High Resolution ISAR Imaging using a low-cost ultrasonic radar

Khumalo, Ndumiso

High Resolution ISAR Imaging using a low-cost ultrasonic radar

Thesis / Dissertation

2024

Publisher

University of Cape Town

Department

Department of Electrical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

Generating high-resolution focused Inverse Synthetic Aperture Radar (ISAR) images is a challenging task that requires advanced motion compensation algorithms to eliminate the blurring induced by the target's motion. However, the high cost of investigating different ISAR algorithms using real-life radars presents a significant challenge. To overcome this challenge, low-cost radar systems have been developed for experimentation and education purposes. This research aims to develop a low-cost ultrasonic ISAR imaging system that is scalable and reusable. The system will provide a practical solution to image objects of interest and allow for testing various algorithms for ISAR imaging. MATLAB Simulations were conducted to determine the most suitable waveform and radar system parameters for the project's implementation phase. The simulations involved small objects rotating uniformly at 13◦/s on a turntable with a radius of 25 cm. An ultrasonic radar was used in the simulation, operating at 40 kHz and transmitting a chirp waveform with a bandwidth of 4 kHz. Using the Range-Doppler Algorithm (RDA), focused ISAR images were generated with a resolution of 4 cm in both the slant-range and cross-range when the rotation angle was 10◦ . However, increasing the rotation angle resulted in target scatterers experiencing migration through resolution cells (MTRC), which led to a blurred ISAR image. A rotational motion compensation algorithm was implemented to reduce the effects of MTRC. After applying rotational motion compensation, the blurred ISAR image was focused, and a cross-range resolution of 70 mm was achieved for a rotation angle of 50◦ . Furthermore, the image contrast after motion compensation increased by a factor of 2 for the simulated ISAR images. A low-cost ultrasonic radar was designed and developed to measure small rotating objects and validate the motion compensation algorithm on measured data. The total cost of the radar was approximately ZAR 2000, which is significantly lower than a similar RF radar system, which usually costs approximately ZAR 7500. The ultrasonic radar could resolve objects at a maximum speed of 0.14 m/s with a slant-range resolution of 7 cm and cross-range resolution of 1 cm when the rotation angle was 46◦ . The measured objects ranged from 3 × 3 × 3 cm corner reflectors to a 25cm × 40 cm aeroplane model. After applying rotational motion compensation to the measured data, the image contrast on the final ISAR image increased by a factor of 1.42. The resulting high-resolution ISAR images were focused and had enough discriminate features to be used for classification. In both measured and simulated results, the rotational motion compensation algorithm's performance decreased when the rotation angle was increased above 46◦ for a target with dimensions 60 cm x 45 cm in the slant-range and cross-range, respectively. This was due to the algorithm's limitation on small-angle approximations. The study demonstrated the successful use of a low-cost ISAR imaging system to generate high-resolution focused ISAR images of small targets rotating on a turntable. Such a low-cost radar system is ideal for experimental purposes to evaluate various ISAR signal processing algorithms and gain insight into radar concepts. Future work could involve using additional ultrasonic receivers to take advantage of the spatial diversity provided by bistatic ISAR. Furthermore, more advanced rotating platforms could be employed to produce three-dimensional motion resembling that of real-world targets.

Keywords

Engineering

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