In recent years, Doppler radar and Doppler lidar are used for the wind shear monitoring in airports. However these monitoring systems are used to detect the horizontal wind shear in the long range about a kilometer to a hundred kilometers. There is no warning system to observe the wind shear in the low altitudes near the decision height of landing. But it is important for the takeoff or landing aircrafts to detect the wind shear and the occurrence of turbulence in the low altitudes around airports.

The authors have developed the ground based acoustic remote sensing system that can observe the time-height variation of air flow in the height range from about 30m to 200m. This height range is important for aircrafts as the decision height. This acoustic remote sensing system monitors the Low Altitude Wind Shear (LAWS) caused from the atmospheric phenomena such as Gust front , Down burst, Lee wave, Low level jet, Local front and Turbulent wake of structures etc.

In principle, this system is the bi-static Doppler sodar using the phased array antennas. The system can measure the wind profiles above a runway every three seconds. The system adopts high-performance array antennas. Those are composed of one vertical Transmitter-Receiver, which has 13x13 honeycomb acoustic elements, and two inclined Receivers, each of which has 13x7(6) honeycomb acoustic elements. The receiver has the function of phase synthesis to irradiate the desired altitudes with an acoustic beam. This function shows the good isolation of received signals between adjacent altitudes. Each element is the newly developed horn speaker with a PZT diaphragm. It has the speaker sensitivity better than SPL100dB/m/wat in 2kHz to 3kHz frequency range used in the present study. This means that the new element is improved about 5dB better than the conventional one in the conversion efficiency.

Electric systems are composed of the Transmitter-Receiver (T-R)unit, which transmits tone bursts and amplifies received signals, the Signal Processor unit, which executes filtering, phase synthesis and Doppler analysis, the Control unit, which receives digital signals from the T-R unit and the R units through LAN connection, and calculates the wind vectors and statistics. Display-Server unit displays and stores the observed data in the main control room.

Data outputs are the vertical profiles of wind speed and direction, intensity of turbulence, gust factor (GF) and upper wind shear factor (UWSF) defined by formula (1).

UWSF= √ ∑ (u'²+v'²+w'²)/N (1)

UWSF is the parameter that includes the effect of vertical wind turbulence additional to wind shear factor (WSF) usually observed by a surface anemometer. WSF shows the shear generated by horizontal wind and direction as formula (2)

WSF= √ ∑(u'•v')/N (2)

GF indicates only the effects of wind speed fluctuations. The newly developed system can supply such data every two minutes calculated from wind vectors in each altitude observed every three seconds. The authors report the compared data of this system with the tower sensors and show the observation data in the airport related with the planes shake.