测控技术与仪器专业英语阅读翻译.docx

1、测控技术与仪器专业英语阅读翻译 测控技术与仪器专业英语阅读翻译篇一：测控技术与仪器专业英语课后阅读翻译(1,510) 第1章课后 Underwater Acoustic Signal In the operation of a sonar system the operator is repeatedly faced with the problem of detecting a signal which is obscured by noise. This signal may be an echo resulting from a transmitted signal over which

2、 the operator has some control, or it may have its origin in some external source. These two modes of operation arise in radar surveillance and in disciplines for techniques and for illustrations of the basic principles. Since there are many ways in which one can think about signal detection , it is

3、 desirable to define a term to denote special cases . The word detection will be used when the question to be answered is, ?Are one or more signals present? when the system is designed to provide an answered to this question , either deterministic or probabilistic, one speaks of hypothesis testing.

4、The case of a single signal occurs so often that many system are designed to provide only two answers, ?Yes , a signal is present,? or ?No, there is no signal.? One can make the problem more complicated by endeavoring to classify the signal into categories. Decisions of this latter kind will be refe

5、rred to as target classification. Normally a piece of detection equipment is designed to operate in a fixed mode and the parameters such as integrating time of rectifier circuits or persistence of the oscilloscope tube for visual detection cannot be changed readily. There will always be some uncerta

6、in signals, which the observer will be hesitant to reject or accept. In these cases the operator might have the feeling that if the integrating time of the detector or the persistence of the oscilloscope tube were longer, he could reach a decision about the existence of the signal. Wald(1950) has fo

7、rmulated this intuitive feeling into a theory of detection. When one is able to vary deliberately the interval over which one stores data in the reception system in order to achieve a certain level of certainty, one speaks of sequential detection. Frequently it is desirable to determine not only the

8、 presence or absence of the signal but also one or more parameters associated with the signal . The parameters of interest can vary widely from a simple quantity such as time of arrival or target bearing to the recovery of the completewaveform . When a system is designed to recover one or more param

9、eters associated with thesignal , one speaks of signal extraction. The word signal was not defined and it was assumed that the reader had an intuitive felling for the word. Some elaboration may be in order since the definition of signal subjective and depends on theapplication . One may say that ?si

10、gnal? is what one wants to observe and noise is anything that obscures the observation. Thus, a tuna fisherman who is searching the ocean with the aid of sonar equipment will be overjoyed with sounds that are impairing the performance of a nearby sonar system engaged in tracking a submarine. Quite l

11、iterally, one man?s signal is another man?s noise. Signals come in all shapes and forms. In active sonar system one may use simple sinusoidal signals of fixed duration and modulations thereof. There are impulsive signals such as those made with explosions or thumpers. At the other extreme one may ma

12、ke use of pseudorandom signals. In passive systems, the signals whose detection is sought may be noise in the conventional meaning of the word; noise produced by propellers or underwater swimmers, for example. It should be evident that one of our problem will be the formulation of mathematical techn

13、iques that can be used to describe the signal. Although the source in an active sonar search system may be designed to transmit a signal known shape, there is no guarantee that the return signal whose detection is sought will be similar. In fact , there are many factors to change the signal. The amp

14、litude loss associated with inverse spherical spreading is most unfortunate for the detection system nut it does not entail any distortion of the wave shape . (Incidentally, where the wave can be approximated locally as a plane wave.) The acoustic medium has an attenuation factor , which depends on

15、the frequency . This produces a slight distortion of the wave shape and a corresponding change in the energy spectrum of the pulse. The major changes in the waveform result from acoustic boundaries and inhomogeneities in the medium. When echoes are produced by extended targets such as submarines, th

16、ere are two distinct ways in which echo structure is affected. First, there is the interference between reflections from the different leads to a target strength that fluctuates rapidly with changes in the aspect. Secondly, there is theelongation of the composite echo due to the distribution of reflecting features along the submarines. This means that the duration of the composite echo is dependent in a simple manner on the aspect angle. If T is the duration of th