This paper describes the detection and classification of targets against clutter by distinguishing between linear and nonlinear scatterers and, further, by distinguishing those nonlinear targets that scatter energy at the even-powered harmonics from those that scatter in the odd-powered harmonics. This is done using twin inverted pulse sonar (TWIPS), which can also, in some manifestations, require no range correction (and therefore does not require the a priori knowledge of the environment needed for most remote detection technologies). The method applies, in principle, to a range of sensor technologies, including the use of radar to distinguish between circuitry, metal and soil; Light Detection and Ranging (LIDAR) to detect combustion products; and Magnetic Resonance Imaging (MRI). A sonar application is demonstrated, detecting objects in bubbly water (including in the wake of a ship of 3953 gross register tonnage). A manmade sonar that can operate in bubbly water is relevant: Cold War sonar is not optimized for the shallow coastal waters that typify many current operations. The US Navy use dolphins in such waters. TWIPS arose as a demonstration that echolocation was possible in bubbly water in response to a video showing dolphins generating bubble nets when hunting: if echolocation were impossible in these nets, then during this hunt, the dolphins would have blinded their sonar.

Dolphins have been observed to blow bubble nets when hunting prey. Such bubble nets would confound the best man-made sonar because the strong scattering by the bubbles generates 'clutter' in the sonar image, which cannot be distinguished from the true target. The engineering specification of dolphin sonar is not superior to the best man-made sonar. A logical deduction from this is that, in blowing bubble nets, either dolphins are 'blinding' their echolocation sense when hunting or they have a facility absent in man-made sonar. Here we use nonlinear mathematical functions to process the echoes of dolphin-like pulses from targets immersed in bubble clouds. Dolphins emit sequences of clicks, and, within such a sequence, the amplitude of the clicks varies. Here such variation in amplitude between clicks is exploited to enhance sonar performance. While standard sonar processing is not able to distinguish the targets from the bubble clutter, this nonlinear processing can. Although this does not conclusively prove that dolphins do use such nonlinear processing, it demonstrates that humans can detect and classify targets in bubbly water using dolphin-like sonar pulses, raising intriguing possibilities for dolphin sonar when they make bubble nets.

Twin inverted pulse sonar (TWIPS) is here deployed in the wake of a moored rigid inflatable boat (RIB) with propeller turning, and then in the wake of a moving tanker of 4580 dry weight tonnage (the Whitchallenger). This is done first to test its ability to distinguish between scatter from the wake and scatter from the seabed, and second to test its ability to improve detectability of the seabed through the wake, compared to conventional sonar processing techniques. TWIPS does this by distinguishing between linear and nonlinear scatterers and has the further property of distinguishing those nonlinear targets which scatter energy at the even-powered harmonics from those which scatter in the odd-powered harmonics. TWIPS can also, in some manifestations, require no range correction (and therefore does not require the a priori environment knowledge necessary for most remote detection technologies).

No abstract

The proposition that the use of twin inverted pulses could enhance radar is tested. This twin inverted pulse radar (TWIPR) is applied to five targets. A representative target of interest (a dipole with a diode across its feedpoint) is typical of covert circuitry one might wish to detect (e.g. in devices associated with covert communications, espionage or explosives), and then distinguish from other metal (‘garbage’ or ‘clutter’), here represented by an aluminium plate and a rusty bench clamp. In addition, two models of mobile phones are tested to see whether TWIPR can distinguish whether each is off, on or whether it contains a valid SIM card. Given that a small, inexpensive, lightweight device requiring no batteries can produce a signal that is 50 dB above clutter in this test, the options are discussed for using such technology for animal tagging or to allow the location and identification of buried personnel who opt to carry them (rescue workers, skiers in avalanche areas, miners, etc.). The results offer the possibility that buried catastrophe victims not carrying such tags might still be located by TWIPR scattering from their mobile phones, even when the phones are turned off or the batteries have no charge remaining.