Passive ranging techniques are used in land-based acoustic surveillance systems and underwater sonar systems to localize sources that radiate acoustic energy into the environment. Passive ranging by wavefront curvature relies on the spherical expansion of the wavefronts as the acoustic energy propagates outwards from the source. A wide-aperture receiving array is used to sense the curvature of the wavefront by estimating the intersensor time delays as the wavefront traverses the array. The time delay estimates are used to calculate the range (which is equal to the radius of curvature of the wavefront) and bearing of the source. The wavefront curvature method is applied here to the passive ranging of sources of four different types of acoustic signals: underwater mechanical transients, underwater biological transients, continuous sound wave transmissions in air and impulsive sounds in air. The method provides precise range and bearing estimates of underwater signal sources. In comparison, large passive ranging errors are observed for in-air
sources because the atmosphere is a nonstationary sound propagation medium. Atmospheric turbulence causes perturbations in the curvature of the acoustic wavefronts and leads to random fluctuations in the source position estimates on time scales ranging from seconds to minutes. Background noise at each sensor has only a small effect on
the positional uncertainty of in-air sources with random fluctuations in the source position estimates occurring on subsecond time scales.
Acoustic sensing systems are used to detect, localize, track and classify sources of military interest in real time with negligible false alarm rates. Automated acoustic systems are able to cue response systems and devices such as cameras for source identification. Two defense applications are demonstrated: one involves remote land-based surveillance where an array of unattended passive acoustic ground sensors automatically cues a day/night camera to observe the passage of ground vehicles, the landing of air vehicles on an isolated air strip, and the transit of motor-powered watercraft in estuarine waters. The video imagery is compressed and relayed via satellite to a central monitoring facility for input to the decision and intelligence processes. The other application is for in-harbor force protection and port infrastructure security where a high-frequency high-resolution monostatic active sonar automatically detects, localizes and tracks fast inshore surface watercraft in real time. A cavitating propeller forms a bubble wake that lasts several minutes and is highly reflective of the incident sonar energy. The wake, which traces the trajectory of the watercraft, is clearly delineated on the sonar display. The active sonar reliably estimates the instantaneous position of the moving source at each point along its path of travel. The sonar can be used to pan an imaging device to aid identification of the moving source or to vector autonomous response craft for intercept purposes.