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Bistatik yapay açıklıklı radar yardımıyla görüntüleme

An Algorithm for bistatic synthetic aperture radar inversion in object imaging

  1. Tez No: 39363
  2. Yazar: SERDAR KARGIN
  3. Danışmanlar: PROF.DR. BİNGÖL YAZGAN
  4. Tez Türü: Yüksek Lisans
  5. Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
  6. Anahtar Kelimeler: Bistatik, Görüntüleme, Monostatik, Sentetik açıklık radarı, Bistatic, Imaging, Monostatic, Synthetic aperture radar
  7. Yıl: 1992
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: Belirtilmemiş.

Özet

DIET Bu tezde bistatik Yapay Açıklıklı Radar (Y.A.R.) yardımıyla görüntülemede ters dönüşüm algoritmasının oluşturulması üzerinde çalışılmıştır, ilk olarak yapay açıklıklı radar kavramı kısa olarak anlatılmıştır. Daha sonra monostatik yapay açıklıklı radarda ters dönüşüm bağıntısı elde edilmiştir. Buna bağlı olarak bistatik radar içinde ters dönüşüm bağıntısı ve algoritması üzerine çalışılmıştır. Burada elde edilen dönüşüm yöntemi, bistatik oluşturulan dizi bilgisinin Fourier analizine ve Doppler bilgisinin faz modülasyon analizine dayan maktadır. Bu yaklaşım gönderilen ve sonuçta yansıyan küresel dalgaların faz bilgisi analizini içermektedir. Doppler bilgisi bistatik açı ve mesafelere bağlı olan bir band içinde yansıma fonksiyonunun uzaysal Fourier dönüşümünün örnekleriyle belirlenmektedir. Elde edilen bistatik yapay açıklıklı radar dönüşümü kullanılarak, hareketli bir cismin görüntüsü tek transmisyon durumu için incelenmiştir. Monostatik ve bistatik Y.A.R. ters dönüşüm algoritmaları ifade edilmiştir. Daha sonra ters dönüşüm algoritmaları ile ilgili uygulama örnekleri gerçekleştirilmiştir. in

Özet (Çeviri)

AN ALGORITHM FOR BISTATIC SYNTHETIC APERTURE RADAR INVERSION IN OBJECT IMAGING SUMMARY The aim of this thesis is to show that an algorithm for mon- static and bi static synthetic aperture radar inversion in object imag ing. The synthetic aperture radar can be regarded as a particular kind of high resolution imaging system operating in the microwave re gion. Its high resolution is achieved in range direction by compres sion of a chirp pulse, and in azimuth direction by using the synthetic aperture principle. Synthetic aperture radar can be mounted either on an airplane, a spacecraft or on a space shuttle. These vehicles move with respect to the detected targets as shown as Fig 1, then collect and process the echo reflected by them by means of digital or optical processing technique. Finally such radar system can generate a two dimensional image, which is similar to the optical photograph, of reflectivety distribution of the detected targets. One dimension of the image is proportional to slant range and the other dimension to along track koordinate position. Synthetic aperture radar has its unique characteristics, that is, synthetic aperture radar use its own energy to illuminate the detected surface, and it generates an image from the backs catter echoes. Therefore SAR is not dependent on illumination from the sun, it means that the weather changes (such as rain, clowd and mist take place) almost do not effect on imaging of synthetic aperture radar. In not limited by and weather factor. In addition, the illumination angle and illumination direction of synthetic aperture radar can be controlled and selected, whereas in optical system these parameters conserained by the sun's location. Consequently, synthetic aperture radar is an all time, all-weather microwave remote sensor. According to the wave theory, though the target is buried the hideaway under ground, the synthetic aperture radar is still able to observe to a limited depth. In fact, the depth D of the penetration depends on frequency and pal ari zati on of incident waves. The image resolution of synthetic aperture radar is independant of the lenght of micro wave, the altitude of the vehicle and the maximum detected range of radar, with the size of a resolution element being 1x1 m to 10x10 m. Synthetic aperture radar employs wavelengths different from photo graphic sensor, and thus provides information on surface rouhness, dielectric properties, moisture. Synthetic aperture radar can ope rate simultaneously in several wavelengths, and thus has a multi- spectral potential. IVCarl Wiley, Goodyear Aerospace Corporation, first put foward Doppler beam-sharpening concept in June 1951. University of Illinois demonserated the above concept in 1952 and produced first synthetic X=Vat *Y s <?/. - / r, Azimuth resolution ja <? # Fig 1. Sidelooking synthetic aperture radar Configuration aperture radar image in July 1S53. Since then, synthetic aperture radar has proved to be an extremely useful surveillance tool for civil and military applications. In addition, synthetic aperture radar has great potential for weather monitaring, assessment of deforestation. Especially since the first spaceborn synthetic aper ture radar, known as SEASAT, was launched in 1978, synthetic aperture radar technique itself has gone ahead by leaps. It has already gone into the spaceborn position from airborne one. Therefore, the appli cation region of synthetic aperture radar will broaden more and more. As the synthetic aperture radar produces 2-dimensional image,so it is necessary for a hi ah resolution imaging radar to process both high range resolution (across-track) and high azimuth resolu tion (along-track). Before inventing the synthetic aperture prin ciple, if the conventional radar is used, we had to increase the dimention of the radar antenna, because the half-power beam width 3, in radians, of the antenna is equal to x/D, the along-track reso lution Pa is R.x/D, where x is the operating wavelenght and D the aperture of the antenna. For example, Let R=200 km, x=2 cm (x band), Pa=T0 m, then D=400 m. So we had to lead to the development of ever larger antenna installations, if one wishes to have fine azimuth resolution at long ranges, the required antenna lenght has to be of the order of hundreds or even thousands of meter. Obviously, it is very unsuited for the practical use, especially for air surveillance. In addition, it is a not good ideal for using at very short wave lenght, because it leads to weather limitation. So, Neither is attractive. At this time, it was urget for the scientist to look for new approach to enhance the angular resolution of an antenna. It is the background to yield the synthetic aperture principle and synthetic aperture radar. Half-power beam width of synthetic an tenna is es=x/2Ls. Length of synthetic antenna is LS=XR/D. D represent the horizontal aperture of the physical antenna, Pa the azimuth resolution at slant range R. The linear along-track reso lution is Pa=D/2. The azimuth resolution Pa in the synthetic an tenna is independent of the operating wavelength and target range and varies directly with the antenna length. It means that a shor ter antenna actually gives better the azimuth resolution. This thesis presents an inversion method for bi static syn thetic aperture radar imaging. The method is based on a Fourier analysis (Doppler processing) of the bi static synthesized array's data followed by a phase modulation analysis of the Doppler data. The approach incorporates the phase information of the wavefront curvature in the transmitted waves as well as the resultant echoed signals. The Doppler data are shown to provide samples of the reflectivity functions's spatial Fourier transform within a band that depends upon the bistatic angles and ranges. Associated resolution, reconstruction, and sampling constraints for the imaging problem are discussed. The bistatic SAR inversion is also utilized to formulate an inversion for multistatic measurements made along a physical linear array due to a single transmission to image a dynamic object. This thesis addresses problem of inverting the data collected in a bistatic synthetic aperture radar imaging problem. This prob lem arises in high resolution terain imaging using transmitting and receiving radars that are mounted on to different aircrafts. Such bistatic measurements are useful when the terrain's monostatic radar cross section is not strong. Bistatic SAR measurements may involve several runs with varying bistatic angles by the radar carrying air- crafts to obtain powerful echoed signals from the terrain that is to be imaged. Similar bistatic data may also be encountered in in verse synthetic aperture imaging problems of radar and sonar. VTThe transmitting and receiving radiation patterns of a radar resemble as spherical wave. The existing SAR inverse methods are based on approximations for spherical radiation pattern of the radar e.g., the Fresnel approximation, or the plane wave approximation. These methods fail in the case of high resolution imaging of a large object area. This thesis has inversion methods that remove these restrictions in monostatic SAR problems; a proof of this in version method is briefly described in thesis (Fig. 2). This thesis presents an inversion for bistatic synthetic aperture radar imaging via a phase modulation analysis of the Doppler data across the synthetic aperture. The formation is based on the Fourier decomposition of a spherical wave analogous to the monostatic case. The bistatic data inversion, however, involves analysis of nonlinear phase functions that do not arise in the monostatic problem. Based on a spatial compression of the SAR signal across the synthetic aperture an efficient sampling proce dure on the synthetic aperture is developed that is invariant of the lenght of the aperture (Fig. 3). Radar's Itijîhl Palh.\,.V,.|.1.;: >r Transmitting RcrcİMns Radar Fig. 2. Imaging Geometry for a Monostatic SAR vnTransmitting Radar's Flight Path (X,.Y,.L) | K. <“2 «,*,> _ Transmitting Radar | (X”Y,*o) f, S J. -' (X..Y.-L) Receiving Radar's Flight Path (X2,Y,»U 1 * (XjY,-») _ S I Receiving Radar T (X2,Yj4.) Fig. 3. Imaging Geometry for a Bi static SAR. The bistatic SAR formulation also brings out certain functi onal properties of a physical array's data that is useful for dev- loping inversion in multi static echo imaging problems. An dynamic object with a single transmission and makes multiple (multistatic) measurements of resultant echoed signal along a physical linear array is discussed in this thesis. It is shown that such multistatic measurements can be translated into the data from a monostatic syn thesized linear array. Reconstruction of simulated targets in the above mentioned imaging problems are presented. vm

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