Recently, sonar signals and other sounds produced by cetaceans have been used for acoustic detection of individuals and groups in the wild. However, the detection probability ascertained by concomitant visual survey has not been demonstrated extensively. The finless porpoises ͑Neophocaena phocaenoides͒ have narrow band and high-frequency sonar signals, which are distinctive from background noises. Underwater sound monitoring with hydrophones ͑B&K8103͒ placed along the sides of a research vessel, concurrent with visual observations was conducted in the Yangtze River from Wuhan to Poyang Lake in 1998 in China. The peak to peak detection threshold was set at 133 dB re 1 Pa. With this threshold level, porpoises could be detected reliably within 300 m of the hydrophone. In a total of 774-km cruise, 588 finless porpoises were sighted by visual observation and 44 864 ultrasonic pulses were recorded by the acoustical observation system. The acoustic monitoring system could detect the presence of the finless porpoises 82% of the time. A false alarm in the system occurred with a frequency of 0.9%. The high-frequency acoustical observation is suggested as an effective method for field surveys of small cetaceans, which produce high-frequency sonar signals.
Detection resolution is crucial for improvement of the measurement precision in the device and instrument. Because of the limited resolution, a fuzzy area with the truth-value as its center is found during the detection. The finding for improving the measurement precision by the border of fuzzy area is first introduced. The higher resolution can be captured by the higher resolution stability which makes the different detection results of the inner and outer fuzzy area on the border reflected more sensitively between the measure and the reference quantity. The system resolution obtained only depends on the stability of measurement resolution, which is much better than the measurement resolution itself. Based on the finding, the measurement precision can be improved two or three orders of magnitude. The finding can be used in various kinds of high precision measurement.
With the improvement of the accuracy of atomic frequency standard and satellite navigation, the high-resolution phase comparison method is necessary. Using the phase synchronous detection principle, a super-high resolution phase comparison method between frequency standards is proposed based on the greatest common factor frequency, phase group processing and a common frequency source and so on. This method is mainly dependent on the stability of the common frequency standard and its frequency. The ±1 count error can be eliminated effectively. Therefore, higher than 1 ps resolution can be easily reached with a simple instrument. Experimental results show higher than 10 −15 /h precision can be obtained in the long-term frequency standard comparison and the measuring precision can reach 10 −17 for several days of comparison.
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