No abstract

Influences of the atmospheric wind and temperature profiles upon the maximum height in temperature measurement by using a radio acoustic sounding system (RASS) were studied. The RASS uses radar to receive echoes (RASS echoes) backscattered from periodic perturbations in the atmospheric refractive index produced by an incident acoustic pulse and to measure the atmospheric temperature from the local speed of sound which is derived from the Doppler frequency shift of the RASS echo signal. One of the important conditions for the RASS echo to be efficiently backscattered (for monostatic radar) is that the radar beam is incident normal to the wave surface of the perturbations in the refractive index, i.e., to the acoustic wave front. This paper gives the results from numerical calculations to simulate the portion of the acoustic wave front where the above condition is satisfied, under a fixed configuration of the radar and acoustic antennas, and under changing shapes of the propagating acoustic pulse wave front influenced by height‐varying background atmospheric wind and temperature. Comparisons between the calculations and RASS experiments were made by using the middle and upper atmosphere (MU) radar with steerable beam under different atmospheric conditions. The numerical estimations of the height range for effective reception of RASS echo agree quite well with actual RASS observations carried out by using the MU radar. RASS echoes were obtained at altitudes up to 8.4 km under strong wind conditions in January 1988 and up to 22 km under calm wind conditions in July 1986.

No abstract

This paper is concerned with the accuracy of temperature measurements with radio acoustic sounding system (RASS) consisting of a pulsed Doppler radar and an acoustic source, where the latter excites short monochromatic pulses. Through the use of a numerical model and middle and upper atmosphere (MU) radar experiments, we found that the accuracy is significantly affected by the acoustic and radar pulse length ratio. When the Bragg condition is not strictly satisfied, a numerical model predicted that the mean frequency shift fm of a RASS echo spectrum is detected between the Doppler‐shifted frequency corresponding to the sound speed fs and the transmitted acoustic frequency fa. When the ratio is close to or larger than unity, fm becomes almost identical with fa, while fm approaches fs as the ratio decreases. RASS experiments involving the MU radar operating at 46.5 MHz (6.45‐m wavelength) and an acoustic transmitter with a frequency of about 100 Hz showed that the observed characteristics of RASS echoes for various acoustic pulse lengths agreed quite well with model predictions. Although the numerical model suggested that a small value of the ratio is preferable for accurate measurement of temperature with RASS, the minimum value of the ratio was determined to be about 0.2 by taking into account the system sensitivity of the MU radar, since the RASS echo intensity decreases as the acoustic pulse length becomes shorter. When the lengths of the acoustic and radar pulses were set equal to about 60 m (18 acoustic wave cycles) and 300 m (l μs in pulse duration), respectively, which gives a ratio of the acoustic pulse length to the radar pulse length of approximately 0.2, we were able to obtain temperature profiles at 5 to 9 km every 3 min with an accuracy of about 0.5°C.

No abstract