Busck, Jens

Et monostatisk laser radarsystem til at optage to- og tredimensionelle billeder med høj afstandsnøjagtighed ved hjælp af ”gated viewing” teknikken er udviklet med det formål at forbedre Søværnets muligheder for optisk identifikation af søminer placeret på havbunden. Søværnet anvender en lille, fjernstyret og ubemandet undervandsbåd til optisk identifikation af søminer. Den optiske identifikation af søminen er sidste led i en længere detektions-, klassifikations- og identifikationsproces. De eksperimentelle resultater fremlagt i denne afhandling indikerer, at det nuværende videosystem kan erstattes af det nyudviklede laser radarsystem og dermed forbedre mulighederne for toog tredimensionel optisk identifikation af søminer og andre objekter. Det nyudviklede laser radarsystem har dog også en række anvendelser i luft, for eksempel ansigtsgenkendelse over flere hundrede meters afstand. Laser radarsystemets hovedkomponenter består af grøn pulset laser og et forstærket og hurtigt CCD kamera. Det nye ved laser radarsystemet er en kombination af anvendelsen af korte laserpulser (0.5 ns), en høj frekvens af laserpulser (32.4 kHz), det hurtige kamera (0.2 ns), de velkontrollerede og korte trinvise kameraforsinkelser (0.1 ns), den optiske fiber (single mode) og den udviklede algoritme til hurtig tredimensionel billedkonstruktion. Laser radarsystemet er en ny variant af eksisterende laser radarsystemer, der også anvender en pulset laser og et CCD kamera. For eksempel optager canadierne ikke tredimensionelle billeder med deres system, og svenskerne anvender ikke en optisk fiber. Det nyudviklede laser radarsystem har vist sig at kunne optage tredimensionelle billeder med en afstandsnøjagtighed, der ikke er set før blandt monostatiske laser radarsystemer. Tredimensionelle undervandsbilleder optaget med et gated viewing laser radarsystem er ikke rapporteret i litteraturen. I denne afhandling præsenteres tredimensionelle undervandsbilleder af en miniature sømineattrap camoufleret med sand for at give en lav kontrast. Undervandsbillederne er optaget på 4-5 m afstand. Undervandsbillederne kan sammenlignes med de bedste tredimensionelle undervandsbilleder, optaget med alternative laser radarsystemer. Iblandt de alternative laser radarsystemer er amerikanernes ”streak tube imaging laser radar” den mest lovende. I afhandlingen er flere forskellige eksperimenter præsenteret, hvor det nyudviklede laser radarsystems ydeevne er testet på korte, mellem og lange afstande. Ydeevnen er testet både i vand og i luft med objekter med lav kontrast. Det viser sig, at tredimensionel billeddannelse har en fordel frem for todimensionel billeddannelse, når objekterne har lav kontrast. Sammenlignet med ydeevnen i luft er ydeevnen i vand forringet på grund af, at vandet spreder og absorberer laserlyset eksponentielt. To nye indfaldsvinkler er fremført med henblik på at forbedre undervandsoptisk sømine identifikation ved hjælp af digital billedbehandling og indsigt i vands optiske egenskaber. For det første er der anvendt en canadisk spredningsfunktionsmodel, og for det andet er vandets ”line spread function” bestemt ud fra to billeder af en optisk slit optaget i luft og i vand. Der er udviklet en digital simulation af laser radarsystemet for at få et større indblik i systemets ydeevne i luft og i vand.A gated viewing high accuracy mono-static laser radar has been developed for the purpose of improving the optical underwater sea-mine identification handled by the Navy. In the final stage of the sea-mine detection, classification and identification process the Navy applies a remote operated vehicle for optical identification of the bottom seamine. The experimental results of the thesis indicate that replacing the conventional optical video and spotlight system applied by the Navy with the gated viewing two- and three-dimensional laser radar can improve the underwater optical sea-mine identification. The laser radar has also a number of applications on land, for example, face recognition at several hundred meters range. The main components of the laser radar system are a green pulsed laser and a fast gating intensified CCD camera. The laser radar system innovation is a combination of the short laser pulses (0.5 ns), the high laser pulse repetition rate (32.4 kHz), the fast gating camera (0.2 ns), the short camera delay steps (0.1 ns), the applied optical single mode fiber, and the applied algorithm for three-dimensional imaging. The gated viewing laser radar system configuration is an innovative variation of other gated viewing laser radar systems applying a pulsed laser and a range gated CCD camera, among which the Canadians do not make three-dimensional images, (e.g. [Fournier et al. 1993]), and the Swedes do not apply an optical fiber, (e.g. [Steinvall et al. 2003]), for example. The innovative gated viewing laser radar system configuration provides a previous unseen high accuracy in the three-dimensional non-scanning laser radar images. The high accuracy of the presented three-dimensional images is unprecedented among the existing gated viewing monostatic laser radar systems. Underwater three-dimensional images recorded with the gated viewing three-dimensional laser radar have never been reported. The underwater images of a low contrast target are recorded at 4-5 m range. The presented underwater images compare well with the best of the reported three-dimensional underwater optical images recorded with alternative laser radar techniques, among which the most promising is the streak tube imaging laser radar developed in United States. A variety of gated viewing three-dimensional laser radar experiments are presented, where the gated viewing threedimensional laser radar performance is tested at short, medium, and long range. The performance is also tested in low contrast conditions in the air and underwater. It is shown that three-dimensional imaging has an advantage over two-dimensional imaging, when the objects have low contrast. Compared to the performance in air the performance underwater is deteriorated by the exponential absorption and scattering. Two new lines of approach have been taken to improve underwater optical sea-mine identification by digital image enhancement and by knowledge of the optical properties of water. The first approach is taken with a Canadian scattering phase function model and an underwater optical data set. The second approach is to determine the line spread function of water from recorded slit images. A gated viewing three-dimensional laser radar digital simulation tool has been developed to gain knowledge and deeper insight into the laser radar performance in air and underwate

Online Research Database In Technology

Optical identification of sea-mines - Gated viewing three-dimensional laser radar

http://orbit.dtu.dk/ws/files/3429200/Afhandling_jbfinal.pdf

Optical identification of sea-mines - Gated viewing three-dimensional laser radar

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