Method and sensor for determining the distance of sound generating targets

#This application claims the priority of application Ser. No. P 42 00 170.6, filed Jan. 7, 1992, in the Federal Republic of Germany, the subject matter of which is incorporated herein by reference.

#The present invention relates to a method and to a sensor for determining the distance of sound generating targets with respect to the sensor by means of acoustic signals subjected to a Fourier transformation.

#DE-OS (German Unexamined Published Patent Application) 3,936,359 discloses the use of a sensor arrangement composed of a plurality of sensors to determine the direction and distance of sound generating targets, with acoustic sensors being employed to determine the target direction. A seismic sensor is additionally employed to determine the target distance. A line characteristic of the target in the air and ground sound spectra is measured and the Doppler shift between these lines as well as the propagation speed of the corresponding waves are used to calculate the radial target velocity and from it, under consideration of the bearing angle, the target distance. Aside from the fact that a number of sensors are required to determine the target distance, the propagation speed of sound waves in the ground relative to the propagation speed of sound waves in the air fluctuates greatly depending on the type of ground involved so that this propagation speed must be determined which requires the formation of a cross spectrum of the signals received from two sensors.

#The propagation of sound signals in air is subject to geometrical as well as atmospheric attenuation. The atmospheric attenuation, whose value is a function of frequency, can be neglected for the frequency and distance ranges of interest so that the attenuation is determined essentially by the geometric propagation attenuation. In this connection, the targets can be considered, in approximation, as point sources (usual target distances up to about 300 m) and thus the propagating sound waves as spherical waves whose amplitude drops proportionally to 1/r while their signal power drops proportionally to 1/r.sup.2. If the sound spectrum radiated by targets is assumed to be constant for target movement paths (armored vehicles and the like) of <10 m, the distance of the target from the sensor can be calculated by way of a determination of the signal power (sum power or spectral power density) and the associated phase spectrum at two suitable points.

#FIG. 1 shows a target 1 which at time t.sub.1 has a distance r.sub.1 and at time t.sub.2 a distance r.sub.2 from an acoustic sensor 2 (microphone). The radial distance between the two points is .DELTA.r=r.sub.2 -r.sub.1. The value of .DELTA.r is calculated by evaluating the phase difference between the sound spectra at the two measuring points r.sub.1 and r.sub.2. Knowing .DELTA.r and the signal powers P(r.sub.1) and P(r.sub.2), the desired target distance r.sub.1 or r.sub.2, respectively, can then be calculated from the signals of the sole sensor 2 by way of the above assumed propagation model.

#It is assumed that S(r.sub.2, kF.sub.G) and S(r.sub.1, KF.sub.G) are sound signals at times t.sub.2 and t.sub.1 which have already been subjected to a Fourier transformation and are therefore displayed in the frequency domain, with F.sub.G representing the frequency resolution and k a running variable. Under the assumption that the target radiates acoustically uniformly in distance region r.sub.2, r.sub.1, the following results for the cross spectrum of the signals: