Measurement of air characteristics in the lower atmosphere

Abstract

Sodar systems and methods for acoustically sounding air are disclosed in which chirps longer than 300 ms-and preferably with durations of tens of seconds-are used along with matched filter and / or Fourier processing methods to derive phase signals indicative of air characteristics in range. A listen-while-transmit strategy is preferred, the direct signal being removed by subtracting the phase signals from two or more receivers located near the transmitter so as to be in the same noise environment. The resultant differential signals can be related to cross-range wind with range distance. In one example, apparatus (100) is employed comprising a reflector dish (102) over which one central loudspeaker (110) and four microphones (112, 114, 130 and 132) are mounted, the microphones preferably being located on cardinal compass points and having their axes (124, 126) slightly angled with respect to the vertical transmission axis (122).

Description

[0001] This application is a continuation of international Application Serial No. PCT / AU02 / 01129 filed 19 Aug. 2002, published under PCT Article 21(2) in English; and claiming priority from Australian patent applications PR 7203 filed 23 Aug. 2001 and PR 7832 filed 21 Sep. 2001, and applicant claims the benefit of Australian patent applications PR 7203 filed 23 Aug. 2001 and PR 7832 filed 21 Sep. 2001.[0002] This invention relates to the use of acoustic signals for atmospheric sounding and is particularly concerned with sodar techniques for measuring air velocity variation--such as horizontal wind speed variation, wind-shear and / or turbulence--in the lower atmosphere. The invention may, however, be applied to measuring local density variation in the atmosphere, such as may be caused by temperature gradients, temperature, thermal inversions and variations in moisture content.[0003] The apparatus and methods of the intention are also applicable to wind profiling in the vicinity of air...

AI Technical Summary

Benefits of technology

"The invention is about methods and apparatus for acoustic sounding in air by transmitting chirps and listening for echoes while sending. This allows for better detection and discrimination of echoes, even at long ranges. The use of long chirps and matched filter techniques improves the ability to detect and process faint echoes. The invention also includes methods for multiple receivers located close to a common transmitter to efficiently remove common signals and wind speed variations. By transmitting two differently coded chirps and manipulating the cumulative phase outputs, the invention allows for the generation of an output that is indicative of variation of the speed of sound with range and temperature. This further improves echo discrimination and cycle time."

Problems solved by technology

Though exclusively acoustic methods for wind profiling and the like have a long history, Coulter & Kallistratova in their 1999 article "The Role Acoustic Sounding in a High-Technology Era" [Meteorol. Atmos. Phys. 71, 3-19] show that these methods have not lived up to their promise.

This appears to have been largely due to an inability to achieve an adequate signal-to-noise ratio [s / n].

Despite the application of such sophisticated techniques to sodar, a review by Crescenti entitled, "The Degradation of Doppler Sodar Performance Due to Noise" [Crescenti, G. H., 1998, Atmospheric Environment, 32, 1499-1509], found that severe problems remain even at modest ranges of 1500 m.

Images