SAR Tomography with Improved Non-Local Means Filtering Based on Adaptive Window

In order to mitigate speckle noise in synthetic aperture radar(SAR)images and enhance the accuracy of SAR tomography,non-local means(NL-means)filtering has been proven to be an effective method for improving the quality of SAR interferograms.Apart from considerations like noise type and the definition of similarity,the size and shape of filtering windows are critical factors influencing the efficacy of NL-means filtering,yet there has been limited research on this aspect.This paper introduces an enhanced NL-means filtering method based on adaptive windows,allowing for the automatic adjustment of filtering window size according to the amplitude information of the SAR interferogram.Simultaneously,a directional window is incorporated to align SAR interferograms,achieving the dual objective of preserving filtering standards and retaining detailed information.Experimental results on interferogram filtering and tomography,based on TerraSAR-X data,demonstrate that the proposed method effectively reduces phase noise while maintaining texture accuracy,thereby improving tomography quality.

Modeling and Analysis of the Impacts of Temporal-Spatial Variant Troposphere on Ground-Based SAR Imaging of Asteroids

The near-Earth asteroid collisions could cause catastrophic disasters to humanity and the Earth,so it is crucial to monitor asteroids.Ground-based synthetic aperture radar(SAR)is an observation technique for high resolution imaging of asteroids.The ground-based SAR requires a long integration time to achieve a large synthetic aperture,and the echo signal will be seriously affected by temporal-spatial variant troposphere.Traditional spatiotemporal freezing tropospheric models are ineffective.To cope with this,this paper models and analyses the impacts of temporal-spatial variant troposphere on ground-based SAR imaging of asteroids.For the background tropo-sphere,a temporal-spatial variant ray tracing method is proposed to trace the 4D(3D spatial+temporal)refractive index network provided by the numerical weather model,and calculate the error of the background troposphere.For the tropospheric turbulence,the Andrew power spectral model is used in conjunction with multiphase screen theory,and varying errors are obtained by tracking the changing position of the pierce point on the phase screen.Through simulation,the impact of temporal-spatial variant tropospheric errors on image quality is analyzed,and the simulation results show that the X-band echo signal is seriously affected by the troposphere and the echo signal must be compensated.

3D Target Localization Based on FrFT from Spaceborne Curve SAR

Synthetic aperture radar(SAR)three-dimensional(3D)imaging technology can reconstruct the complete structure of observed targets and has been a hot topic.Compared with tomographic SAR,array interferometric SAR,and circular SAR,curve SAR can use less data to achieve 3D positioning of targets.Most existing algorithms for estimating Doppler frequency modulation(FM)rate are based on sub aperture partitioning,resulting in low computational efficiency.To address this,this article establishes a target height estimation model,which reflects the relation-ship between the height and the residual Doppler FM rate for spaceborne curve SAR.Then,a fast SAR 3D localization processing flow based on fractional Fourier transform(FrFT)is proposed.Experimental verification demonstrates that this method can estimate the Doppler FM of the target column by column,and the 3D position error for non-overlapping targets is controlled within 1 m.For overlapping points with an intensity ratio greater than 1.5,the root mean square error(RMSE)of the estimation results is around 5 m.If the separation between overlapping points is greater than 35 m,the RMSE decreases to approximately 2 m.

New Acquisition Algorithm for Direct Sequence Spread Spectrum Signals

Aiming at improving acquisition performance for direct sequence spread spectrum (DSSS) signals, a new acquisition algorithm based on fast Fourier transform (FFT ) is proposed. The code phases are parallel searched throughout the Doppler freq uency domain. Since the influence of Doppler frequency errors on peak-to-avera ge ratios of correlation power shows a pattern similar to the sinc function, it is our belief that by judging on peak-to-average ratios for different Doppler frequency errors false alarm probabilities should be lowered. Analyses have supp orted the underlying principle of this new algorithm. Simulation results have al so indicated that the new algorithm can effectively improve the acquisition perf ormance for DSSS signals.

New acquisition decision algorithm for multiple access interference suppression in DSSS system

For decreasing the multiple access interference of weaker signal acquisition in direct sequence spread spectrum(DSSS) systems,a new single decision algorithm is presented.The maximum value of correlation results is conventionally detected.However,there may be not only one strong peak among correlation results when the cross-correlation noise is strong enough to affect the correlation results.The proposed algorithm decreases the false alarm probability through the decision of the ratio of the maximum value and the second maximum value of the correlation results.Theoretical analysis and simulation results indicate that the proposed algorithm effectively suppresses the acquisition problem of multiple access interference in DSSS system.

Evaluations of the Characteristics of the Tropo-Strato-Mesopause Height and Temperature Variability over Bahir Dar,Ethiopia(11.60 N,37.30 E)Using SABER

The height profile of atmospheric temperature data between 12 km and 100 km was obtained from SABER/TIMED satellite instruments during the year 2016 and used to characterize the three atmospheric pauses temporal variability of height and temperature over Bahir Dar,Ethiopia(11.60 N,37.30 E).The daily,monthly,and frequency distributions of tropopause-stratopause-mesopause height and temperature are investigated.From the frequency distribution,we had found that of the tropopause-stratopause-mesopause height 17 km,48 km,and 98 km with the corresponding temperature 192 K,268 K,and 148 K.The decrement(cooling)trend lines of tropopause height 0.7 K/year and its corresponding tropopause increment temperature has been~1.5 K/year.The stratopause and mesopause trend lines of height are insignificant and the corresponding decrement(cooling)temperatures are~3 K/year and~13 K/year respectively.The mean monthly maximum heights of tropopause 19 km in May with a corresponding maximum temperature of 201 K in September.The maximum stratopause height 49.5 km in February and July and its temperature 268 K and 267 K in February and April respectively.The maximum mesopause height 98 km,95 km,97 km in March,Jun,and November respectively,and its maximum temperature 196 K and 198 K in January and July respectively.

Analysis of Moving Target Detection in Space-Surface BiSAR

Based on the geometric structure of multi-channel space-surface bistatic aperture radar(SS-BiSAR),the time-frequency and spatial-frequency relationships of clutter and moving target are detailedly analyzed.It is found that the prepositive stationary receiver can improve the spatial separation between moving target and correspondingly competitive clutter,furthermore the processing scheme of space-time adaptive processing(STAP) in SS-BiSAR echo domain is given.Meanwhile,compared to the STAP in monostatic synthetic aperture radar(SAR),it is firstly proved that the minimum detectable velocity in SS-BiSAR system can improve R0/R1(transmitter-to-target distance over receiver-to-target) times.Finally,the computer simulation verifies the effectiveness of the STAP processing scheme.

Ground moving target signal model and power calculation in forward scattering micro radar

Forward scattering micro radar is used for situation awareness;its operational range is relatively short because of the battery power and local horizon,the free space propagation model is not appropriate. The ground moving targets,such as humans,cars and tanks,have only comparable size with the transmitted signal wavelength;the point target model and the linear change of observation angle are not applicable. In this paper,the signal model of ground moving target is developed based on the case of forward scattering micro radar,considering the two-ray propagation model and area target model,and nonlinear change of observation angle as well as high order phase error. Furthermore,the analytical form of the received power from moving target has been obtained. Using the simulated forward scattering radar cross section,the received power of theoretical calculation is near to that of measured data. In addition,the simulated signal model of ground moving target is perfectly matched with the experimented data. All these results show the correctness of analytical calculation completely.