Research project: Dolphin-inspired radar for finding bombs, bugs and catastrophe victims

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Following the successful invention of the world’s first sonar that can operate in bubbly water (click here for that project page), the same waveforms were used to devise a new radar system for the detection of bombs, bugs, and catastrophe victims.

Project Overview

What is it for?

A normal radar image shows the location of many objects. It can be difficult to distinguish the target you are interested in from some other objects that also give off strong radar scatter, but are of no interest to you (“clutter”). Twin Inverted Pulse Radar (TWIPR) allows particular types of circuits to be clearly distinguished from one another, and from clutter.

Where would you use it?

Imagine you are looking at the radar image:

of a building, searching for embedded bugging devices;
of a roadside, searching for buried explosives;
of a collapsed building, searching for buried rescue workers or catastrophe victims;
of woodland, searching for discarded phones, buried digital watches etc.

In such cases, the problem is twofold:

getting a strong signal from your targets of interest (strong compared to background noise);
distinguishing the target of interest from other strong radar scatterers (such as, air conditioning ducts, discarded trash such as the lead changed hands, nails, etc. – “clutter”).

Different types of circuit give particular "TWIPR fingerprints" in terms of the frequency spectrum they emphasize when scattering a TWIPR pulse. For example, a small target (figure 1) that was made to resemble certain types of bomb trigger, produced a TWIPR echo that was 100,000 times more powerful than the echo from an aluminium plate the size of a kitchen tea tray. This allows that circuit to be readily identified and distinguished from other objects ("clutter") that would show up more strongly in normal radar images.

How does TWIPR work?

The radar system TWIPR grew out of TWIPS (twin inverted pulse sonar)an explanation of which can be found by clicking here .

In TWIPS, the targets of interest were fish, and the clutter was provided by underwater gas bubbles blown by dolphins from their blowholes when they create nets of bubbles in order to hunt fish.

Both TWIPS and TWIPR send out a pair of pulses, the second pulse being identical to the first but having reversed polarity. We can represent the first pulse by +1, and the second pulse by a -1. In TWIPS, the fish scatter back echoes that are very similar to the sonar pulses that hit them, so that if we subtract the second echo from the first, we get a strong signal [1-(-1)=2]. However, the bubbles scatter back a lot of energy as the square of the pulses that hit them, so that subtraction of these echoes generates a very small signal [12-(-1)2=1-1=0]. The fish (or other linear scatterers, like mines in bubbly coastal seawater) can therefore give a very strong TWIPR signal compared to the bubbles, which would be the stronger scatterers for normal sonar.

In the case of TWIPR, it is the circuits of interest (mobile phones, bugging devices, bomb triggers) that scatter the square of the pulses that hit them, so in this case it is addition of the echoes which makes such targets [12+(-1)2=1+1=2] stand out much more strongly than would linearly scattering clutter (such as buried cans, bicycle parts etc.): [1+(-1)=0].

Why do bubbles scatter the square of sonar signals, and electronics scatter the square of radar signals?

Bubbles and circuits scatter energy a lot of ways, only some of it as the square of the incoming pulse, but that is enough to make TWIPR and TWIPS work. This is because they do not respond 'linearly' to the incoming pulse: if they did, then doubling the strength of the incoming pulse would double the strength of the echo, and furthermore the scatterers would produce echoes of the pulse that showed the same positive/negative symmetry as the incoming pulse.

Bubbles do not do this, because the gas in the bubble gets stiffer the more it is compressed. Put in simpler terms, the pressure variations in the sonar pulse consist of alternating half cycles of rarefaction (reduced pressure) and compression (increased pressure). The rarefactions can in principle cause the bubbles to expand as much as they like, but the compressions cannot squash the bubble indefinitely or it ceases to exist - therefore the bubble motion is asymmetrical compared to the positive and negative half-cycles of the incoming pulse.

Electronic components like diodes, for example, are designed to allow current flow in one direction but not the other. Therefore when the oscillating electromagnetic fields in the radar pulse try to induce electrical current flow one way and then another through the device, only one of those currents flows - again the positive/negative symmetry that was present in the incident pulse is lost.
What are the applications?

TWIPR was initially designed to identify certain types of bomb triggers. However the fact that trigger-like circuit shown in figure 1 measured 6 cm in length, weighed 2.8 g, costs less than one Euro and requires no batteries, opens up other possibilities. Specifically, that TWIPR could be used with easily-manufactured small, lightweight and inexpensive location and identification tags for animals, infrastructure (pipelines, conduits for example) and for humans entering hazardous areas, particularly where they might be underground or buried. These tags can easily be tuned to scatter-specific resonances to provide a unique identifier to a TWIPR pulse, the 'TWIPR fingerprint’.

Buried catastrophe victims not carrying such tags might still be located by TWIPR, as it can carry the bandwidth to search for mobile phone resonances, offering the possibility of locating victims from their mobile phones, even when the phones are turned off or the batteries have no charge remaining, or have been crushed.
Although the tags are small, all radar systems are sizeable - however TWIPR does not require manufacture of new radar hardware 'from scratch': Converting currently-available radar systems to TWIPR means replacing the current waveforms ('tunes') played by today's radar systems, with a TWIPR waveform, and processing the echoes the TWIPR way. The inventors have published the method so that it is freely available, so that current radar manufacturers are not restricted from using it.

One important restriction for TWIPR is that the TWIPR pulse must be strong at the location of the target. If the radar source cannot be brought close to the target (e.g. using a robot roller, snake, or micro-copter), then a larger, more powerful, radar source must be used to sweep for possible targets from a distance.

Webs, blogs, and press material on this story

Professor Tim Leighton talking to camera

Further details:

For an overview see the University Press Release ( https://southampton.likn.co/mediacentre/news/2013/oct/13_188.shtml ), and for technical detail see the publication tab.