and Marangoni force are included in the momentum equation. In addition, the continuity equation and energy equation are modified to allow for the phase change. A thin superheated layer is considered in the numerical model. The vapor-liquid interface is captured by the phase field method. The variations of bubble dynamics and heat transfer during subcooling pool boiling in microgravity can be obtained by solving the coupled equations. The results show that the bubble changes from a hemisphere to an ellipsoid and eventually becomes a pear shape. The bubble can randomly move on the surface. Besides, the bubble exhibits non-axisymmetrical shape during growth period. Due to the effect of fluid flow, the temperature field above the bubble appears to be a mushroom shape. The bubble departure diameter and departure time are proportional to g-0.488 and g-1.113 respectively. The average heat flux on the heater surface is proportional to g0.229.
Cortical Microtubule Reorientation and Its Relation to Cell Surface Texture of Epidermal Cells of Arabidopsis Thaliana Hypocotyls under Simulated Microgravity Conditions
XIE Junyan, ZHANG Yue, ZHENG Huiqiong
2019, 39(4): 478-488. doi: 10.11728/cjss2019.04.478
Gravitropic curvature growth of Arabidopsis hypocotyls mainly occurred in the rapid growing Elongation Zone (EZI), not in the slow-growing Elongation Zone (EZⅡ). By examining reorientation of Microtubules (MT) and phenotype of the cell wall in the EZI and the EZⅡ of Arabidopsis hypocotyls under normal gravitational condition, it is found that MTs in the rapid growing epidermal cells were mainly in the transverse direction, while those in the non-growing epidermal cells were in the longitudinal directions. However, this difference in cortical MT arrays between the EZI and EZⅡ cells disappeared when the seedlings were exposed to the simulated microgravity condition on a horizontal clinostat. Field emission scanning electron microscopy revealed that the surface texture of epidermal cells, like the direction of the MT, in the EZI and the EZⅡ also became similar when exposed to the simulated microgravity condition. This result indicated that simulate microgravity could modify the potential differentiation between the EZI and the EZⅡ by affecting the orientation of cortical MT in the epidermal cells.
Research on Transfer Orbit Based on Electric Propulsion Satellite to Halo Orbit
WANG Shen, XIONG Shujie
2019, 39(4): 489-493. doi: 10.11728/cjss2019.04.489
Energy-saving optimization was achieved by using the characteristics of low-thrust electric propulsion and the orbital dynamics. The transfer orbit of the spacecraft from the Earth parking orbit to the Sun-Earth L2 point Halo orbit has been studied by combining the invariant manifold in the restrictive three-body problem with the low thrust trajectory optimization. Firstly, the transfer orbit is divided into burn and coast phases. Tangential acceleration was used to decrease Jacobi integration constant rapidly, and then Radau pseudo spectral method was adopted in the burn phases to optimize the orbit so that the spacecraft could reach the stable invariant manifold of the target orbit. When the spacecraft reached stable invariant manifold, it could coast to target halo orbit. The initial transfer orbit was guessed by Jacobi integration constant. Finally, a simulation example of transfer orbit was given to verify the effectiveness of the method. The results show that it is of guiding significance to practical engineering application.
Optimal Design of Constellation Configuration for Irregular Imaging Area Coverage Based on Improved Simulated Annealing Algorithm
YU Yanjun, WANG Feng, MIAO Yue
2019, 39(4): 494-501. doi: 10.11728/cjss2019.04.494
The requirements of Earth observation through satellites is increasing in recent years. An optimization design method of constellation configuration for irregular imaging area coverage is proposed. For a single point target, an improved simulated annealing algorithm is used for constellations configuration design which can satisfy the revisiting time requirement. The optimizing objectives are minimum number of satellites and the requirement of revisiting time. In order to meet the requirements of observing a large and irregular shape area using less satellites and with a specific revisiting time, a new algorithm which is based on the equal-area grid point coverage method as well as the improved simulated annealing algorithm is proposed. The new algorithm can be used to optimize the constellation configuration and realize imaging irregular area targets with full coverage. The influence of target illumination condition is analyzed. A comparison is made between the optimal results ignoring illumination and with illumination. Taking some specific area as examples, the simulations are performed to prove the feasibility of the algorithm. The results show that the optimal constellation configuration for the designated area with certain constraints can be obtained by using the algorithm.
Calculation of Accuracy and Integrity Parameters Based on WAAS Monitoring Data
NI Yude, ZHANG Tianyang
2019, 39(4): 502-511. doi: 10.11728/cjss2019.04.502
The Wide Area Augmentation System (WAAS) is a Satellite-based Augmentation System (SBAS) developed by the United States. It broadcasts correction information in the form of message and enhances the Required Navigation Performance (RNP) for the Global Positioning System (GPS). WAAS supports most of the airspace in its service area from the RNP of route to Localizer Performance with Vertical Guidance (LPV)-200 approach. The process of decoding and the parameter extraction of WAAS message is given. The accuracy and integrity parameters in RNP indicators are analyzed, and the algorithms of accuracy and integrity indicators are described. The measured data of 34 sites in Alaska and the Continental United States are selected to calculate the accuracy and the integrity parameters. The results show that the accuracy and integrity calculation of the selected sites are basically consistent with the statistical data released by the National Satellite Test Bed (NSTB). It verifies the correctness of the message decoding and the parameter extraction algorithm, and provides reference for the construction of China's Beidou Satellite-based Augmentation System (BDSBAS).
Real-time Precise Point Positioning Ambiguity Resolution Based on Phase Biasesormalsize
ZHAO Shuang, YANG Li, ZHU Enhui, GAO Yao
2019, 39(4): 512-519. doi: 10.11728/cjss2019.04.512
The real-time phase biases products released by CNES are used to realize real-time precise point positioning with ambiguity resolution. Firstly, the timing characteristics of real-time phase biases products are analyzed, which have high continuity and stability. Then the RTPPP solution is performed on the 10-day observation data of 10 IGS stations, and the first fixed time and positioning accuracy of the ambiguity are calculated respectively. It is concluded that the real-time phase biases product can achieve the first fixed ambiguity within an average time of 30 minutes. When the ambiguity is fixed, the horizontal position error can be quickly reduced from 6 cm to 2cm, and the three-dimensional position error can be quickly reduced from 10cm to about 5cm. At the same time, as the RTPPP ambiguity resolution keeps the result in 3h measurement, the positioning accuracy is about 3cm in 2D and 5cm in 3D. The results show that the real-time PPP ambiguity resolution technology based on phase biases has high positioning accuracy and stability, and can realize centimeter-level positioning quickly. It can promote the application of real-time phase biases products in real-time precise point positioning.
Moving Point Target Detection Based onHigh Frame-rate Image Sequence ormalsize
NIU Wenlong, WU Yong, YANG Zhen, ZHENG Wei, LIU Bo
2019, 39(4): 520-529. doi: 10.11728/cjss2019.04.520
A high frame-rate based framework is presented to detect moving point target in very low SNR. A novel target detector based on higher order statistics is proposed to analyze the time domain evolution of visual image sequence for distinguishing the background and target. Our method is formulated to detect a time-domain transient signal and the bispectrum is used to characterize the temporal behavior of pixels. The method is evaluated using both simulated and real-world high frame-rate data, and a comparison to other widely used point target detection approaches is provided. Experimental results demonstrate that the proposed framework can be used for robust moving point target detection in very low SNR.
Approach to Suppress Range Sidelobe by the Point Target Response
LAN Ailan, YAN Jingye, ZHAO Fei, WU Lin
2019, 39(4): 530-536. doi: 10.11728/cjss2019.04.530
Range sidelobes are induced by pulse compression, which may contaminate effective signals. In order to reduce the impact from range sidelobes, an approach based on deconvolution is proposed. In this method, the radar system response is assumed to be known. In practice, a point (or quasi-point) target response can be used as system response. Therefore, the point target response of a radar system must be obtained firstly. In this paper, how to pick up a quasi-point target response from a large number of echoes as the system response is proposed. Then process echoes by deconvolution to eliminate the impact of range sidelobes. In order to prove the validity of the sidelobe suppression method, Hainan Coherent Phased Array Radar (HCOPAR) is taken as an example to show how to suppress range sidelobes by the point target response. And the results show that range sidelobes can be decreased about 5dB, which illustrate the ability of the method. Moreover, compared to the typical methods, this approach is simpler and easier to apply.
Atmospheric Correction Method for Ground-based Headwall Hyperspectral Imagery
WU Xiaoming
2019, 39(4): 537-543. doi: 10.11728/cjss2019.04.537
Atmospheric correction of remote sensing imagery is one of the prerequisites and difficulties of the quantitative remote sensing research. There are many methods and models for atmospheric correction. Several common methods of atmospheric correction, including the FLAASH model, 6S model, ELC method, and QUAC method etc are discussed in this paper. These atmospheric correction methods are applied to Headwall hyperspectral imagery. The apparent reflectance, corrected reflectance, and the measured reflectance of four kinds of typical objects in the study area are compared. The result shows that these four models can effectively remove the influence of the atmosphere, and it can basically restore the representative characteristics of typical objects spectrum. The performance of the ELC method is superior to the other methods in the atmospheric correction of ground-based Headwall hyperspectral imagery.
Space-to-space Radio Interferometry System from Medium Earth Orbits
2019, 39(4): 544-562. doi: 10.11728/cjss2019.04.544
This paper presents a new concept to perform space-to-space Very Long Baseline Interferometry which enables the imaging of cosmic sources at high-resolution and high-sensitivity with small antennas. Several individual apertures are embarked on separate identical satellites staggered in height into Polar or Equatorial Circular Medium Earth Orbits (PECMEO orbits). These orbits are stable and allow GNSS-based on-the-fly centimeter-level relative positioning. Coherent operation is possible by exchanging local oscillator components and measured signals through Inter-Satellite Links (ISL). On-board cross correlation is performed at each satellite over a delay window compatible with the accuracy of the on-the-fly relative positioning and the result sent to the ground. Image reconstruction is completed on the ground thanks to sub-millimeter baseline retrieval from accurate GNSS orbits, ISL ranging and spacecraft attitude information. The application of this concept to image the Super Massive Black Hole Sgr A* is hinted.