فصلنامه رادار
سال سوم شماره 1 (پیاپی 7، بهار 1394)

Ships’ magnetic anomalies are one of their main problems encountering sea mines and for the cases of submarines are one of the prominent factors, which causes their fast detection. The main aim of this paper is to design a degaussing system in order to minimize magnetic anomalies of ships. To achieve this goal, first, magnetic signature of a simple ship is simulated. Then by utilizing three groups of coils which are located inside the ship’s hull, an anomaly with the same magnitude and inverse direction will be produced to minimize the total anomalies. In presented work according to the linearity of the whole system and by using particle swarm optimization, the current value of each coil will be calculated. Less complexity and higher speed of this method in comparison with direct optimizations of simulators are the most important advantages of the proposed method.

The main purpose of this study is the fusion of polarimetric SAR (PolSAR) data and hyperspectral data for improving concealed target detection in forest area. The proposed method based on the polarimetric signatures, the spectral signatures and the pattern recognition matching methods. The combination of SCM, that measures the similarity between the shapes of two signatures, and SAM, that measures the difference between the intensities of signatures were selected for this study. The results of the proposed method were evaluated using two methods. First, the output of the proposed detector using a fusion of the two source data was compared to the output of the proposed detector using each of the two sources in detection of concealed targets. The number of false detected targets in these methods are 3, 9 and 12. Second, the algorithm was compared to ACE algorithm inhyperspectral data and CFAR method in PolSAR data using the ROC curves. The area under the curve of these methods are 0.87, 0.91 and 0.69. As the results, the proposed method has detected concealed targets with the minimum false targets. Also, this method presented well accuracy in other targets.

According to high costs of phase shifters in antenna arrays, new methods for beam steeringwithout phase shifter are introduced. Frequency Diverse Array (FDA) concept is another approach to beam steering problem. In this structure, the subsequent antenna elements are fed with stepped discrete frequencies causing continuous scanning of space in time. So a range-angle dependent scanning is made possible. There are several ways to implement, which the simplest of them is using Linear Frequency Modulated Continuous Wave (LFMCW) source. In this paper, a new implementation scheme is proposed and analyzed where a Square Frequency Modulated Continuous Wave (SFMCW) source is used for feeding a special beam forming network. Also radiation pattern of SFMCW and LFMCW was compared.

The radars that use medium PRF, have either range ambiguity or Doppler ambiguity, but these ambiguities can be resolved with selecting a PRF set. Avoiding ghost targets detection and minimizing blind zones are two important points that should be considered in targets detection. Then, using this set, coincident algorithm and clustering the closely detected points, we resolve ambiguity and detect the targets. After that, to decrease ghost target detection, deghosting algorithm is used. In this paper, a new definition of uniqueness condition is presented to be robust against errors when clustering is employed. Also, it is shown that the medium PRF range depends on the maximum range and Doppler as well as range bin and Doppler bin.

An important section of range-Doppler imaging algorithm in SAR systems which needs large number of resources, memories and solution time, is FFT on the azimuth dimension. In this paper, by decomposing the FFT relation, and converting to smaller dimension FFTs, implementation has been simplified. The proposed method, based on removing the extra phase factors reduces the resources, memories and solution time in hardware. Evaluation of the proposed method show that optimum architecture depends on image resolution, next processing stage and the number of FFT points. Using the proposed architecture leads to implement the whole range-Doppler imaging algorithm in smaller hardware.

Due to difference between azimuth phase history and range migration model in circular strip-map synthetic aperture radar and linear SAR, imaging algorithms of linear system are not valid in circular system. In this article using a range approximation in a ground-based circular strip-map SAR, the signal model at 2-D frequency domain is extracted and due to stability of path and speed of antenna, a frequency domain imaging algorithm is suggested based on extracted model that despite of accuracy has a high speed. Simulations verify the accuracy of suggested algorithm at image formation in polar coordinates. It is shown that for targets with ranges near to reference, the processing burden and needed hardware can be reduced by omitting a phase correction step. Finally the location of these targets is extracted as “fast imaging region” according to the geometry and parameters of the system.