ABSTRACT
According to the principle of synthetic aperture ladar, high-resolution imaging can be achieved if the relative motion exists between the target and the ladar. The imaging system has characteristics including a large field of view, narrow-band laser signals applied, and easy engineering implementation. The complex image reconstruction and the synthetic aperture laser imaging method for moving targets based on the spatial light modulator and the direct-detection detector array are proposed. The far-field simulations and the near-field experiments for the stop-and-go target and the continuous-moving target were carried out. It is verified that the complex image reconstruction method can equivalently realize coherent detection for the target and reflect its phase information corresponding to the laser wavelength. Multi-frame complex images reconstructed can be applied to the synthetic aperture laser imaging, which forms high-resolution images for moving targets under far/near-field conditions.
ABSTRACT
Inverse synthetic aperture ladar (ISAL) has the capability to achieve high-resolution imaging of long-distance targets in a short time because of the laser's short wavelength. However, the unexpected phases introduced by target vibration in the echo can cause defocused imaging results of the ISAL. How to estimate the vibration phases has always been one of the difficulties in ISAL imaging. In this paper, in view of the echo's low signal-to-noise ratio, the orthogonal interferometry method based on time-frequency analysis is proposed to estimate and compensate the vibration phases of ISAL. The method can effectively suppress the influence of noise on the interferometric phases and accurately estimate vibration phases using multichannel interferometry in the inner view field. The effectiveness of the proposed method is validated through simulations and experiments, including a 1200 m distance cooperative vehicle experiment and a 250 m distance noncooperative unmanned aerial vehicle experiment.
ABSTRACT
A dual-channel inverse synthetic aperture ladar imaging experimental system based on wide-pulse binary phase coded signals and its moving target imaging are introduced. The analysis, simulation, and experimental data processing results of binary phase coded signal Doppler compensation and pulse compression are included. The method of motion phase error estimation based on interferometric processing and the imaging method with small computation in the case of large squint angles are proposed, and the simulation results are presented. The effectiveness of the imaging method is verified by experimental data processing. Doppler frequency curves are estimated based on time-frequency analysis of echo signals, and the coarse compensation of motion phase error is realized. According to the interferometric phase and coherence coefficient of dual-channel echo signals' time-frequency analysis, the coherence of the dual-channel echo signals is checked, and along-track interferometry can be applied to the precise compensation. The stable interferometric phase and increased coherence coefficient of actual dual-channel data imaging results indicate the effectiveness of the motion phase error compensation method proposed. Considering characteristics of inverse synthetic aperture ladar (ISAL) imaging, after dividing echo signals into multiple sub-apertures, range-Doppler algorithm and sub-aperture stitching are adopted, the stitched image is corrected geometrically through Stolt transformation, and the computation is reduced.
ABSTRACT
According to the self-heterodyne signal obtained by lidar under different fiber delay times, the model of the local oscillator signal was established, and the maintenance method of signal coherence in lidar based on the digital delay was improved by using multiple sinusoidal frequency modulation components. An imaging detection experiment was carried out at a distance of 5.4 km. The coherence of the lidar signal was maintained by combining the transmitting reference channel correction method and the local oscillator reference channel compensation method, accompanied by the use of a phase spectrum to analyze the improvement effect. The processing results of the echo signal showed that the method could remove the high-order phase errors that cannot be compensated by the phase gradient autofocus algorithm and improve the signal coherence, which could be used for the detection and imaging of long-range targets.
ABSTRACT
Aiming at the wide-spectrum chromatic aberration problem of membrane diffractive lenses, the concept of the digital chromatic aberration correction infrared imaging system was proposed. The principle of digital chromatic aberration correction was given, and the chromatic aberration correction of this system was completed on a computer, which is based on a wavelength-tunable laser local oscillator coherent detection detector. Compared with the chromatic aberration correction method for traditional lenses, this membrane diffractive optical system exhibited characteristics of small size, low weight, and low complexity. The digital chromatic aberration correction application conditions of this membrane diffractive optical system were analyzed. In addition, the main parameters and imaging simulation results were given. The center wavelength of this membrane diffractive optical system is 1.55 µm, and its spectral range is 1.50-1.60 µm. The application of autocorrelation processing to infrared complex images after digital chromatic aberration correction followed by incoherent accumulation of the received corrected complex images based on multiple stepping wavelengths could significantly improve the imaging signal-to-noise ratio.
