A ground-based narrow-band passive acoustic technique for estimating the altitude and speed of a propeller-driven aircraft

The Doppler effect is the change in the observed frequency of an acoustic or electromagnetic wave due to the relative motion of source and observer. The acoustical Doppler effect is utilized here to estimate the altitude and speed of a propeller-driven aircraft. The acoustic energy emitted by the aircraft is received by a microphone located just above ground level. The acoustic spectrum of the aircraft is dominated by a spectral line corresponding to the propeller blade rate which is equal to the product of the propeller shaft rotation rate and the number of blades on the propeller. For the present experiment, the aircraft is flown at a constant altitude and speed over the microphone. A frequency estimation technique is applied to the acoustic data from the microphone so that the Doppler shift in the propeller blade rate can be observed at short time intervals during the aircraft’s transit overhead. Using the altitude and speed of the acoustic source as the variable parameters, a simple model is fitted to the observed variation of the blade rate with time; estimates of the aircraft’s altitude and speed correspond to a least-mean-squares curve fit of the model to the observations. These estimates are then compared with the actual altitude and speed recorded by the aircraft itself as it flew over the microphone at each of the nominated altitudes: 250, 500, 750, 1000, 1250, and 1500 ft, and at each of the nominated speeds: 150, 200, and 250 kn.