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The use of autonomous underwater vehicles (AUVs) for the detection of buried mines is an area of current interest to the Mine CounterMeasures (MCM) community. AUVs offer the advantages of lower cost, stealth, reduced operator risk, and potentially improved coverage rates over more traditional mine hunters. However, AUVs also come with their own set of difficulties, including significant error in navigation and low communication rates with the mother platform and each other. In the case of bistatic detection scenarios, AUVs will therefore have difficulty knowing where exactly in space they are and the trigger time of sources on other platforms, be they ships or other AUVs. However, the potential improvement in detection and coverage rates offered by bistatic sonar concepts makes resolution of these issues a high priority. In this paper, the problems of inaccurate navigation and source timing information are addressed for the Generic Oceanographic Array Technology data set. In this experiment, conducted off Marciana Marina during June 1998, a MIT AUV with a SACLANTCEN acoustic array and acquisition system was used together with a TOPAS parametric sonar to explore issues of buried target detection using AUVs. In this paper, solutions to the navigation and timing problems are proposed which enable the effective use of bistatic synthetic aperture sonar (SAS) concepts for the detection of buried objects in the mid-frequency regime of 2-20 kHz. 相似文献
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A new coherent reverberation model developed at the Naval Research Laboratory, Washington, DC, and the Supreme Allied Commander Atlantic Undersea Research Centre, La Spezia, Italy, is exercised in the 17-750-Hz band to estimate the degree of non-Rayleighness of shallow-water reverberation envelopes as a function of waveguide multipath, system bandwidth, directivity, and frequency. Findings suggest that reverberation from diffuse, but non-Gaussian, scatterer distributions is significantly more Rayleigh for multipath environments than for equivalent environments excited by a single or small number of modes or for broadside receiver array processing that extracts narrow angles of reception. These findings suggest that the problem of non-Rayleigh reverberation in shallow-water waveguides can be ameliorated through the use of tuned ensonification and reception schemes, which retain high probabilities of detection while reducing the associated probability of false alarm. 相似文献
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We measured the δ18O of cellulose (δ18Ocel) extracted from fossil wood collected at 9 sites in the northern and southern hemispheres as a potential source of information about precipitation δ18O (δ18Oppt) in the past and paleotemperatures. The samples had been buried in fluvial sediments for periods of time ranging from ca. 45 million to 250 years. At the oldest localities (high latitude, Eocene- through Pliocene-age sites in Canada and Russia), mean annual temperature (MAT) estimates derived from the modern relationship between MAT and δ18Ocel are 6-16 °C lower than the MAT estimates derived from other biological proxies. Estimates of Pleistocene and Holocene mean annual temperatures are close to the modern values at those sites. These results are consistent with other recent findings that the MAT/δ18Oppt relationship across North America was not constant throughout the Cenozoic. Paleo-δ18Oppt estimates derived from fossil cellulose and the modern North American relationship between δ18Ocel and δ18Oppt are within the current annual range of δ18Oppt values at all locations. The middle Eocene δ18Oppt we determined from arctic cellulose samples (−21.9‰) is consistent with river water δ18O determined in two other studies (−19.1‰ to −22‰). These findings provide some evidence that a precipitation δ18O signal may be retained in wood cellulose during millions of years of burial, and that latitudinal patterns in δ18Oppt may not have changed much during the past 45 Ma. These interpretations depend, of course, on the assumption that the isotopic composition of the cellulose has not changed during burial, an assumption for which it is difficult to gather direct evidence. XRD analysis shows that the crystalline form of the fossil cellulose we used to estimate paleoprecipitation δ18O and paleo-MAT is the same as that of modern cellulose, and that the samples are free of quartz and iron oxide contaminants that result in negative errors in measured δ18Ocel. 相似文献
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