Study On System Design And Algorithm Of Airborne Synthetic Aperture Ladar

As the combination of laser technology and synthetic aperture technology, Synthetic Aperture Ladar (SAL) is the only optical remote sensing in centimeter-level. Most recently, the development of SAL has received increasing interest. In 2006 Raytheon and Northrop Grumman successfully demonstrated airborne synthetic aperture ladar experiments. In 2011 Lockheed-Martin Corporation (LMCT) completed airborne synthetic aperture ladar flight demonstration independently. The cross range resolution is 3.3 cm at a down look angle of 45 degrees to the target 1.6 km away. These experiments show that the SAL has tremendous potential in space remote sensing and high-resolution imaging applications.The imaging principle of the synthetic aperture ladar is basically the same as the synthetic aperture radar’s. However, there are still some difficulties in the process from theory to application because the laser wavelength is smaller 4 orders of magnitude than microwave wavelength. There are two big problems need to be paid attention to. Firstly, the swath of SAL is small because of the micro-meter wavelength. Secondly, due to the limitation by the velocity of the lasers tunable component, the tunable band of ladar contradicts with the pulse repetition frequency (PRF) in SAL. Based on the above background, the Airborne Synthetic Aperture Imaging Ladar Imaging Algorithm is discussed through conceptual analysis and theory derivation. Meanwhile, the simulations validate the feasibility and effectiveness of the proposed methods.This paper is organized as follows:Chapter 1 is the introduction. It reviews the history of both synthetic aperture technique and synthetic aperture ladar respectively. And then, the research background and significance are introduced.Chapter 2 introduces the basic theory of the synthetic aperture ladar. Then, it gives the key techniques of the synthetic aperture ladar in practical application.Chapter 3 employs the Scan-mode of aperture synthesis technology in ladar. Due to the limitation by the velocity of the lasers tunable component, the tunable band of ladar contradicts with the pulse repetition frequency (PRF) in SAL. Meanwhile, the swath of SAL is small because of the micro-meter wavelength. In order to solve these contradictions, this chapter gives a novel imaging mode of SAL, which combines multi-channel technology and Scan mode imaging system, which can provide a wide-scene imaging. This system utilizes multi channels, i.e., a few of the same lens located in the azimuth direction, which are provided spatial information with spatial filtering to remove the ambiguity caused by the low PRF in the Doppler domain. And then, by means of controlling the laser beams which scan in different swaths, a better imaging width can be obtained in spite of accompanying with a reasonable loss of resolution in azimuth. Then, SPECtrum ANalysis (SPECAN) analysis is adopted to remove the ambiguity caused by the Scan mode. As such, the image would focus in the Doppler domain. Finally, simulation with three lens, i.e. three channels, can reduce the PRF equal to one third of the constraint of the Nyquist theorem.Chapter 4 proposes a suitable imaging algorithm for Terrain observation by progressive scans mode (TOPS) imaging mode, to obtain a wide imaging area. Firstly, the azimuth pro-filtering based on the SPECAN is adopted to eliminate the aliasing caused by the steering of the antenna beam and the signal spectrum without aliased is obtained. Then the improved Frequency Scaling Algorithm (FSA) is used to complete the range compression and Range Cell Migration Correction (RCMC). Also, the Doppler frequency shift induced by the continuous moving of the platform is compensated. Finally, combined the Dechirp technique, the signal is focused in the Doppler domain and the SAL image without ambiguity is obtained.Chapter 5 proposes a novel 3-D imaging system, which is based on synthetic aperture technique. High resolutions in 3-D are achieved by matched filtering of LFM signal in range, synthetic aperture technique in along-track dimension and real aperture array in across-track dimension respectively.Chapter 6 is the summary of the whole dissertation. And then, it also puts forward corresponding suggestions on the research to be further studied.