كليدواژه :
رادار روزنه تركيبي , SAR , پردازش سيگنال , تشكيل تصوير , سكوي زميني
چكيده فارسي :
امروزه سيستمهاي تصويربرداري رادار روزنه مصنوعي در مسائل گوناگوني از جمله سنجشازدور كاربرد دارند. جهت استفاده از سيگنال دريافتي سيستم SAR نياز به پردازش سيگنال خام دريافتشده و ايجاد سيگنال تصوير مختلط تكمنظر (SLC)[1]. از جمله كاربرديترين اين الگوريتمها ميتوان به الگوريتم RMA اشارهكرد. اين الگوريتم مختص سيستمهاي تصويربرداري فضابرد و هوابرد است. با اينحال ويژگيهاي اين الگوريتم باعثشده تا در كاربردهاي بردكوتاه نيز نتايج مناسبي داشتهباشد. در اين مقاله عملكرد اين الگوريتم بر يك سيستم تصوير برداري رادار روزنه تركيبي زميني با برد خيلي كوتاه مورد آزمايش و بررسي قرار ميگيرد. الگوريتم Range Migration بر چندين هدف شبيهسازي شده طبق شرايط مورد نظر اجرا و در نهايت طبق معيارهاي PSLR و ISLR مورد بررسي قرارگرفت. طبق اين دو معيار، مقدار ميانگين PSLR در راستاي برد برابر با 1143/13- دسيبل و در راستاي آزيموت برابر با 2153/13- دسيبل و مقدار ميانگين ISLR در راستاي برد برابر با 9726/5- دسيبل و در راستاي آزيموت برابر با 1159/6- دسيبل گرديد. با مقايسه نتايج حاصل با حالات تصويربرداري با برد بلندتر ميتوان به عملكرد مناسب و قابل قبول الگوريتم RMA براي سيستمهاي تصويربرداري با برد خيلي كوتاه پيبرد.
چكيده لاتين :
Nowadays synthetic aperture radar (SAR) imaging systems are used in a wide range of applications. Synthetic aperture radar signal processing is processing radar signals with specific data acquisition geometry, in order to forming high resolution radar images in range and azimuth directions. In order to SAR processing become possible, radar antenna must move over the imaging scene. With antenna motion and SAR signal processing technique, we can generate an analytical antenna with narrow beamwidth in the along-tack direction. In order to make use of the SAR image in different applications, in the first step, it is needed to process the raw signals, acquired by the SAR system, and produce the single look complex (SLC) image. Different SAR signal processing algorithms have been developed to create the SLC image. Range migration algorithm (RMA) is one of the best and popular image formation algorithms from SAR signals, which operates in frequency domain. RMA is mostly used for imaging the Airborne and Spaceborne systems. Furthermore, other capabilities of RMA made it possible to use this algorithm for ground-based SAR (GB-SAR) systems. GB-SAR imaging follows the same imaging geometry principles of airborne and spaceborne systems but, the differences are in shorter analytical antenna in azimuth direction of GB-SAR, look angle of antennas, and targets range to sensor, which are very close, in GB-SAR scenario. In ground based systems, radar sensor is mounted on a straight rail. Radar sensor moves step-wise on the rail and acquires data from the corresponding range profile. At the end of data acquisition process, all the raw signals are stored in a two dimensional array. This two dimensional array in used as input of image formation processor and then the final image will be derived. Range Migration algorithm, consists of four main steps: (1) One dimensional long-track Fourier transform (2) Matched filtering (3) Stolt interpolation (4) Two dimensional inverse Fourier transform in azimuth and along-track directions. In this paper, we are going to develop the RMA for image formation of a Ground-based SAR system, which is used for very close range purposes. Our simulated SAR system operates in S-band and modulates signal form 2.4 GHz to 2.5 GHz. It mounted on a three meters long rail and acquires data every two centimeters. We simulate the different distributions of point targets, such as presenting one point target or presenting nine point targets in the imaging scene, based on the reflected signal model of targets. All the targets’ ranges are less than 40 meters. After simulating acquired two dimensional raw signal of targets, RMA is used to extract the focused targets. In post processing step, Hann window in used to suppress the sidelobes of compressed signals. PSLR and ISLR are used as parameters to analyze the quality of detected targets. The mean PSLR of all examined targets in range direction is -13.1143 dB and in azimuth direction is -13.2153 dB and also the mean ISLR of nine targets in range direction is -5.9726 dB and in azimuth direction is -6.1159 dB. Obtained results are acceptable compared to other imaging modes in higher ranges such as Airborne and Spaceborne imaging systems.
