Target detection for low angle radar based on multi-frequency order-statistics

For radar targets flying at low altitude, multiple pathways produce fade or enhancement relative to the level that would be expected in a free-space environment. In this paper, a new detec- tion method based on a wide-ranging multi-frequency radar for low angle targets is proposed. Sequential transmitting multiple pulses with different frequencies are first applied to decorrelate the cohe- rence of the direct and reflected echoes. After receiving all echoes, the multi-frequency samples are arranged in a sort descending ac- cording to the amplitude. Some high amplitude echoes in the same range cell are accumulated to improve the signal-to-noise ratio and the optimal number of high amplitude echoes is analyzed and given by experiments. Finally, simulation results are presented to verify the effectiveness of the method.

Modified GIT model for predicting wind-speed behavior of low-grazing-angle radar sea clutter

A modified GIT model for describing the variational trend of mean clutter reflectivity as a function of wind speed is proposed. It uses two slope adjustment factors and two critical wind-speed factors to define and adjust the increasing slope of reflectivity with respect to wind speed. In addition, it uses a constant factor to compensate the overall amplitude of clutter reflectivity. The performance of the modified GIT model has been verified on the basis of the L-band low-grazing-angle radar sea clutter data. The results are in good agreement with the experimental data, indicating that the model is more effective in predicting the wind-speed behavior of clutter reflectivity than the conventional GIT model, especially for lower and higher wind speeds. We believe that the proposed model can provide deeper insights into the relationship between radar sea clutter reflectivity and sea state conditions.

Stability of High Slope Interbedded Strata with Low Dip Angle Constituted by Soft and Hard Rock Mass

Slopes consisting of interbedded strata of soft and hard rock mass, such as purplish red mudstone and grey brown arkosic sandstone of Jurassic age, are very common in Sichuan basin of China. The mudstone is soft while the sandstone is hard and contains many opening or closing joints with a high dip angle. Some are nearly parallel and the others are nearly decussated with the trend of the slopes. Many natural slopes are in deformation or sliding because of those reasons. The stability of cutting slopes and supporting method to be taken for their stability in civil engineering are important. In this paper, the stability and deformation of the slopes are studied. The methods of analysis and support design principle are analyzed also. Finally, the method put forward is applied to study Fengdian high cutting slope in Sichuan section of the express way from Chengdu to Shanghai. The results indicate that the method is effective.

Low velocity zone of upper mantle and its effect on PdSwr phase related to 670 kmdiscontinuity

This paper studied some properties of PdSwr phase related to 670 km discontinuities in detail, and theoretically processed a preliminary analysis to this phase. We discussed the relationships between the incident angle ih of PdSwr phase with its path, epicentral distance, travel-time and relative amplitude due to low velocity zone (LVZ) of upper mantle, and preliminarily pointed out the main characters of PdSwr phase recorded in seismogram. The PdSwr phase is concentrated in range of 13.5~96.5. When epicentral distance is greater than 33, the start point of PdSwr phase is relatively well distinguishable and could thus be determined more easily. When the epicentral distance is between 13.5 to 33, the triplication of PdSwr's travel-time curve could be slightly distinguished due to the low velocity zone and 220 km seismic velocity discontirluity of upper mantle. The relevant observed PdSwr phase should be in a more complex pattern and it should be more difficult to determine its start point

Improving the strength and SCC resistance of an Al-5Mg-3Zn alloy with low-angle grain boundary structure

The strength of traditional Al-Mg alloys is relatively low because it mainly relies on solid solution strengthening.Adding a third component to form precipitation can improve their strength,but it usually leads to high-stress corrosion cracking(SCC)sensitivity due to the formation of high-density precipitates at grain boundaries(GBs).So far,it is still challenging to improve the strength of Al-Mg alloys without re-ducing SCC resistance.Herein,a nanostructured Al-5Mg-3 Zn alloy with a good yield strength of 336 MPa and good elongation was successfully produced.By dynamic plastic deformation and appropriate anneal-ing treatment,near-equiaxed nanograins were introduced in the nanostructured Al-5Mg-3 Zn alloy with a high proportion(71%)of the low-angle grain boundary.TEM statistical investigations show that the pre-cipitation of active T’phase at GBs has been greatly suppressed in the nanostructured Al-5Mg-3 Zn alloy at sensitized conditions,and the area fraction of GB precipitates is reduced from 72%to 21%,which sig-nificantly decreases the SCC susceptibility.This study provides guidance for developing advanced Al-Mg alloy with high SCC resistance.

Overview: Electromagnetic Scattering from Ocean Surface

Understanding the sea surface scattering process is very important in the development of models to detect the target above or under the surface. In this paper, both the analytical and the numerical methods applied in sea surface scattering are summarized. Some important problems concerned in this field are discussed. For numerical study, edge effect brings artificial nonrealistic scattering and therefore must be suppressed. Different edge treatment methods are compared in this paper. Scattering of breaking wave surface at very low grazing angle always needs more attentions than other scattering problems. Some numerical results show the existence of the special phenomena at very low grazing angle, for example, the ＂sea spikes＂ and the Doppler splitting.

Graded index profiles and loss-induced single-mode characteristics in vertical-cavity surface-emitting lasers with petal-shape holey structure

The 850-nm oxide-confined vertical-cavity surface-emitting lasers with petal-shape holey structures are presented. An area-weighted average refractive index model is given to analyse their effective index profiles, and the graded index distribution in the holey region is demonstrated. The index step between the optical aperture and the holey region is obtained which is related merely to the etching depth. Four types of holey vertical-cavity surface-emitting lasers with different parameters are fabricated as well as the conventional oxide-confined vertical-cavity surface-emitting laser. Compared with the conventional oxide-confined vertical-cavity surface-emitting laser without etched holes, the holey vertical-cavity surface-emitting laser possesses an improved beam quality due to its graded index distribution, but has a lower output power, higher threshold current and lower slope efficiency. With the hole number increased, the holey vertical-cavity surface-emitting laser can realize the single-mode operation throughout the entire current range, and reduces the beam divergence further. The loss mechanism is used to explain the single-mode characteristic, and the reduced beam divergence is attributed to the shallow etching. High coupling efficiency of 86% to a multi-mode fibre is achieved for the single-mode device in the experiment.

Microstructural evolution and oxidation inα/βtitanium alloy under fretting fatigue loading

Coupling effects of fretting wear and cyclic stress could result in significant fatigue strength degradation,thus potentially causing unanticipated catastrophic fractures.The underlying mechanism of microstructural evolutions caused by fretting wear is ambiguous,which obstructs the understanding of fretting fatigue issues,and is unable to guarantee the reliability of structures for long-term operation.Here,fretting wear studies were performed to understand the microstructural evolution and oxidation behavior of anα/βtitanium alloy up to 108 cycles.Contact surface degradation is mainly caused by surface oxidation and the generation of wear debris during fretting wear within the slip zone.The grain size in the topmost nanostructured layer could be refined to~40 nm.The grain refinement process involves the initial grain rotation,the formation of low angle grain boundary(LAGB;2°–5°),the in-situ increments of the misorientation angle,and the final subdivision,which have been unraveled to feature the evolution in dislocation morphologies from slip lines to tangles and arrays.The formation of hetero microstructures regarding the nonequilibrium high angle grain boundary(HAGB)and dislocation arrays gives rise to more oxygen diffusion pathways in the topmost nanostructured layer,thus resulting in the formation of cracking interface to separate the oxidation zone and the adjoining nanostructured domain driven by tribological fatigue stress.Eventually,it facilitates surface degradation and the formation of catastrophic fractures.

Comparative New Insight into the Tectonic Origin of Folds and Thrust Faults of an Extensional Basin: Soke-Kusadasi Basin, Aegean, Western Turkey

The Aegean area of the western Anatolian region of Turkey,controlled by the low-angle detachment normal fault system,forms an extensional province,the West Anatolian Extensional Province(WAEP).The tectonic deformation which occurred in the Miocene Period,including the Plio–Quaternary Period has created different structures in both the basement rocks and intra-basin deposits of the crust.One of these structures,high-angle normal faults,controls the supradetachment Soke-Kusadasi Basin(SKB).Within this basin,there are folds with different axes and thrust faults with a north-northwestnortheast(N,NW,NE)trend.These folds and thrust faults in the SKB deformed the sedimentary structures of intra-basin deposits.The folds and thrust faults,which caused the rotation of beddings and imbrications in the SKB,are mainly associated with the tectonic process of the low angle detachment normal fault,which affected the SKB and the Aegean part of western Anatolia.In the SKB,during the process of extensional deformation associated with primary low angle detachment normal faulting,the ramp-flat and inversion geometry observed in the basement rocks and basin deposits of the crust caused folds and thrust faults in only intra-basin deposits.In the WAEP,it is determined for the first time that the folds and thrust faults causing limited shortening deformed the Plio–Quaternary sediments.