tailieunhanh - Atmospheric Acoustic Remote Sensing - Chapter 4

Bản chất của một hệ thống từ xa cảm ứng âm thanh tạo ra âm thanh vào một chùm wellformed tương tác với không khí một cách được biết đến và sau đó phát hiện tương tác. Trong chương 2, chúng tôi đã học được về bản chất của không khí vào âm thanh dự, và trong Chương 3 cách thức mà âm thanh đi. Trong chương này chúng tôi mô tả làm thế nào để hình thành một chùm âm thanh, âm thanh phân tán như thế nào được phát hiện, và làm thế nào hệ thống được thiết. | 4 Sound Transmission and Reception The essence of an acoustic remote-sensing system is in generating sound into a well-formed beam which interacts with the atmosphere in a known manner and then detecting that interaction. In Chapter 2 we learned about the nature of the atmosphere into which the sound is projected and in Chapter 3 the way in which sound travels. In this chapter we describe how to form a beam of sound how scattered sound is detected and how systems are designed to optimize retrieval of various atmospheric parameters. The main emphasis of Chapter 4 is on geometry and timing but details on some of these aspects are left to Chapter 5. GEOMETRIC OBJECTIVE OF SODAR DESIGN The boundary layer atmosphere is often strongly varying in the vertical but horizontally much more homogeneous. The geometric design objective for vertically profiling instruments is therefore to localize the acoustic power sufficiently in space so that atmospheric properties are obtained from well-defined height intervals at a particular time. This means that the vertical resolution has to be defined typically by using a pulsed transmission. But since sound will spread spherically from the source height resolution also depends on angular width of the beam transmitted. Here we concentrate on SODAR SOund Detection And Ranging systems for which the acoustic beams are often non-vertical as shown in Figure . Here the pulse duration is T and the angular width of the acoustic beam is Aộ in azimuth angle and A0 in zenith angle. From Figure the vertical extent of the pulse volume is CT cos 0 2zAsin0A0 which has a term increasing with height z. Taking CT 20 m and 0 20 the vertical extent of the pulse volume near the ground is CT cos 0 m but for a beam half-width of A0 5 this increases to 50 m at z 500 m. This emphasizes the need to keep the product sin0A0 small. Also if A0 is too large then the pulse volume will include a wide range of radial velocity the Doppler spectrum will be