Meteor radar temperatures over Collm (51.3°N, 13°E)

The method of obtaining absolute temperature estimates by measuring the ambipolar diffusion coefficient with meteor radars in the mesopause region is basically well known. However, there is still a need to refine and adjust the background temperature gradient model which is necessary to calculate the temperature values. Therefore, a detailed comparison with independent temperature measurements is necessary to evaluate the performance of the method and to obtain more information about the temperature gradient. Recent studies provide some evidence that the impact of the gradient model on temperature estimates affects the absolute temperatures, but that it is of minor importance for wave analysis. This paper focuses on a detailed evaluation of the meteor radar temperatures by comparing them with SABER satellite and OH-emission mesopause region temperatures. The seasonal variation of the observed temperatures is well reproduced by the COMMA general circulation model.     

隐身卫星的现状及其在空间攻防中的应用

空间攻防是空间技术的一个主要研究内容。隐身卫星正以独特的优势在空间对抗领域中发挥重要作用。首先论述了卫星隐身的主要手段,其次分析了国内外隐身卫星的发展现状,最后详细介绍了隐身卫星在未来空间攻防中的应用前景。     

Detection of regolith buried water stream channels on Mars with the help of synthetic aperture radar

A major theme in the study of Mars is the search for evidence that water was present in the past or is present today, either at or below the surface. Biological life is connected to water. Hence much research is focused on the detection of water stream channels, which in the past flowed on Mars. In these areas, the petrified remains of the former life on Mars may be found. These channels may be under the regolith layer; however, the radio wave penetrating ability allows for the detection of these channels under the regolith.     

NOVEL RADAR SIGNAL SORTING METHOD BASED ON GEOMETRIC COVERING

With the increase of complexity of electromagnetic environment and continuous appearance of advanced system radars, signals received by radar reconnaissance receivers become even more intensive and complex. There- fore, traditional radar sorting methods based on neural network algorithms and support vector machine （SVM） cannot process them effectively. Aiming at solving this problem, a novel radar signal sorting method based on the cloud model theory and the geometric covering algorithm is proposed. By applying the geometric covering algo- rithm to divide input signals into different covering domains based on their distribution characteristics, the method can overcome a typical problem that it is easy for traditional sorting algorithms to fall into the local extrema due to the use of complex nonlinear equation to describe input signals. The method uses the cloud model to describe the membership degree between signals to be sorted and their covering domains, thus it avoids the disadvantage that traditional sorting methods based on hard clustering cannot deinterleave the signal samples with overlapped param- eters. Experimental results show that the presented method can effectively sort advanced system radar signals with overlapped parameters in complex electromagnetic environment.     

Transponder calibration of the Envisat RA-2 altimeter Ku band sigma naught

Although the history of spaceborne altimeters goes back to the early seventies, the absolute calibration of the backscattering coefficient has never been deeply investigated. This information has been primarily used to infer the wind speed via an empirical model, and the intercalibration among different satellite altimeters has revealed to be suitable for this purpose, being the wind retrieval based on an empirical relationship. As far as Ku band system is concerned, the sigma naught absolute calibration of the Envisat altimeter (RA-2) has been performed using an active reference target provided by a transponder. This has been exploited during the 6-month Commissioning phase to generate early calibration results. In order to monitor the RA-2 backscatter calibration during the Envisat lifetime, a continuous calibration effort has been carried out by operating the transponder as much as possible. This paper aims to review the entire effort for calibrating the RA-2 sigma naught measurements, which lasted for almost seven years. It presents in detail the adopted methodology and the final outcome of the activity, providing the users with the correction (bias) to get the calibrated sigma naught and analyzing its stability during almost the entire Envisat lifetime. Specifically, it is concluded that the RA-2 backscatter measurements were quite stable, even if a bias of about 1 dB should be considered with respect to the actually released product. Some small changes in the bias as function of time can be identified during most of the Envisat lifetime, consisting in a slight increase in the first two years, followed by a more stable period and a final drop observed at the end of 2009, until the conclusion of the calibration activity (corresponding to the change in Envisat orbit).     

An overview of radar soundings of the martian ionosphere from the Mars Express spacecraft

The Mars Express spacecraft carries a low-frequency radar called MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) that is designed to study the subsurface and ionosphere of Mars. In this paper, we give an overview of the ionospheric sounding results after approximately one year of operation in orbit around Mars. Several types of ionospheric echoes are commonly observed. These include vertical echoes caused by specular reflection from the horizontally stratified ionosphere; echoes from a second layer in the topside ionosphere, possibly associated with O⁺ ions; oblique echoes from upward bulges in the ionosphere; and a variety of other echoes that are poorly understood. The vertical echoes provide electron density profiles that are in reasonable agreement with the Chapman photo-equilibrium model of planetary ionospheres. On the dayside of Mars the maximum electron density is approximately 2 × 10⁵ cm⁻³. On the nightside the echoes are often very diffuse and highly irregular, with maximum electron densities less than 10⁴ cm⁻³. Surface reflections are sometimes observed in the same frequency range as the diffuse echoes, suggesting that small isolated holes exist in the nightside ionosphere, possibly similar to those that occur on the nightside of Venus. The oblique echoes arise from upward bulges in the ionosphere in regions where the crustal magnetic field of Mars is strong and nearly vertical. The bulges tend to be elongated in the horizontal direction and located in regions between oppositely directed arch-like structures in the crustal magnetic field. The nearly vertical magnetic field lines in the region between the arches are thought to connect into the solar wind, thereby allowing solar wind electrons to heat the lower levels of the ionosphere, with an attendant increase in the scale height and electron density.     

半主动雷达寻的空空导弹作战技术运用研究

导弹的攻击区是衡量导弹的攻击能力的最重要的指标之一。通过仿真计算得出不同情况下半主动雷达寻的空空导弹的攻击区。具体分析了不同因素对攻击区的影响，以此为依据，讨论了在战术上如何运用该导弹，给出了充分发挥导弹作战能力的有效途径。可为飞行员使用该种导弹进行空空作战提供正确的理论指导和参考。     

ANALYSIS OF THE RETURNED SIGNAL MODEL IN BISTATIC RADAR SYSTEMS

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Bayesian edge detector for SAR imagery using discontinuity-adaptive Markov random feld modeling

Synthetic aperture radar（SAR）image is severely affected by multiplicative speckle noise,which greatly complicates the edge detection.In this paper,by incorporating the discontinuityadaptive Markov random feld（DAMRF）and maximum a posteriori（MAP）estimation criterion into edge detection,a Bayesian edge detector for SAR imagery is accordingly developed.In the proposed detector,the DAMRF is used as the a priori distribution of the local mean reflectivity,and a maximum a posteriori estimation of it is thus obtained by maximizing the posteriori energy using gradient-descent method.Four normalized ratios constructed in different directions are computed,based on which two edge strength maps（ESMs）are formed.The fnal edge detection result is achieved by fusing the results of two thresholded ESMs.The experimental results with synthetic and real SAR images show that the proposed detector could effciently detect edges in SAR images,and achieve better performance than two popular detectors in terms of Pratt＇s fgure of merit and visual evaluation in most cases.     

基于多载频MIMO雷达的Radon-Fourier变换盲速旁瓣抑制

 长时间相干积累方法Radon-Fourier变换(RFT)中盲速旁瓣(BSSL)现象会导致雷达虚警增加、目标检测性能降低。针对BSSL问题,给出了一种基于多输入多输出(MIMO)雷达多载频设计的BSSL抑制方法。首先根据载频与BSSL分布的关系,详细推导了BSSL不交叠的约束条件;然后基于该约束条件给出了具体的载频设计公式。利用设计的载频可得到具有不交叠BSSL的两个RFT输出,通过联合处理这两个RFT输出,可实现BSSL抑制。给出了BSSL抑制性能的评价方法。理论分析和数值实验结果表明,本文算法能够在不降低RFT相干积累性能的同时,有效实现BSSL抑制。