ABSTRACT
Inverse synthetic aperture imaging ladar (ISAL) can achieve high-resolution images, and yet it faces pulse-to-pulse high-order phase errors that the microwave radar can ignore. The high-order phase errors are almost caused by mechanical vibrations in general, which blur the azimuth focusing effect. This paper presents an ISAL imaging model to obtain high-resolution images. A novel modified cubic phase function (CPF) algorithm is proposed to compensate the additional high-order phase errors. Some high-resolution well-focused ISAL simulation images and real target images are shown to validate the methods. It is shown that the third-order phase errors are compensated by the distinctive digital signal process and the image entropy of real target images is reduced significantly.
ABSTRACT
Synthetic aperture ladar (SAL) is a newly developed imaging device for remote sensing application. Owing to its short wavelength (3-5 orders of magnitude shorter than radar), SAL is very sensitive to platform vibration. For frequency-modulated continuous-wave SAL (FMCW-SAL), the platform vibration induces an additional range cell migration (RCM) to the SAL image. The vibration-induced RCM (VI-RCM) deteriorates the image quality. The VI-RCM is a unique problem for the FMCW-SAL imaging. To address this problem, a raw-data-driven method is proposed to correct the VI-RCM in this paper. First, the signal model was developed to show the VI-RCM in FMCW-SAL echo. Then, based on the model, the differential phase function (DPF) is constructed for the adjacent range profiles. The DPF is a single-frequency signal with its frequency being proportional to the relative range shift between the adjacent range profiles. Based on the DPF, the relative range shift is estimated. After the estimation of all the relative range shifts, the VI-RCM is calculated and corrected. Experiments are performed. The simulated experiment demonstrated the feasibility, accuracy, and efficiency of the proposed method, and the real data processing result verified the effectiveness of the proposed method for FMCW-SAL in practical applications.
ABSTRACT
Doppler tomography is an important means to obtain two-dimensional (2-D) images of remote targets. It is especially suitable for imaging spinning targets such as space debris, warheads, and aircraft blades. However, related research is mostly focused on the microwave band rather than the laser. Higher resolution can be achieved by implementing Doppler tomography in the laser band compared to the existing Doppler tomography in the microwave. Moreover, existing imaging methods are mostly directed at point targets. When these methods deal with extended target echoes, the image quality is unsatisfactory. These problems severely limit the application of Doppler tomography. Here, a novel laser Doppler tomography method has been proposed. The method is based on a single-frequency laser radar (LADAR) that does not require any form of wideband modulation of the transmitted signal. The imaging process is based on the precise relationship between the scattering coefficient of the target and the statistical characteristics of the Doppler spectrum and finds the maximum a posteriori (MAP) estimate of the scattering coefficient distribution. The imaging resolution depends on the Doppler frequency resolution, which exceeds the diffraction limit and is independent of the imaging distance. A laser Doppler tomography experimental system was established. With this system, high-quality laser Doppler tomograms of extended targets were obtained for the first time. In the experiment, the targets have different rotational speeds from 100 to 1000 r/min. The images of these targets with a resolution of 0.4 mm are obtained at a distance of 5 m indoors. In these images, the target details such as textures on the surfaces can be rendered. The quality of these images is greatly improved compared to existing processing methods. The experimental results confirm the effectiveness of the proposed laser Doppler tomography method.
ABSTRACT
A long-distance inverse synthetic aperture LADAR (ISAL) imaging experiment outdoors over 1 km for cooperative targets is demonstrated, which gets a two-dimensional high-resolution image with resolution exceeding 2.5 cm. The system utilizes an electro-optic in-phase and quadrature modulator to output a linear frequency-modulated continuous waveform (LFMCW) with a bandwidth of 6 GHz and pulse repetition frequency (PRF) of 16.7 KHz. For the problem of the coherence of the laser, the effects of the coherent processing interval (CPI) and time delay of the local oscillator (LO) on the coherence are discussed. The fiber delay line is set and the CPI is reduced to lower the requirement of the frequency stability of the laser source. The images are formed by two-dimensional Fourier transform and joint time-frequency transform methods, respectively. In this paper, we present the system structure, imaging processing, and the experiment result in detail. The experiment result validates the performance of our system for ISAL imaging.
ABSTRACT
A novel and high-efficiency linear frequency-modulated continuous-wave (FMCW) ladar system for synthetic aperture imaging is proposed and experimentally demonstrated. This novel system generates wide-bandwidth linear FMCW ladar signals by employing an electro-optic LiNbO3- in-phase and quadrature modulator with an effective bias controller. The effectiveness of the proposed system is experimentally validated. Optical synthetic aperture images are obtained by using two 0.41 cm aperture diameter telescopes at the distance of 1 km. The resolution of these images can reach to 4 cm. A resolution improvement by about 10 times is achieved when compared with the conventional real aperture imaging system.
