Research On New Methods Of ISAR Imaging Of Non-cooperative Targets

Because of characteristics of all-time and all-weather, Inverse Synthetic Aperture Radar are of great value in the applications of civil and military use. Recently, in order to obtain high-quality ISAR images, researchers all around the world did a lot of work in ISAR imaging methods. This paper mainly works on the following problems:1) ISAR imaging theories and ISAR image cross-range scalingWhen we obtain ISAR images, we usually do not consider the proper echo pulses for imaging, so that the observed objects cannot be represented as the same scale with the real objects. Due to the certain resolution of range profile, we should set the resolution of cross-range to be the same. Therefore, we can obtain the observed objects with real scale. This is the ISAR image cross-range scaling problem. Focused on this problem, the paper proposes a new method-Pseudo Time Dechirp Method and uses curve fitting method to get the proper number of pulses for imaging.2) ISAR imaging for maneuvering targetsFirstly, the paper introduces the theories of ISAR imaging, and analyses the signal processing method in the direction of cross-range and range. Next, we present the cross-range scaling problem for ISAR imaging. Traditional range-doppler(RD) algorithm is usually applied for ISAR imaging, when the observed targets rotate uniformly. If the targets rotate with acceleration or accelerated acceleration, the form of the signal in a range bin can be considered as linear frequency modulation signal(LFM) or cubic phase function signal(CPS). When we do cross-range compression, ISAR images obtained by RD algorithm are often blurred. So we should analyse the complex signals such as LFM or CPS and use methods such as time-frequency and parameter estimation to obtain instantaneous ISAR images for the maneuvering targets.3) ISAR imaging for targets with rotational partsThe complex targets introduced in this paper refer to the observed targets with rotational parts, such as rotating blades in helicopters and the propeller s or turbofans in airplanes. The rotational parts have their own moving at the sam time, when they rotate with the main body. The rotational scatters in each range bin generate sin frequency modulation signals, which cause stripe interferences in ISAR imag es obtained by RD algorithm. To solve this problem, in this paper, we use time-frequency methods such as short time fourier transform(STFT) and local mean decomposition(LMD) to obtain the ISAR images for the main body after removing the rotational parts.4) ISAR imaging based on compressive sensing methodsWe also introduce the application of compressive sensing(CS) methods in ISAR imaging. Compared with the large observed field, the targets are very small. We can consider the scatters of the targets are sparse, so that we can use CS methods to obtain ISAR images. When the signals are sparse in one domain, CS can recover the original signals at a high probability from limit samples. Therefore, ISAR images with a long imaging time can be obtained by CS methods by a small duration.What’s more, due to the unstability of the real ISAR systems, random missing observations may happen in the real scenarios. To solve this problem, this paper proposes a new algorithm based on CS method and use the mehod into the application of real data ISAR imaging.5) ISAR imaging based on spectral estimationSpectral estimation has always been highlighted in the signal processing field. With higher resolution and lower side-lobes, spectral analysis methods are more effective than tranditional Fourier transform. Therefore, these methods have been applied in a wide range of engineering fields. We use APES and IAA methods to obtain high resolution ISAR images in this paper. Moreover, IAA method can revover the whole samples of the signal from limited random samples. Also, the paper uses the IAA method to obtain ISAR images with limited random pulses.